J 6 I
FIT TO MCE
Crossbreeding of tlie ocean going yacht witli a sailing dingliy does not appear to be promising from tlie viewpoint of
seawortliiness. Unseawortliy beliaviour in heavier wind is more tlie nde tiian tiie exception witii contemporaiy boats built to tlie International Offshore Rule (IOR). It is not only tlie strong, active crews of racing yachts who bear the bnint of current design trends -it is also the average cruising man wiio is directly influenced by the IOR fashion. Past experience clearly indicates that tiiis type of boat is becoming increasingly difficult to sail and unmanageable in winds of Force 6 and above. (Photo R. Tomlinson)
No matter how sophisticated the CAD/
CAM programs available to
boatbuilders, it is impossible to design
a flat-out IOR racing yacht that is also
suitable for offshore cruising, argues
Antony Marchaj.
.. •
"As a critic, I have had to deplore many of the characteristics of the present generation of racing and cruising yachts. As I had a rather large part of the formulation of the International Offshore Rule (IOR), which has fostered many of the faults, I must admit my mistakes and try to encourage corrections."
The words are those of Olin Stephens who, as chairman of the Technical Committee of the Offshore Rating Council, was a prime mover i n the introduction of the rule i n 1970. The intenüon behind the rule was to encourage "...the development of seaworthy offshore racing yachts" and discourage "...developments which lead to excessive costs, or reduced safety or suitability, of yachts for cruising."
In other words, the I O R was aimed at the development of dual-purpose yachts
which could sausfy two kinds of people: those primarily interested i n racing and also the cruising folk - a laudable aim, but can it practically be achieved?
It is beyond the scope of this article to discuss all implications of this question comprehensively. However, experience, both before the rule was formulated and, often tragically, since i t came into operation, suggests that i t cannot.
The accumulated experience of the Cruising Club o f America goes back a hundred years before the introduction of the I O R . The conclusion of the club is that the cruising man "...chooses the type of hull, the rig and all the details w i t h an eye single to the comforts, convenience and safety of cruising. After all of these ends have been accomphshed he will add all features contributing to speed that are not consistent w i t h the demand of ideal cruiser. The real fact must always remain that the cruising boat cannot be a racer."
A cruising boat should be designed to be sailed by a small amateur crew and, as far as possible, to be handled without exceptional skill or endurance - a kind of seakindly and seaworthy boat which should be able to take care of her crew and defend herself against the perils of the sea.
On the other hand, bearing i n mind the nature, purpose and spirit of compedtion, the racing man is not prepared to accept any t r a d e - o f f f o r speed performance because speed is the single virtue of the boat that matters most - it wins races. Designers of racing boats w i l l therefore . naturally seek to increase speed performance even at high risk of failure, which is bound to occur i n heavy weather conditions.
Such hair-raising failures of lightly constructed hulls, particularly affecting those made of exodc and composite materials, occurred during the Sydney-Hobart race i n 1984 and the Fastnet race in 1985, i n addition to the tragic Fastnet
Antony Marclmj is an independent aerodynamic and researcli scienlist who was also nalional Finn Champion in his native Poland. He is a former researcli fellow at Southampton Universily and winner of the Silver Medal awarded by the Royal Inslinite of Naval Architects. He lias written extensively on the technical aspects of sailing and several of his books are now standard reference works.
lids in liydrostatic stability characteristics <achts designed since I960. It sliould be ne in mind that the positive area (Ap) 'er tiie rigliting moment curve (Fig 2A and is a measure of energy required to capsize boat in calm water; this area can be much •iced in waves. The negative area (A„) is 'easure of energy required to right the boat n inverted attitude. A glance at graphs B, D and E will suffice to see that the littion of yachts since the early 1960s has n orientated towards less and less iDorthy craft, with lower stability and liter angles of vanisliing stability. • of 1979 i n which 15 people lost their s.
vn integral part of offshore racing as 1 as cruising is impending danger. JS the h u l l shape, together with its endages and other characteristics, aid correspond to efficient and safe cüon i n order to meet this danger w i t h igh probability of survival,
fortunately, seaworthiness and kindUness requirements are usually
tradictory to those of speed, ouraged by the current rating rule, 'he intense preoccupadon w i t h speed ch dominates today permits no h a l f -• approach. The designers' and Iders' reputation is made by the racing :ess of their creations. Inevitably, the I g n conflict between speed and .vorthiness is nowadays always resolved avour of speed performance.
