(A)IMS - an Almost Ideal Measurement System

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(A)IMS - an Almost Ideal Measurement System

Presented by

David Pedrick Pedrick Yacht Designs, Inc. Newport, Rhode Island, USA

at the "Yacht Vision '94" Symposium Auckland, New Zealand

16-20 February 1994

This paper is intended for sailors interested in the Jundamental working principles and current state of the art of the International Measurement System (IMS)- The strengths and weaknesses of the IMS are described and priorities for its improvement are proposed Goals of an ideal measurement system for handicapping yacht races are presented It is then shown how the IMS is effective in approaching those goals, with the potential to converge ever closer to ideal yacht handicapping.


The equitable scoring of racing yachts is an ideal that has eluded rulemakers for as long as there have been races. Except in one-design classes, yacht owners have been eager to pursue a competitive advantage through rule-exploitative design, and designers have obliged. It evokes a decades-old piece titled "Men Against the Rule."

The fiindamental problem with scoring yachts against each other is that the assumed performance criteria have been based on human mterpretation of limited information. A fixed rating rule, such as the IOR', computes specific hull and rig measurements through a series of empirical formulae as developed by the rule's technical committee, yielding an all-encompassing, single number for each yacht. This number ~ the rating ~ is meant to represent the yacht's relative performance potential in all wind and sea conditions against diverse types and sizes of competitors. Popular handicapping systems, such as PHRF^ and CHS3, reduce the algebraic clutter to a smaller number of equations, but supplement the

1 International Offshore Rule ^Performance Handicap Racing Fleet ^Channel Handicap System

resulting technical madequacies with subjective adjustments "guesstimated" by regional handicapping committees. Both of these approaches have technical failings that invite a better solution.

Goals of an Ideal Measurement System -In a perfect world, what would an Ideal Measurement System be able to do? Perhaps the following:

1. Assess the speed potential of all yachts flawlessly for any and all strengths and directions of the wind and sea, allowing for diversity in hull shape, proportions, displace-ment, appendage efficiency, rig configuration, sail efficiency, interior outfit, hull and deck construction, longitudmal weight distribution and overall stability;

2. Know the precise, actual wind and sea conditions that existed for each yacht throughout a given race; and,

3. Cnmhine 1 and 2 for the fleet of a given race, and rank each yacht according to its elapsed time relative to its predicted potential time, or race-specific handicap; i f the System is based on accurate science, a yacht having a better (lesser) ratio of (elapsed time/predicted


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

time) indicates a higher level of sailing skill (luck being ignored) than one having a worse (greater) ratio of actual to target performance, and is, therefore, the winner.

Under this Ideal Measurement System, yachts could evolve according to the highest and best knowledge of hydrodynamics and construction, as well as to current styling trends and the whims of each owner, without obsoleting last year's or last decade's model on the race course. There would be no rule biases, and therefore no "horses for courses" or type-forming design pressures. Progress would keep pace with rapidly improving technology, unfettered by rule restrictions, and sailors would race equitably in any kmd of yacht they chose.

I f only the problem were as easy as the "1-2-3" above, racing sailors would flock to the Ideal Measurement System as the obvious choice for fair racing. So, why don't we have one?

Enter the I M S -¬

No, not the Ideal Measurement System. The other IMS, the International Measurment System ~ an almost ideal measurement system.

The search for the ideal yacht handicapping system began 20 years ago ~ during the early heyday of the lOR - when two M I T ^ hydronamics professors, Nick Newman and Jake Kerwin, attracted the support of some forward-looking, senior statesmen of US yachting, notably Pat Haggerty, Irving Pratt, Clayton Ewing and Lynn Williams. The profs proposed a new, computer-based prediction approach to yacht handicappmg, and the yachtsmen provided the leadership and funding to turn on the most serious research project that has ever been undertaken for the common good of yacht racing.

'•Massachusetts Institute of Technology, USA

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The I M S Concept ~

The founders of what has become the International Measurement System began with a vision of a yacht handicapping system very much like the "1-2-3" ideal above. It was driven by an underlying concern to protect the competitiveness and long-term value of the existing cruiser-racer fleet.

The notions of computer-based hull lines and empirically derived performance predictions were brand new concepts in the early 1970's. The founding visionaries proposed that each yacht's configuration would be entered into the computer, which would have a program to predict its performance according to theroetical principles of hydro- and aerodynamics, and supporting, empirical data. The computer modeling of the physics that produce sailing equilibrium would be updated continually as the science improved, or as needed to adapt to design pressures. The intention was to be broadly inclusive of diverse yacht types.

By defining the race course and wind conditions, yachts would be handicapped according to their predicted speed potential much more fairly than through the use of fixed ratings and standard time allowances. Furthermore, this system would use integrated values of principal parameters, avoiding the extreme sensitivity to limited, local point measurements common among all existing measurement rules. But, as the song of the same period warned, easier said than done!

Great things can be accomplished when smart people are hired to tackle a major, intellectual challenge, however. In three, very fertile years, from 1975-78, the Pratt Project ~ a collaboration of professors and graduate students at M I T , Prof Gerritsma and his colleagues at Delfl University, Olin Stephens and other designers in the US, and additional, technically inclined yachtsmen - produced the first working version of this scientifically


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modeled system for yacht handicapping. It was introduced as the "Measurement Handicap System" (MHS), and was first used in the 1978 Newport-Bermuda Race. Its name was chosen carefully. I t is a system of electronic and manual hull and rig measurement, processed through a computer program that calculates race-specific handicaps, using integrated measurements (rather than subjective, human adjustments) for the primary handicapping parameters. Such a technically sophisticated, computer-based approach to yacht handi-capping was especially extraordinary, considering the era ~ keep in mind that the Apple I I was introduced only in 1977, and the I B M PC did not arrive until 1981.

After several years of use in the US, the Offshore Racing Council (ORC) asked the United States Yacht Racing Union (now named the US Sailmg Association) to admmister the rule internationally. Renamed the International Measurement System in 1985, it is now mamtained through the ORC's International Technical Committee (ITC), which responds to internationally organized submissions for improvement.

Coupled to the performance prediction approach to handicapping has been the requirement that yachts racing under this system would have to incorporate a reasonable degree of cruising accommodations. These were intended originally to be characteristic of bona fide, dual-purpose cruiser/racers. (More about this later.)

Although the MHS didn't set the world on fire with its immediate brilliance, it was a noble start. Indeed, the basic underpinnings of today's IMS have changed very little in a further fifteen years. The focus of continuing, extensive work has been on refining the process and improving the physics of the prediction model, and adding secondary factors as necessary to treat diverse types of yachts more equitably.

Elements of the I M S

-The heart and genius of the IMS is the Velocity Prediction Program (VPP). The VPP is the central "solver" of all of the hydrodynamic and aerodynamic forces for a given yacht's configuration, flotation and stability. It receives data of various types, especially the yacht's hydrostatic data fiom the Lines Processing Program (LPP), and computes an optimized equilibrium solution for a matrix of true wmd speeds and angles, adjusting the sail "trim" to operate the yacht at its most efficient condition at each point of the wind matrix. Since it was first published in 1978, the Pratt Project VPP has spawned many variants of the code inside and outside the IMS, so "the VPP" has become a generic reference. Aside from its handicapping importance, it has been established as an essential yacht design tool for over a decade. In this paper, however,

"VPP" refers only to the IMS version.

Figure 1 is a much-simplified flow diagram of how the IMS goes from a yacht hauled out "on the hard" to a race committee's prediction of handicap data for different courses and winds. The next few sections of this paper will expand on the basic elements of the IMS.

Yacht Measurement: Accommodations:

From the start, the Pratt Project was intended to give dual-purpose yachts a fair shake on the race course. So, a broadly applied prerequisite for racing under the IMS has been to meet the minimiun standards of accommodation established in the system's associated regulations. These exist because the prediction process has not yet been able to make an accurate allowance for the performance penalty caused by differences in accommodation weight. The IMS is getting closer to having a solution to that, however.


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The IMS acommodation regulations stipulate minimum values for each of many interior features: headroom dimensions; number, type and sizes of berths; galley features; dining table; stowage; enclosed head; and tankage capacities. However, generally greater than minimum interior features are necessary to achieve the mmimum accommodation rating recommended for racing under IMS. Base values of each feature, scaled to the length of the yacht, produce a score of 100 points. Trade-offs can be made within upper and lower boimds to achieve the score. Although a valid IMS certificate may be obtained for a yacht that does not comply with these standards, the acceptance of such a yacht in a particular event is ultimately at the discretion of the local event organizers.

As the lOR faded from interest, increased competition under the IMS began to put strong design pressure on all features of the rule, mcluding the interior. A 100-point IMS interior, while being a pronounced step up fi'om what stripped-out lOR racers had become, does not measure up to the dual-purpose objectives of the rule's founders. This has led to more stringent requirements for a recently created "cruiser/racer" division, whose minimum requirement in each feature has been raised to the IMS base value, so there are no trade-offs in reaching the 100-point score. Those yachts that fall short of that, but meet the original point-score criteria, now qualify as "racers." Hull and Rig Configuration:

The most significant breakthrough in the IMS measurement process is the ability to record the entire surface of the yacht's hull and appendages. This enables the use of the yacht's complete geometry in modelmg performance potential, and allows upgrades in the handicap system's prediction formulations without requiring further measurements of the yacht to be taken.

A fundamental part of the Pratt Project was the development of a machine to semi-automate the "taking of lines" from the sheerline to the bottom of the keel. A clever, microprocessor-based instrument allows the measurer to align and quickly pick o f f typically thirty points on each of about forty stations (depending on the yacht's size) along the hull's length, at generally ahematmg intervals port and starboard. The entire surface of the hull, keel and rudder is recorded electronically for submission to the rating office. Additional, manual measurements of details, such as the propeller installation, are taken, as well. With particular attention to "freeboard stations" at the bow and stem, this is a remarkably time-efficient process to document the entire hydrodynamic shape of the yacht for later computation of hydrostatic characteristics and "integrated" parameters.

Rig measurements are more straight-forward, taking traditional, linear dimensions of the spars, fore triangle base, and sails. An added feature of the IMS is the inclusion of mast diameter and taper, to be used for an aerodynamic drag allowance.

The in-the-water measurements are essentially the same as for the lOR. Freeboards are measured to establish the flotation plane in measurement trim, and an inclimng test is performed for later determination of the yacht's vertical center of gravity and stability curve.

The measurer submits all of this data to the rating office, which runs it first through a "lines acquisition program"(LAP). The lines are checked to filter out any spurious data, and the complete file of input data of the yacht is made ready for the serious number-crunching part of the IMS.


(A)IMS -- an Almost Ideal Measurement System A review of the IMS by David Pedrick

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O u t — o f — w a t e r m e a s u r e m e n t s : • H u l l M e a s u r i n g I n s t r u m e n t • A d d ' ! m a n u a l m e a s u r e m e n t s C I n — t h e — w a t e r m e a s u r e m e n t : ^ • F r e e b o a r d s • I n c l i n i n g t e s t H u l l , f l o t a t i o n , a n d r i g L A P ( L i n e s A c q u i s i t i o n d a t a e n t r y : P r o g r a m ) H y d r o s t a t i c s cSc I n t e g r a t e d c h a r a c t e r i s t i c s • L P P ( L i n e s P r o c e s s i n g P r o g r a m ) rr E q u i l i b r i u m S o l u t i o n : ( I n c l u d e s h y d r o a n d a e r o m o d e l s ) • V P P ( V e l o c i t y P r e d i c t i o n P r o g r a m ; P o l a r P e r f o r m a n c e F r o m V P P O u t p u t : " ^ H a n d i c a p s : ( F o r I M S — s p e c i f i c c o u r s e s ) ^ • F r o m V P P CScoring: • B y R a c e C o m m i t t e e • P C S ( P e r f o r m a n c e C u r v e S c o r i n g ) p r e f e r r e d j


C o u r s e M o d e l s W i t h i n V P P A c c o m m o d a t i o n r a t i n g : • R a c e r • C r u i s e r / R a c e r

Figure 1. Basic flow diagram of the International Measurement System.

Lines Processing Program (LPP):

This is where the integrated fun begins. Now that the computer knows the entire shape of the yacht (at least, from the sheerline down), it can be programmed to compute any number of hull and appendage characteristics. To start with an easy one, it computes the displaced volume of the yacht up to the measurement plane (according to the actual fi-eeboards). It does this basically by slicing the boat into many, closely spaced elements, and then

summing (or integrating) their individual contributions. The calculated volume, multiplied by the density of the water (which was checked at the time of floatation), has been demonstrated to give the weight of the yacht to quite acceptable accuracy. The computer also determines wetted surface area, the longitudinal and vertical centers of bouyancy, and the vertical center of gravity indicated by the inclining test.


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Figure 2. Hull Profile with sailing & sunk waterlines

The next step begms to reveal some of the IMS' cleverness in hydrodynamic modeling. It has always been difficult to devise a rated representation of effective sailing length. The IMS defines effective length in a way that assesses the volume in the ends of the yacht with proportionately greater weightmg than amidships. (More about this later.)

The effective length in measurement trim is called LSMO. This length is used to determine an addition for crew and gear weight to reach sailing trim. It is then a straightforward procedure to refloat the yacht at different immersions and heel angles, with equilibrium trim and sinkage automatically calculated for each condition. These are generally at sailing displacement, and at a range of heel angles all the way to 165°. The heeled attitudes of the yacht are used to calculate the stability curve from 0-180 degrees, as well as hull characteristics within the sailing range of heel angles. There is a "sunk" condition, as well, in which the hull is immersed artificially and trimmed to simulate "squatting," to account for the effect of overhangs on sailing length at higher speeds. (See Figure 2.)

In addition to sailing length, the LPP calculates an effective beam and canoe body draft by a "weighted" calculation, to be described later. It also makes an adjustment to the keel span to represent its hydrodynamic efficiency better in the calculation of upwind and reaching performance results. The various measurements of the propeller mstallation yield

an equivalent flat plate area for drag calculations.