vD/CAM design
myth that the conflicting demands of racing yachtsman and the cruising itsman can be resolved to the antage of both and within the I O R is ig perpetuated by C A D / C A M . omputer—aided design and lufacturing equipments are being oduced i n yacht design offices and naking lofts everywhere. The iputer — by which we mean both iware and software - seems universal, ertainly, C A D / C A M has many actions. Any acdvity involving ntitadve i,..umerical) manipulation can lided by computerisadon. I n this lect, and i n cost—effectiveness, the iputer transcends the capabihties of nan beings. Possible savings i n terms lanpower i n the small boat industry are
l i g h that the only alternatives are to
iputerise or shut up shop. The scope, iracy and speed of modern computer ems is now such that the non - C A D ipped offices can no longer compete ously.
teady reductions i n the prices of iputers, coupled w i t h the availability of sorts of software packages, accelerate
proliferation of computer systems, i although computers are being
tinually improved and superseded, no tter what technical advances are made
sequence of operation and some
Continued on page 73
Figure 1
Angle of vanishing slabillly 8 .
20 W 60 80 too I20\ liO 160 Heeling angle 6 degrees
©
AREA OF NEGATIVE STABILITY An Ï .3 -Angle of vanishing slabilily e. 75 «0 rear Angle ol vanishing slabilily =115° 71YACHTDESIGN
Continued from page 7!associated implications will remain the same. There are four basic parts of the computer system: the program; data input; the computer used; and the output, program; data input; the computer used; and the output.
0.03 ,0.02 Beamy 2.3 2L 2.5 2.6 2.7 2.B 2.9 Length/Beam ratio 3 0 o 300 I960 65 70 75 Year 80 Light Heavy 125 150 175 200 225 250 275 DispiacemontjLenglh ratio Figure 2
Effects of beam and displacement/lengtii ratios on capsize probability. The lighter and beamier the boat the higher probability of knockdown in heavy seas.
Note: Displacement/length ratio = ^/(O.OlLf
where A is displacement in tons L is waterline length in feet
This prompts several remarks relevant to the end product of C A D / C A M systems. The first, and probably the most important and far-reaching, conclusion is that ultimately the output (specifically, the performance of a C A D craft) will largely depend on the program used and the initial input data. There is no direct qualitative link between the computer used and output.
A computer is not capable of
independent thought. Neither does it have judgment superior to the men who wrote the program nor the industrial designer who runs the system. And although some
people may beheve otherwise, the computer cannot compensate for human mental deficiencies contained either i n the program or input data.
In broad terms, every program can be considered as the mathematical model of a real physical concept - the basic premise is that the real physical situation can be mathematically modelled to produce a solution to a specifically defined technological problem. JMoreover, i t is tacitly assumed that subsequently the laws of nature will conform to the mathematical model defined by a set of equadons incorporated i n the program.
Continued from page 74
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Viewed from another angle, all design programs are always conditioned by the underlying concept or theory which may or may not be realistic i n its assumptions. I t may happen that the mathematical model is so heavily loaded with simplifications, idealisations,
approximations and wrong assumptions as to be unrealistic, i f not worthless. After all, i t is unlikely that pure mathematics can accurately reflect the imperfect world of reality. Consequently, the end product (in this case, a C A D boat) will not behave as one might wish or expect.
I n boat design, just as i n aeroplane design, we must assume that breaking natural laws may end i n disaster. O f course, design failure will influence a boatbuilder's market share. Boatyards and designers with more responsible attitudes than their competitors should, i n the long r u n , w i n ; buyers' satisfaction will i n the end resolve the issue.
The pitfalls of unquestioning faith i n C A D / C A M aside, one might righdy expect that the C A D product, say an offshore sailing boat, should be cheaper and better than that produced by traditional method. After all, by its very nature, the computer holds great potentialities for a better integration of Figure 3
Hydrostatic stability curve of a traditional cruiser/racer built to the earlier RORC nde which was replaced in 1970 by tlte IOR. Note tliat the 'danger zone' is negligible as compared with the 'safe zone'. A boat with this kind of stability lias a good citance of recovering quickly from inversion if a capsize occurs. Because of iter large 'safe zone' and for other reasons which are discussed in tiie autiior's booli 'Seaworthiness - tlie forgotten factor', boats with a long keel and low centre of gravity are much less susceptible to knockdown than contemporary cruiser/racers.
available experience and naval architecture knowledge i n the design process.