Velocity Prediction Program:

The IMS VPP combines the yacht characteristics produced by the LPP with the hydrodynamic and aerodynamic modeling necessary to compute sailing equilibrium. Its output is the predicted speed of the yacht at all points of sail, and in velocities ranging from 6 to 20 knots. Higher and lower winds are not computed, as the the data is currently considered neither sufficiently accurate nor necessary by the ITC. However, some sailors would like to see the upper range of the true wind speed extended to 25 knots.

Sail Model:

IMS sail plan dimensions have followed lOR. However, the use of the information is very different. The IMS includes a mathematical model of the lift and drag for the main, jib and spinnaker (and mizzen and staysail) separately, and includes blanketmg effects when sailing downwind.

The sail plan of a given yacht has a specific set of formulations for lift and drag over the range of apparent wind angles, adjusted for the aspect ratios of the yacht's respective sails. Allowances are made for the "parasitic drag" of the mast and the sails themselves, as well as for the region of the hull above the wateriine. The objective is to determine the net driving force available to propel the yacht, and its associated side force.


(A)IMS -- an Almost Ideal Measurement System A review of the IMS by David Pedrick

A subtlety is a correction for the wind gradient of the earth's boundary layer, so that all yachts, regardless of size, are referenced to a wind velocity at a height of 10 meters above sea level. Wind speeds measured at mastheads above this will be greater. In its calculations, the VPP uses the wind velocity relative to the wind gradient at the "center of effort" of each yacht's sail plan.

Hydrodynamic Model:

This is the most ambitious aspect of the VPP's modelmg. Most of the data on which the IMS resistance formulations are based have come from an extensive systematic series of yacht models tested at Delft University in the Nedierlands between 1975 and 1991. (Refer to the bibliography for suggested references on these tests.) The models were tested in four significant stages, the first two of which combine to be the Delft I Series of 22 models, published in 1977 and 1981. Delft I I (1988 and 1991) and I I I (1993) brought the total number of models to 39, covering a very broad scope of hull parameters ~ light, heavy, narrow, wide, as well as varying prismatic coefficient and LCB. There are two parent huUforms, but both are antiquated by 1990 standards.

The resistance boatspeed data of these models, as well as a limited number of proprietary models more recently, have been "regressed" mathematically, fitting coefficients to terms that include primarily the volumetric coefficient (analogous to displacement-length ratio), beam-draft ratio and prismatic coefficient. (The combination of volumetric coefficient and beam-draft ratio appears sufficient to account for length-breadth ratio.) The upright resistance formulations have been updated at various times, and while greater accuracy has been achieved, there is still significant room for improvement.

A yacht sailing at a course other than dead downwind creates two other components of resistance that become critical when sailmg

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to windward. The smaller component is the additional drag due to the change in form of the immersed volume as the yacht heels. The more critical drag term for a heeled yacht is the "induced drag" associated with the generation of side force produced by the appendages, primarily to counteract the side force generated by the sails. The more efficient the appendages, the less the induced drag. Keel efficiency is represented by a "reduced draft," or TR, which the VPP determines approximately from keel span and the canoe body's maximum sectional area.

The discussion so far has applied specifically to the resistance associated with a yacht sailing on a totally cahn sea. However, vrind makes waves, and rough water drag can be significant. This has been a priority area of study for several years by the International Technical Committee (ITC), but it is a major task. There has been a lack of non-proprietary, rough water added resistance model test data, as well as real sea state data pertinent to yachts, and measurements of actual pitch mertias of yachts (more about this later). The US Sailmg Association has fiinded a Pitchmg Moment Project to develop the missmg data, so that the influence of added resistance m waves can be properly accounted for. A provisional algorithm for added resistance is now bemg used in the IMS VPP, and it will contmue to be unproved over the next few years.

Solution Algorithm:

The VPP solves for three of the six degrees of freedom ~ forward speed (drag-thrust), roll moment and side force. Not solved are pitch moment from sail thrust, heave changes due to the heeled sail forces, and yaw moment (hehn balance). The program finds sailing equilibrium for each true vrind speed and angle in the solution matrix. This means that the driving force of the sails equals the total drag of the hull; and the side force of the sails, which is matched by an equal and opposite force on the hull, produces a heeling


(A)IMS - an Almost Ideal Measurement System A review of the IMS by David Pedrick

moment that is balanced by an equal and opposite restoring moment at a specified heel angle. Crew weight is shifted outboard from centerime so that by 6° heel a majority of it is on the weather rail.

Because true vrind speed and direction are held constant m the iteration for sailing equilibrium, there are tricky trade-offs of speed, sailing angle, heel angle and sail forces in order to find the optimum solution. To add to the degrees of freedom, the sail forces are "trimmed" to get the best performance from the specific yacht m the given vrind condition. The sail coefficients start out at maximum, practical lift. Then, as heel angle grows, the sail coefficients are "flattened" by depowermg them at fiill sail span, and, when needed, "reefed," as well, by shortening down the sail area and height in the iteration. While other data is found, the real reason for all of this is a single number m each vrind condition ~ boat speed.

Polar Curves:

When the speeds calculated at each vrind angle for a constant wind speed are strung together, a polar curve results. The 1994 IMS certificate gives more data than before, but only as seconds per mile, instead of knots, to be more helpful in scoring. Speed in knots is found simply by dividing each of these numbers mto 3600 secs/hr. There is enough data to draw complete polar diagrams.

Time Allowances:

This is the real, bottom lme of the IMS VPP. Now that race committees have become more sophisticated about the use of IMS' extensive data, and are hnplementmg Performance Curve Scoring (PCS, described later), the level of fairness in race handicapping is continually unprovmg, and, vrith it, the degree of satisfaction of sailors racmg under IMS.

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Some countries are now able to download yacht data from the rating office to race committees' computers by modem or floppy disk. Then, a race committee, usmg PCS software, can specify each race's actual course conditions and compute course-specific, unplied winds and corrected times with relative ease. The 1994 IMS certificate lists time allowances at 10 wind angles and 7 vrind speeds, which may be used for manual input for PCS scoring. There are also "wind-averaged" allowances for six standard courses: windward only, leeward only and four standard courses.

"Wmd-averaging" uses a bell-curve distribution of wind speed, centered on the nominal wind, but includmg a bandwidth of several knots of wind, plus-and-mmus, that blows for a share of time according to the bell-curve distribution. This allowance for cycling of the wind speed is another example of the degree of detail that can be hnplemented within the IMS.

The first two wmd-averaged courses are all upwind and all downwind. The Olympic course has three beats, two 135° reaches and a run. The circular random (CR) course corresponds to sailing a speed circle, where the wind stays from a single direction and the yacht sails a 360° circle, beatmg and usmg jibe angles where appropriate. Non-spmnaker is circular random without spmnakers. The "ocean" course, meant for PCS scoring of general, open-course races, is a variable-mix formula using 100% CR at 12 knots; 30% W L plus 70% CR at 6 knots; and 20% CR plus 80% reach at 20 knots. This is believed to follow the trends of a slower or faster point-to-point ocean/coastal race, especially when used with PCS.


One of the bugaboos of the MHS and, mitially, the IMS was the way that tune allowances were applied to races. Because the


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sailing performance of each yacht tends to be skewed m one direction or another ~ especially with respect to light or heavy air ~ correct choices of wind velocity and course composition for a given race enables the IMS to improve corrected time results significantly compared to fixed ratmgs. However, i f one or both choices are wrong, IMS' corrected time results will diverge, and may be worse off than fixed ratings.

One issue about scoring that has gone through years of controversy is the best way to establish the correct wind speed. Deciding after the race, based on actual conditions, makes the best use of the IMS' technical potential, but has been unpopular for at least two reasons. Not knowing how much you owe your competitors while the race is in progress is unacceptable to most sailors, and some competitors are bound to disagree vrith the race committee's choice of vrind strength.

A clever solution has been to let the performance of the yachts themselves determme an implied wmd speed by comparing their actual elapsed tunes against their IMS target speeds. This requires the correct course mix and distances, which can normally be controlled within acceptable tolerances in round-the-cans racmg. The faster a boat sails, the higher its unplied wind speed. I f the actual, average vrind speed is consistent among the fleet (and i f the VPP is sufficiently accurate), a yacht that is sailed slower than her potential will see that as a lower bnplied wind speed. It will be charged as a reduction in its time allovvance corresponding to the seconds/mile difference between its performance curve at its own implied wind and at the greater implied wind of the winner.

A software package, available free to national authorities for distribution to race committees from the ORC, or for a price from commercial code developers, makes PCS

handicappmg easy. With the only "string" being the user's description of the day's course content for handicappmg purposes, PCS automatically scores corrected time finishes according to the wind strength that is evidenced by the yachts themselves. The implied wind speeds presently tend to. seem lower than sailors' observations during the race, however, which would affect the local slope of yachts' performance curves for scoring purposes. Although some "tuning" of the procedure is apparently necessary. Performance Curve Scoring has been a generally consistent, fair and welcome addition to the International Measurement System.

Note that this is sounding very much like the "ideal measurement system" postulated in the beginning of this paper!

A Closer Look —

Although the technical approaches mcorporated m the IMS are well founded in physical principles, and the methodologies are reasonably well based on various forms of experimental data, there are some hnportant areas of the rule that are not as accurate or refined as they need to be. Some of the most important ones are on 1994 agenda of the International Technical Committee - the hard-working, volunteer arm of the ORC that maintams the technical aspects of the IMS. The following sections elaborate on the rule's primary performance parameters, with comments about potential improvement.

These are very complex topics, and the discussions here will attempt to convey the way that the principal formulations work without getting unduly technical. For those with an appetite for coefficients, exponents and integrals, there is deeper readuig in Report 78¬

11, "A Velocity Prediction Program for Ocean Racmg Yachts . . .," by Prof Kerwin.


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Figure 3. Sectional area curves used to calculate L S M l & LSM4.

Sailing Length:

One pf the substantial improvements that the Pratt Project made over previous measurement rules is the method of computmg effective sailmg length. Most rating rules base length on the centerline profile at a prescribed height, with perhaps girth corrections of some kind at each end. The IMS, instead, derives length from the hnmersed sectional area curve in the following way.

Figure 3 shows several characteristic curves for a representative yacht. The first curve to understand is the sectional area of the yacht m sailing trun (the innermost, dotted lme). A point on this curve equals the area of a transverse section of the yacht below the sailmg waterlme at that particular position along the

displaced length. Actually, the scale shown has divided all areas by the maximum area to collapse them into nondmiensional form; this emphasizes the length effect that is of mterest m these curves. Integrating the area under this curve (multiplied by the value of the maxunum area) yields the hull's displacement.

Even before the build-up of waves on a yacht's overhangs at higher speeds, the hull generates wave crests at the ends of the waterline and a trough amidships. Because code limitations of the VPP make the calculations of volume relative to a planar water surface, it is desirable to artificially increase the area m the ends of the yacht as a means of sunulating the wave build-up effect for calculatmg purposes.


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

Systematic techniques used by the IMS to emphasize the ends of die area curve m computing sailing length are to modify the areas exponentially, and to use a non-linear property to characterize the length of the curve. The IMS uses the square root of the area curve in its length determination. Comparing the solid and mnermost dotted Imes on figure 3, the shape of the solid curve bulges substantially near the ends of the waterime where the values are small, and very little amidships where the values are closer to unity, mcreasing the apparent length of the area under the curve.

The next step is even more significant. The "radius of gyration," or gyradius, of the area under the square-root curve is used to characterize its length. The gyradius is a measure of the moment of mertia of the square-root curve about the longitudmal position of its centroid, vrith any particular element contributing its value times the square of its distance from the centroid. The moment of inertia of a body is also known as its "second moment," so the IMS calls the length obtained in this way is the "second moment length", or LSM.

Direct computation of the gyradius of a body yields a value much less than the body's overall length, though, so this scientific representation of displaced length is multiplied by a bald-faced "fudge factor" to produce an L S M whose magnitude is similar to the yacht's waterlme length. An L S M is computed for each of several different flotations.

As a subjective refinement, it was considered that the effects of the rudder and keel were being overemphasized by including them in the L S M , but that some effect due to them should be left in. The grand compromise is that the LSM's are calculated at each flotation both vrith and without the appendages, and then averaged.

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The bottom lme, "average length" (L) used by the IMS is a sunpie, lmear average of the LSM's found for three specified flotations.

L = .3194*(LSM1 + LSM2 + LSM4) Condition 1 is the static, upright flotation at the sailmg displacement. Condition 2 is also at sailmg displacement, but heeled to 2°. Condition 3 is the "sunk" condition shown in Figure 2. The constant is .3194 mstead of .3333 as another "fiidge" to make the number come out similar to the static waterline length.

Figure 3 shows square-root-of-area curves typical of a newer IMS hullform at flotation conditions 1 and 4. (Condition 2 would be virtually identical to condition 1.) Even for a hullform with a stem knuckle above the waterime and well forward of the end of the static waterlme, the additional volume forward in the LSM4 flotation makes only a modest change m the nondmiensional, square-root-of-area curve compared to L S M l . A substantially greater change is made aft.

Wliile this writer believes that the methodology for length is good, the judgement of using the simple, linear average of these three specific LSM's dates from the Pratt Project, and warrants a more rigorous review today. The way that length is assessed obviously has a strong, type-forming mfluence on hull shape.

LSM4 is the only term that picks up the effects of the overhangs, and it has only a one-third weighting m L. Design trends suggest that it is undervalued, encouraging designers to reduce LSM's 1 and 2 m relation to LSM4 for minimum rating relative to performance. Not-so-very-old boats vrith stem knuckles below the waterlme and skeg/bustles aft are penalized, by comparison. A sunpie mcrease m the weightmg of the LSM4 would help fix this inequity.


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

Also, the LPP "sinks" light and heavy yachts by the same percentage of length, although, logically, heavier yachts tend to sink more and have a greater growth of elevation of the bow and stem waves, causmg increased dynamic length. The suitability of adding a displacement relationship mto the sinkage for LSM4 is among the ITC's 1994 studies.