Such expectations are well reflected i n glossy catalogues lavishly illustrated with hull lines generated by computers. The following example is by no means isolated;... "exceptional breakthrough cruising boats with extraordinary ease of handling and maneuverability....The result is perfectiy controlled... Outstanding sailing performance i n all weather conditions."
The fact that these boats have been designed by computer is considered to be a guarantee of quality. I n practice, however, many C A D / C A M
'breakthrough' performance cruisers chffer little i n hull shape either f r o m each other or f r o m the host of other boats built to the International Offshore Rule (IOR). Sadly, they also display similar bad habits in strong winds and call into question claims that this type of boat can be "perfectiy controlled" i n all weather conditions. Indeed, their discouraging misbehaviour indicates that there must be a fundamental error within the rule and related C A D programs which produce these yachts. Unfortunately, the saihng boat market is dominated by just that type of ocean-going yacht. The I O R fashion has set the design pattern i n general.
Dangerous breed
It has been argued that racing improves the breed; cruising people may benefit f o r m developments made i n racing boat design. Such an argument cannot be supiported. The crux of the matter is that racing may improve the breed for racing, but not for seaworthiness, not for seakindliness, not for safety, and not for comfort.
The reason is that the hull shape designed for speed is different f o r m that
Typical CAD cruiser/racer built to tlte Intemational Rating Rule (IOR). Computers can speed up the design work but tiiey cannot improve the quality of tiie CAD boat. Computers alloio bad designers to produce bad boats quicker.
designed for cruising. Therefore the claims made by designers and salesmen that the same hull can serve two purposes ("...a fast cruising vessel i n one version and a quick racer i n another" as they are or after interior conversion) are clearly nonsensical.
The search for speed under the rating rule has produced a dangerous breed of yacht, prone to heavy rolhng, broaching and capsize; inflicting large and devastating accelerations which quickly break crew morale in heavier sea. These light displacement, beam.y, flat bottom hulls w i t h cUnghy—like sections, fin keels and ineffective (when really needed) rudders, can no longer be regarded as seakindly and seaworthy. They may be raced around the buoys as day sailing machines, but they are not boats fit to go to open sea i n higher winds.
Figure 2 illustrates the relationship between the hull shape as it evolved over the past 25 years and the probabihty of capsize, it will be seen that, as the beam/ displacement ratio increases and the displacement/ length ratio decreases, the occurrence of capsize is more frequent. Once capsized, the boats w i t h decreasing range of positive stabihty are more Ukely to remain i n the dangerous upside—down attitude for a sufficiendy long time to reduce the chance of crew survival to n i l .
This alarming trend towards hulls w i t h decreasing stability, both i n terms of available righting moment and stability, range, is demonstrated i n Figure 1. Hydrostatic stability of traditional 'displacement type' cruiser/racers (Figure 3) was largely determined by heavy ballast and the position of the centre of gravity below the waterline; while the stabihty of
Continued on page 77
YACHTDESIGN
Continued from page 74
IOR type of boats depends on beamy hulls and the crew contribution; that is a number of crew members distributed along the weather rail as 'live ballast'. Such a means of augmenting stability could not possibly be recognized by the older generation of sailors and yacht designers who would rightly argue that today's fashionable ploy of manning the rail hke a row of sparrows on a telephone wire is simply bad seamanship.
There are horses for courses. Application o f expensive aerospace technology to sailing craft i n order to produce faster but fragile, air-bubble-hke structures operating on rough seas makes no sense f r o m the cruising man's point of view. Even i f a hght hull made of extraordinarily strong exodc materials may survive heavy sea impacts, the crew inside is likely to become physically and mentally incapacitated by large neck-breaking acceleration to which light displacement boats are suscepuble.
Such a consequence of Newton's elementary law (force = mass X acceleration) was widely experienced during the 1979 Fastnet race storm. Evidently, the lighter the boat the higher die acceleration, other things being equal.