Another feature affecting performance is the fineness of the bow entry angles, helpmg especially in penetratmg a chop or larger waves. The hydrodynamics here are quite different from those of the stem, and perhaps volume should be treated differently in the bow and stem. Instead of modifying the area curve by the power of 0.5 over the entire length, a smaller power might be used forward, locally mcreasmg the bulge of exponential curve, and then making a transition to appropriate power(s) midships and aft. Another approach might be to assess the angle of fineness of the forward topsides, but it would require quite a bit of research to introduce a new term such as this mto the VPP's hydrodynamic model.

A more complex approach might be to make lengüi dependent on speed vrithm the VPP's resistance predictions. At lower speeds, L S M l corresponds reasonably to the wet part of the boat. As speed builds, however, the dynamic length grows, and an mcreasing share of LSM4 would do a better job of representmg the effective sailmg length.

Improvement m the treatment of LSM4 is one of the higher priority items m the ITC's agenda for 1994. The purpose-buih IMS yacht, with its long, fine forward overhang, and broad, powerfijl counter, is clearly receivmg an unfair advantage relative to the bulk of the fleet that the IMS is mtended to serve. I f there is an obvious mequity m the newer boats, it exists to a less conspicuous degree among the older ones, as well. The science on which this aspect of the LPP and VPP is based appears flawed, and needs to be fixed.

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Given the unportance of gettmg this parameter technically correct and the amount of study needed to do it, a long-term solution will be more than a one-year project. However, interim improvements to diminish the type-formmg advantage, such as adjusting the weighting factors m averaging "L," should be vrithm the ability of the ITC to propose for the November, 1994, ORC meetmgs.

Beam (B):

Now for somethmg less controversial! Beam is another mtegrated parameter, usmg the "second moment" approach again, plus another concept ~ a reduction of the value of the beam at a given depth below the flotation plane by an "attenuation" factor. This is an exponential decay fimction that adjusts for the fact that the more submerged portions of the hull and appendages have less to do vrith causmg yacht-induced wavemaking and its associated drag than do those near the free surface. For a typical canoe body, the lower portions have their physical beam attenuated (shrunk) by a multiplier m the order of about 0.7, and the appendages by about 0.2.

The effective beam is found by applying the second-moment concept transversely, combined vrith the depth attenuation factor. This emphasizes the effect of beam near the maxhnum, static waterime, but does so without encouragmg hull distortions. The directly mtegrated value is multiplied by another fudge factor to yield a number whose value is m the order of the maximum beam of the yacht.

Beam-Depth Ratio (BTR):

BTR is mtrinsically linked to the calculation of attenuated sectional area, vrith the effective, or attenuated, depth bemg an mtermediate result. The maxhnum attenuated sectional area is calculated, usmg the attenuated beams described above. Dividmg this area by B and multiplying by another


(A)IMS - an Almost Ideal Measurement System A review of the IMS by David Pedrick

fudge factor gives the effective hull depth, Tc. BTR is simply B/Tc, both bemg expressed as integrated terms. Presently, yachts with a relatively higher BTR appear favored on the race course.

"Reduced" Draft (TR):

The draft of the keel is fundamental to windward sailmg performance. However, the effective span of the keel is dhnmished by the presence of the canoe body. So-called slender body theory, supported by experimental results, has shown that the canoe body's effect on reducmg the effective span of the keel, expressed roughly, is directly related to the square root of the maximum sectional area of the hull m way of the keel.

For fin-keeled yachts, this is the only correction for span efficiency that is made according to the yacht's specific geometry. There are adjustments for centerboards and wmglets, both of which are mtended to enable yachts having them to compete under IMS with reasonably fair treatment of their effects, but biased away from encouragmg them as desired performance features.

The purpose of TR is to calculate the mduced drag that is produced as a byproduct of creatmg lift on the keel. For different keels of equal IMS TR, the VPP can not distinguish the actual induced drag differences between an efficient and an mefficient design. This can be determmed today by another form of analysis called computational fluid dynamics (CFD). It may not be long before the VPP can mclude CFD for computmg a more accurate value of TR for the specific keel of a measured yacht.

The IMS TR is presently based on the keel only. However, it is recognized that there is a significant contribution to mduced drag efficiency by the rudder. Although it is not on the ITC's agenda for 1994, a correction for the relative span and loadmg of the rudder relative to the keel is bemg discussed by the US IMS

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Committee. The use of conventional biplane theory would be far more fair to the racmg fleet than ignoring this difference among yachts, as is the case now.

VPP Calm Water Hydrodynamics:

An overview of the hydrodynamic resistance model of the VPP was given on p. 7. To discuss this subject fully could fill volumes, so only a few, specific pomts will be addressed here.

The bulk of a yacht's resistance at any point of sail is the basic, upright drag. This consists of two different, primary components -- friction and wavemaking. Actually, these terms have taken on more specific meaning m marine hydrodynamics since the time that the Delft Series tank tests were begun. Secondary components of drag are now examined in more sophisticated ways, although their treatment is still debated among the experts.

In the past two years, especially, there have been substantial shifts m IMS-predicted course tunes. I f all yachts shifted uniformly, and tune allowance differences remamed the same, the changes m numbers would be puzzling, but the net effect on the race course wouldn't matter. This is not the case, however. Changes m the IMS fi-om 1992 to 1993, and now again to 1994, have affected yachts of differing characteristics by different magnitudes of ratmg shifts. Where previous mequities may exist, it is a strength of the IMS to focus on mtentional modifications to the mathematical modelmg which, when done correctly, moves the system closer to the ideal.

The recent changes may have mixed some steps forward and backwards. They have been conspicuous and somewhat unsettlmg, and the matter of resistance prediction formulations is still m a state of flux, so some explanation may help readers understand the magnitude of the problem.


(A)IMS - an Almost Ideal Measurement System A review of the IMS by David Pedrick

As models have been added to the Delft Series since the first nine were tested nearly twenty years ago, there has been general unprovement m the accuracy with which "regressed" parametric equations correspond to mdependent model test data from other sources. A major unprovement was made following the Delft I I Series, whose complete results were first published m early 1991. A reanalysis of this data usmg IMS parameters of length, beam, depth and other prmcipal terms ~ a major undertakmg -- preceded within the ITC, with a new resistance model completed for adoption by the ORC for use m the 1993 season. This model was substantially more accurate than the resistance model that existed through 1992, which was based on only the first 22 models of Delft I .

Aldiough the 1993 IMS tune allowance predictions were a big step m the right direction, the ITC kept workmg on additional unprovements. In practice, it appeared that heavier boats were gettmg an undue advantage m their time allowances. It was decided to review the upright resistance curve fits, and to merge the extension of resistance predictions mto very high speeds that was newly mtroduced m 1993. Also, unprovements were desired m the allowance for tlie mcrease m resistance due to heel at zero side force.

The fleet of models used for regressmg the resistance coefficients was also added to and screened more carefully. The Delft I I I data became available, and the entire series was examined to remove models that had either unrealistic proportions or test data of questionable accuracy. Also, five, more recent, proprietary models were added to the base fleet.

Despite the unproved quality of the experimental database and the considerable efforts of the ITC and other contributors to the effort, the results of the 1994 formulations appear mixed. Predicted upright resistances

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have been generally mcreased compared to 1993, and an mcrement for resistance due to heel seems to be overpredictmg its drag component, as well, producmg larger tune allowances for most yachts. The smallest increases and the minority of decreases m tune allowances tend to be among the heavier yachts that were, m fact, thought to have been advantaged m 1993. This might be a reasonable adjustment. The biggest gams m tune allowances for 1994, which may prove to be excessive, are among lighter yachts.

The reanalysis of resistance m the past few years has raised a number of issues about more accurate scaling of the model tests on which the entire VPP process is based. The fiindamental means of expandmg the Delft test data to full scale still follows some procedures that are now known to be deficient. The unprovements made for 1993 mclude mdividual computations of drag on the keel and rudder usmg their respective, local geometric properties. However, the wetted length assumed for determining the frictional drag of the canoe body is illogically short, producing too high a drag component. Furthermore, current practice in hull resistance analysis considers adjustments for viscous drag (a subtle but significant distinction from frictional drag) due to differences m the bulkuiess of hullforms; blunter shapes tend to have a higher form factor, and corresponduigly higher viscous drag, than more slender shapes havuig equal wetted, or frictional, surface.

In the upright, non-liftmg case, the drag remaming after subtractmg the compo-nents above is prunarily wavemaking drag. Gettmg the viscous drag right is a prerequisite to making sense of the wavemaking drag. At the present tune, the IMS mcorporates some inaccuracies.

Only after the wavemakmg drag component is properly calculated from the model tests can the correct parametric relationships for it be determined. This is the


(A)IMS - an Almost Ideal Measurement System A review of the IMS by David Pedrick

highest priority of the US IMS Committee's agenda, but is not even on the ITC's. This is a concern, to say the least.

The drag adjustment for heel m the 1994 IMS appears quite excessive. The ITC is likely to unprove on that problem this year.

These are not trivial problems to correct. Re-expanding all the tank data to the new procedures and regressmg the revised wavemaking and heeled resistance components vrill be a very large undertakmg. Much to their credit, the members of the ITC work very hard without bemg paid for their efforts, and the budget to fimd important research tasks beyond the ITC's own manpower has been very lunited. This writer recommends that a major overhaul of the cahn water resistance data be committed by the ORC for hnplementation by the end of

1995. This will require an operatmg budget for significant thne by research professionals. Also, additional, high quality, model test data of modem yachts, provided m confidence by the design community, would be very helpfiil to the cause.

Another way of establishmg hull resistance, as well as lift, is by computational fluid dynamics. Predictmg wavemaking drag vrith good accuracy is a fairly recent breakthrough, but is now a normal part of America's Cup research. As the time and cost to run these codes comes down, CFD vrill someday supplant the regression of model data as the preferred way to predict hydrodynamic forces.


Predicting the speed loss of yachts sailing m rough water is a particularly difficult hydrodynamic challenge. There is very little experimental data on which to base parametric studies because rough water tank testmg is quite expensive to conduct. Computer codes that have been developed for ships have been more concerned vrith pitch and roll behavior

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than added resistance, as their operatmg lunits are govemed more by slamming damage lunits and cargo shiflmg than by horsepower thresholds. Furthermore, most of the codes have been developed for symmetrical, "wall-sided" ships. Consequently, the application of seakeepmg codes to yachts has been generally madequate.

To address this important element for eventual mclusion m the IMS, the US IMS Committee began a "Pitching Moment Project" m 1989. This project is conductmg research mto the three, primary elements that must be solved to be able to make accurate allowances for added resistance m waves.

The first is to establish the parametric relationships that affect added resistance, such as length-displacement ratio, length-beam ratio and radius of gyration, and create the algorithm that predicts added resistance from a yacht's hullform characteristics and weight distri-bution, as well as its speed and angle of mcidence to the waves.

An mitial parametric algorithm for added resistance m waves has been developed by this project, and is now included in the IMS. Although added resistance would be best established through model tests, the computer code values bemg used m the mterim by the IMS are better than none at all. This is a very good start to the challenge of mcluding added resistance, but vrill be subject to significant unprovement over the next few years.

The evaluation of resistance (or thrust) in following seas is also being studied, especially to understand surfing conditions for future handicapping adjustments. There is a very small amount of experimental data available, but, once agam, a start on lunited data is far better than failing to address a significant, known phenomenon.

For now, the VPP uses an esthnated value of longitudinal radius of gyration in its


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

added resistance calculation. The gyradius is a measure of the yacht's pitchmg mertia, which sailors know has a substantial effect on boat speed U l rough water. It is another "second moment," and its physical meanmg is as follows. I f you could concentrate the entire weight of the yacht mto two pomt masses, placed equidistant fore-and-aft from the overall center of gravity by the gyradius distance, the mertia would be equal to that of the overall yacht. The more concentrated the weights of a yacht, the smaller the gyradius, the less the mertia, and the less the added resistance (except Ul perhaps very short waves).

Ideally, Imking the ability to determine radius of gyration of a yacht with its effect on added resistance and speed loss m waves could quantify the handicappmg requirements of a stripped-out racer compared to a fuU-mterior cruiser/racer. It would obviate the need of accommodation regulations to categorize performance potential. Owners would be able to put anything they wanted mto the interiors of their yachts, with their tune allowances reflecting the correspondmg effect on speed through gyradius measurement and the VPP's added resistance algortithm.

The adjustments to the base gyradius bemg mtroduced mto the IMS for the weight distribution advantage of carbon fiber rudder stocks, hulls and decks are the beguinmg of a means of hnproving the VPP's accuracy by mcluduig realistic weight distribution effects on added resistance. This is a good example of the IMS' potential to provide rational handicappmg of features that have caused unfair advantages m other ratmg fleets.

The next task of the Pitchmg Moment Project is to be able to determme the yacht's actual, longitudinal gyradius. Project researchers have produced a measurmg device and analytical software to tune the natural pitching period of yachts afloat. By making allowances for the added mass of the water that moves with the pitchmg yacht according to ship

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codes, the yacht's radius of gyration can be calculated. Actual yacht pitchmg data is startuig to be measured to advance the eventual use of each yacht's actual gyradius m its performance prediction.

The third essential element for seakeepmg calculations is knowledge of the sea conditions to be used for handicappmg. Unlike cahn water, which has only one defined state, there are mfinite combuiations of rough water. Wave lengths and heights of a random, open-water sea state can be quantified in a "sea spectrum." Because added resistance is reasonably proportional to the square of wave height, the accurate prediction of added resistance requires that the sea state used ui the calculations correspond closely to what exists in the real world.

To help understand representative wave length and height content on actual race courses, the US IMS Committee has collected wave data at several sites, usmg an oceanographic bouy for that purpose. A t the size of waves that affect sailmg yachts, the character of sea spectra tend to be very much different than the generally established ocean spectra for ship hydrodynamics, so this is a valuable step toward accurate allowances for added resistance and speed loss of yachts due to rough water.

The challengmg area of seakeepmg is on a very vvorthwhile track. It's a big job, though, and IMS sailors will have to be patient for a few years as the procedures for predictmg speed loss upwuid and gam downwind ui rough water contmue to be developed.