Habitability is more than available space below the deck, no matter how
5 wo
I
«01I
60 c i fI
-0 -Slightly elevated — Sleep normal — Some increase in effort I Sleep difficult, fatigue I V« Additional manpower required-Strenuous effort required - Normal eating difficui:
- Normal sleep impossible ; Incapacitation
®
W° 20' 30' W° Angle of roll
tfor prolonged period}
6 S 10 12 Period of oscillation in sec
Figure 4
Effects of rolling and accelerations wlien pitcliing and heaving on crew capability. If boat motion becomes unbearable, the crew would not sail her to best advantage and, when the worst comes to tiie worst, the chances of survival can be seriously impaired. Habitability is more than tlie available space below decii.
comfortable a boat may appear when exhibited in the marina or at the boat show. Here are just two relevant complaints: "Crews dishke long races... because I O R boats, particulariy the smaller ones, are so uncomfortable... it takes much longer to physically recover f r o m an overnighter on today's I O R boats than i t did in years past" {Saü Magazine, May 1987). I n a letter to the same
magazine an experienced ocean-racing sailor and president of Hood Sail makers, USA - Ch. Bouzaid - was even more direct in his appeal: "Rule-makers, do something to stop us enthusiast f r o m drowning ourselves!"
Figure 4 illustrates the point. I t will be seen that high accelerations experienced when rolling and heaving i n waves provokes accelerations rapid deterioration of mental and physical effectiveness is related to seakindliness of the boat. Ideally, she should respond to sea with relatively low rates of acceleration, without sapping the w i l l and energy of the crew.
I t has been estabhshed that the resulting vertical accelerations experienced by fishing vessels heaving i n waves may change up to one g below or above the acceleration due to gravity (g). Such variation is much larger than that at which
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seasickness occurs (Fig 4B). I t may be added that a boat which is hit by a breaking wave (while broadside on to the sea) may experience acceleration up to 5 g, depending on the area of the hull exposed to the wave impact and displacement o f t h e boat. This explains why the safety harnesses and lifelines may break and the crew catapulted far away f r o m the boat.
As might be expected the shape of the hull determines verdcal acceleradons induced by waves and Fugure 5 shows the effect of hull cross-secdon and its depth on acceleradons of a vessel running in stern seas at different speed/length rados ( V . V L W I T ) . I t will be seen that flat-bottom hull with circular secdon may experience about 10 dmes higher acceleration than deeper hull w i t h V-form section characterisdc of traditional type of yachts (Fig 3).
This direct relation between the hull shape and its behaviour in rough sea does not appear to be understood or appreciated by contemporary yacht designers. One of them, of world-wide fame, when asked: "Do the design measurement formulas produce distorted boats?" gave the following answer: " N o , I think that today the boats are better than before. I n general, design has no bearing on safety." (Sailing Magazine, March 1987).
This view is widely shared by designers, in particular those to whom the IOR book is the only textbook on naval architecture they know.
Fortunately there are exceptions. O l i n Stephens, quoted at the beginning of this article, is notable among them.
Modern consumer law lays stress on Titness for purpose', and what a boat buyer needs to know is the purpose for which given design is intended. Is it going to be built with strength enough to last a lifetime, to be seakindly and seaworthy? Or will i t be light enough to w i n in this
1.0 • ShoUovt hull Df Deeper hull Df 6.2% LWL .8 U-form tion tr .6 / / form j era j ' ' 1 :cei .4 y /
\ ;
o ^ / ' U-form o u 2 ' f V^/orm j Oi o.a 1.2 IB 2.0 Figure 5Effect of hull cross-section and depth (D,) on vertical accelerations in fore part of a vessel running in stern seas at different speed/length ratios (V, /VLWL ). It is assumed tliat the wave length is equal to or greater than liull length. Variation in vertical acceleration in waves is given in terms of acceleration due to gravity g which in the static condition on eartli's surface is constant and equals 32.2 fti sec? (9.81mlsec^).
season's races and then become obsolete and of much reduced second-hand value? This is of crucial importance which should not be ignored by responsible boatbuilders and designers.
There are two distinctly different groups of potential buyers with two different requirements: those interested primarily i n racing and those in recreation and cruising. How large are these groups? Circulations of representative sailing magazines may give some clue. Take for example the U K market - the circulation (independently audited) of the Practical Boat Owner k about 68,500 and rising. On
the other end of the spectrum we have the Seahorse magazine dealing exclusively with racing matters. Its circulation (unaudited) is claimed to be 20,000 and has been decreasing, although its owner. The Observer newspaper group, will no doubt want to reverse that trend. But i f those figures are any indication, i t would seem that the number of people interested i n buying safe cruising-type boats is about seven times larger than those oriented towards racing.
It is imperative that boat production should be diversified accordingly to meet successfully these different demands,
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