Sail Force Coefficients:

The sail force model m the VPP has a rational, theoretical basis, with values adjusted to reasonable ranges accordmg to practical experience and judgement. Confirmation on the validity of sail force coefficients can be checked by making sure that speeds and heel


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

angles at sailmg equilibrium are close to the VPP's predictions. The hydrodynamic resistance model needs to be overhauled before the sail force coefficients can be reanalyzed comprehensively.

Meanwhile, the ITC is studying various details of the existing coefficients for possible adjustment durmg 1994. One of these is the uncoupling of the present dimensional Imkage between jibs and sphmakers. Smce the aero model already separates the forces of the main, jib and spmnaker, it may not be very difficult for the VPP to predict the effects of spmnakers whose hoist is higher up the mast than the headstay. Like the winged keel, this is a feature that the IMS would not encourage as a "must-have" item, to protect the majority of the existuig fleet. The goal would preferably be to permit taller-than-fore-triangle sphmakers, but to bias the aerodynamic modelmg so that the existmg fleet remams fairly treated and that, for the tune bemg, equal-hoist spmnakers remam preferred for IMS use.


Yachts that are competing m close quarters are affected by transient responses, such as tacking, roundmg marks, and mterfering with other yachts. The fact that some yachts fare better than others when tacking (boat handling being comparable) is bemg discussed by the US IMS Committee. General:

To quote the Introduction of the rulebook, the stated mission of the IMS is as follows:

• Weigh each factor used m the formulae to accord with its effect on speed.

• Reduce obsolescence caused by the design of yachts which beat a rule and thereby render older yachts not competitive.

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• Devise a system which is designer-proof m the mception i f possible, but by correction i f this proves necessary.

• Provide fair thne allowance for yachts of the dual-purpose type (for cruismg and racmg). It is intended that production yachts of good design should be able to compete with custom yachts.

Overall, the IMS is domg a very good job of fulfillmg this mission. It's not perfect,

but it handicaps diverse types of yachts rather well. Work underway or otherwise possible to improve the IMS has the potential to keep broadenmg the range of yachts that can compete equitably under the system.

Limits and Restrictions: Stability:

In addition to the mam focus of the IMS on the prediction of sailmg equilibrium speeds and thne allowances, there are parts of the rule that deal with separate lunits and restrictions.

A very hnportant byproduct of the system of hull measurement and calculation of properties at large angles of heel is the stability curve, hi the early 1980's, the US Sailmg Association (formerly USYRU) analyzed many hundreds of yachts, vrith special attention to some of the tragic cases of the '79 Fastnet Race. Model tests of a yacht being rolled by large waves were also conducted, to develop a better understanding of the important factors that reduce the risk of capsizuig.

An IMS yacht's stability curve's positive range must exceed 103°, and typically extends up to a magnitude of 107-120°, after which it is negative to 180°. The heel angle at which the curve crosses zero rightmg moment mdicates the "lunit of positive stability" (LPS). The ratio of the areas of the positive region of stability to the negative is another measure of


(A)IMS -- an Almost Ideal Measurement System A review of the IMS by David Pedrick

resistance to capsize. Further, a stability mdex considers the effects of the yacht's length, beam and displacement m altering its probability of capsizuig. Incremental terms applied to the LPS yield the "stability mdex," which must meet the minimum values given m the IMS Regulations for the respective ORC categories of races.


The IMS Regulations specify the maxunum mventory of different types of sails that may be carried while racmg. The list is rather mmimal, although adequate, for competitive sailmg. A reduction m the number of permitted jibs and sphmakers used in 1993 has been reverted to the 1992 allowances.

Mamsails may have battens longer than the lOR lunits. The sail model m the VPP has an adjustment to the sail coefficients for this, mcreasing the lift and decreasmg the drag of the sails.


At its Annual Meetmg hi November, 1993, the ORC overcame years of conservatism to accept carbon fiber hull and deck construction for the Racmg Division. With the declming cost of carbon fiber, reasons for banning it have greatly duninished. A means of compensating for its weight and stifl&iess advantage has been made hi the IMS' surrogate gyradius. Also. the curing temperature permitted for all yachts has been raised to 80° C, appropriate to pre-preg lammates. However, the ORC shut the door on any further changes to the permitted materials for four more years.

The 1994 rule also specifies mhumum remforcement material for the outside skm of sandvrich construction yachts due to concern about hnpact resistance. Various oüier Ihnits on materials and construction have been added. Carbon fiber rudder stocks are now allowed for

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all yachts, with a small reduction of the surrogate gyradius to reflect its weight distribution advantage ui rough water.

As dynamic effects due to pitch mertia are accounted for accurately m the VPP, this writer would like to see greater freedom for more rational progress in yacht materials and construction, unfettered by ancillary handi-cappmg regulations.

Level Racing ~

There is a popular tendency among competitive sailors to race m yachts that are closely matched hi speed, although wantmg at the same thne to have a controllmg edge. Level racmg provides this type of competition. The IMS has addressed the organization of this format of racmg vrith the debut of the hitemational Level Class 40 (ILC 40) Rule m April, 1993.

The ILC 40's are to be grand-prix-caliber yachts, racing among themselves vrithout tune allowance. The interior requirements will be those of the Racing Division, although it is mtended that yachts constructed to this class vrill be "fast, sound and seaworthy," able to have prolonged lives after their racing careers vrith a minimum of modification.

The parameters of the ILC 40 are controlled m two ways. The first is through upper and/or lower Ihnits on LOA, IMS L , maxhnum beam, IMS B, keel draft, displacement, LPS and minunum freeboards. Once the design fits mside the dhnensional constramts, the real distmction of the ILC 40 is the matrix of performance lunits to which it must comply. Mmimum tune allowances (corresponding to maximum predicted speeds) have been specified for a nme pomt, pro-rated vrind matrix. An ILC 40 must not have any tune allowance faster than the mdividual specifications for each true wind speed (6, 10 and 20 kts) and direction ( V M G beat, 110°


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

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reach & V M G run). Furthermore, there is an overall minimum allowance that, for 1994, is set about 11 secs/mi slower than the overall, weighted average of the mdividual minima. This allows some practical performance tolerance, but prevents breakaway speed extremes. As the IMS goes through its annual adjustments, a "control" fleet of ILC yachts m the ORC's database vrill be used to preserve consistency when resettmg the handicaps.

The first ILC 40 world championship event vrill be held m Marstrand, Sweden, m August, 1994. The Cercle de la Voile de Paris, which is the trustee of the One Ton Cup, has designated the trophy to the ORC's ILC 40 championship beghming m 1996.

The ILC concept has generated hnmediate enthusiasm, and the ORC has set nominal sizes of additional classes ~ the ILC 30, ILC 46 and ILC 70. Parameters sunilar to the ILC 40 must be determmed by the ITC, which is one of the hottest priorities on its agenda for a 25-27 February 1994 meetmg m London.

In addition to the level racmg of developmentally designed ILC yachts, the success of the Mumm 36 class deserves attention. This is a worldvride, IMS one design racer created by the commitment of a sponsor and an event organizer to the advancement of yacht racmg. Champagne Mumm has enjoyed the mutual benefits of its sports marketing in yachtmg, and m early 1993 saw the opportunity to fill a void in international competition. Havmg witnessed the success of the IMS racmg m the 1992 Commodore's Cup m England, the Admiral's Cup Committee decided to switch from lOR to IMS for its

1995 event, and Champagne Mumm helped "jump-start" participation at the 36-foot size by sponsoring the creation of a new, one design class.

During the second quarter of 1993, candidate designs were solicited in a very short

tune frame (which a sponsor can do ~ an mtemational committee can't.I). A Farr 36 production design, newly launched at Carroll Marine m the US, fit the objectives as well as could be hoped, especially from the standpomt that yachts could be delivered munediately. Three additional builders were promptly licensed worldwide to support the global marketplace. The success of this kind of sponsorship cooperation can be measured by nearly 100 Mumm 36's to be buih and sailmg m the class's maugural year!

Professionalism and Sponsorship:

The previous example should make the facts of life on this subject obvious. To preach against commercialism in the high end of the sport is like pontificatmg agamst birth control --totally out of touch with reality. One can look at any sport with a professional category and pomt to the positive hnpact that it has had on widespread, amateur participation.

There are mdividuals whose passion and talent for sailmg drive them to a level of excellence beyond the reach of most, regular, "working stiffs." Indeed, the pro's are a lucky minority, but amateurs are lucky to have them around. They make the sport better.

Sponsors and sailmg have begun to enjoy a symbiotic relationship. Event sponsors can gam a high profile and help all participants m an event uniformly, and perhaps sweeten the pot with cash or other valuable prizes. Individual yachts and their sailors offer a sponsor the opportunity to draw focused attention to themselves i f their projects perform well, or are othervrise newsworthy m a positive way. The ILC classes will be allowed to advertise m their own events accordmg to Category B of the International Yacht Racmg Rules. Regardless of one's emotions on this issue, "media unpressions" and "column mches" are now well established factors m grand prix racmg.


(A)IMS ~ an Almost Ideal Measurement System A review of the IMS by David Pedrick

IMS ratmg authorities have worked constructively wiüi event organizers to accommodate the mterests of the grand prix element of the sport while, at the same tune, protecting the primary objective of the rule to foster equitable racmg among dual-purpose yachts. The IMS Regulations for the Racmg Division provide scope for developmg finely honed racmg machines under the competitive pressures of grand prix racmg, while utilizmg the same, performance-based handicappmg system as the cruiser/racers. Knowledge gained where the sport is driven by the enthusiasm and commitment of professional sailors does trickle down to benefit everyone who races under IMS.

Constituency and Income:

Smce the hitemational adoption of the IMS for the 1986 season, the number of certificates issued annually has grown from about 1000 in the US to 5500 woridwide m

1993. With the global economy beginnmg to strengthen agam, and especially given the clear direction that IMS has established ui venues ranguig from top hitemational team regattas to yacht club summer cruises, it appears that the International Measurement System is headed for its glory years.

Fuiances present a double-edged sword to growth, however. One is that the cost of obtauiing an IMS certificate is perceived to be expensive. In the US, it costs about US$375 to take specific measurements and process the certificate for a standard, production hull. For a custom yacht, a full, machme measurement brings the certificate total to about US$750. Considering the magnitude of knowledge and accuracy that goes into makmg racing under the IMS equitable, and the cost of operatmg a yacht, measurmg and certificate costs are a bargam. However, the ORC is challenged to market the value of the IMS' good standard of racmg as justification for the certificate costs.

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The other edge is the cost of contmually unprovmg the performance prediction process and regulations, as well as admmistrative matters. The ORC has to exist as a fiscally sound busuiess, and does not have deep resources to fimd the research and other projects that will msure the future health of the IMS and the racmg that h enables. It needs to mcrease its working funds by any of several ways. Increasmg the constituency, raisuig the cost of a certificate, and fmdmg commercial sponsors to fund visible projects are all viable possibilities.

Another one, bound to be controversial, is to assess a surcharge on Racmg Division certificates, smce it is that element that exploits the rule the most and puts the greatest pressure on the IMS' technical development and other costly research activities. As the more lavish spenders on their racing programs and the more nettlesome users of the rule, it would seem appropriate to let them assume a proportionately greater share of its mauitamance.


After several years of meandering between the demise of the lOR and the acceptance of the IMS, it now appears that the IMS has reached a period of rather good racmg built on a reasonably accurate system of measurement, prediction and scoring. Defining separate classes and requirements for grand prix racers and dual-purpose cruiser/racers helps serve the different, yet legithnate needs of each. Also, establishmg focused classes for level racmg fills a very popular style of participation. Further hnprovements are needed, and are bemg worked on as diligently as can be done by volunteer committees augmented by a lunited amount of funded research.


(A)IMS - an Almost Ideal Measurement System A review of the IMS by David Pedrick

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1. Kenvm, J. E., "A Velocity Prediction Program for Ocean Racing Yachts Revised to June, 1978," H . Irvmg Pratt Ocean Race Handicappmg Project, July, 1978. (Available through IMS ratmg offices.) The Hitch-hiker's Guide to the VPP. It also gives a complete list of its background references.

2. "International Measurement System Rule," Offshore Racmg Council, amended through November, 1992.

3. "Mrnutes of the Annual General Meeting," Offshore Racmg Council, November, 1993. 4. "IMS Regulations," US edition. Offshore Racmg Council, 1993.

5. "Recommendations for Offshore Sailing, includmg ORC Special Regulations . . .," United States Sailmg Association (USSA),


6. "hitemational Level Class (ILC) Rule, The ILC 40," Class Rule and Design Specifications, Offshore Racing Council, April, 1993.

7. Kirkman, Karl L . , "Progress Report on USYRU Pitchmg Moment Project," United States Yacht Racmg Union (now USSA), November, 1990.

8. Poor, C.L., "The International Measurement System, A Description of the New hitemational Ratmg System," prepared personally, August,


9. "An hitroduction to IMS," US Sailmg Association, July 1993.

10. Sleeman, Gail Scott, "Gettmg hito IMS," United States Yacht Racmg Union, February,


11. Unpublished proceedings of the US IMS Conunittee, January, 1994.

12. Gerritsma, J., Moeyes, G. & Onnmk, R., "Test Results of a Systematic Yacht Hull Series," Fifth HISWA Symposium, Amsterdam, November 1977.

13. Gerritsma, J., Onnmk, R. & Versluis, A., "Geometry, Resistance and Stabihty of the Delft Systematic Yacht Hull Series," Seventh HISWA Symposium, Amsterdam, November


14. Gerritsma, J. & Keuning, J.A., "Performance of Light- and Heavy-Displacement Sailmg Yachts m Waves," Second Tampa Bay Sailmg Yacht Symposium, February 1988.

15. Gerritsma, J., Keunmg, J.A. & Onnink, R., "The Delft Systematic Yacht Hull (Series II) Experunents," Tenth Chesapeake Sailmg Yacht Symposium, February 1991.

16. Gerritsma, J., Keuning, J.A. & Versluis, A., "Sailmg Yacht Performance hi Cahn Water and m Waves," Eleventh Chesapeake Sailmg Yacht Symposium, January 1993.






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