14th International Symposium on "Yacht design and yacht construction"

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14th International Symposium on

"Yacht Design and Vacht Construction"

Amsterdam, 11 November 1996

PROCEEDINGS

Organized by HISWA - National Association of Watersport Industries inThe Netherlands, the International Trade Show for Marine Equipment METS 97 .

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14th International Symposium on

"Yacht Design and Yacht Construction "

Amsterdam, 11 November 1996

PROCEEDINGS

Organized by mSWA - National Association of Watersport in The Netherlands, the International Trade Show for Marine Equipment METS 97

and the Delft University of Technology

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14th International Symposium on

"Yacht Design and Yacht Construction "

Amsterdam, 11 November 1996

PROCEEDINGS

Edited

by

P.W. de

Heer

June 1997

Organized by mSWA - National Association of Watersport in The Netherlands, the International Trade Show for Marine Equipment METS 97

and the Delft University of Technology

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Uitgegeven en gedistribueerd door: Delft University Press

Mekelweg 4 2628 CD Delft

telefoon: 015-2783254

fax: 015-2781661

E-mail: DUP@DUP.TUDelft. NL

CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG 14th

14th International Symposium on 'Yacht Design and Yacht Construction': proceedings of the 14th International Symposium on 'Yacht Design and Yacht Construction' , Amsterdam 11 November 1996. I P.W. de Heer (editor) - Delft: Delft University Press. - Illustrations.

ISBN 90-407-1505-X NUGI841

Trefw.: Yacht design, Yacht Construction.

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Comfon analysis for a 62-meter motor yacht.

A thorough investigation to quantify the comfort of motor yachts in

waves, with special attention for the roll motions at zero speecl. Break

N. McDonald, Carbospar HambIe, United Kingdom

AeroRig@the rig of the future.

The paper summarizes some of the recent technical fmdings on the innövative AeroRig.

G. Dijkstra & M. Carr, Ocean Sailing Development, Pendennis Shipyard Ltd., United Kingdom.

The recreation of the classic boat.

The paper explains the logic and the theory bebind the recreation of the classic sailing boats of the late 1800s and the early 1900s.

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INTRODUCTION

The 14th International Symposium on Yacht Design and Yacht Construction has been

organized under the auspices of the METS 97, the

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A National Association of

Watersport Industries and the Delft University of Technology.

The present symposium deals with various topics related to hydrodynamics and aerodynamics

of sailing yachts, the construction of the hulls of high speed motoryachts and high

performance rigging for sailing yachts, the genera! design philosophy behind high performance cruising and racing yachts and new design tools and, finally, where is aspects of classification and safety with particular reference to the new international standardisation wich will soon be imposed by the ISO.

The Organizing Committee of this 14th symposium is grateful to the authors who were prepared to share their expert know ledge with the yachting community and spend so much of their valuable time on their presentations.

Once again the Organizing Committee's aim is a symposium that is as fruitful and interesting

as its predecessors. This international gathering of representatives of well-known yacht

designers and manufacturers along with the people active in research and end-users has an

important role to play in the exchange in information and knowledge to the benefit of all

active in this field. Dr. ir. J.A. Keuning

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Sailmaking for the Superyacht

by W.H.I. Bullimore

The interrelationship of load analysis, fabric selection and design technicus used insail construction flor large luxury yachts. by Bill Bullimore, Doyle Sailmakers UK

Introduction

The evolution of a megayachtl superyacht sail inventory takes a very special route compared with their smaller cousins and involves many more disciplines, particularly if the boat is unusual in terms of rig, size, type of sailing or crew handling. For example, not every large yacht today will have the ubiquitous hydraulic/electric roiler furling headsails - many large yachts return to hank-on sails and this poses special sail handling problems with fabric, hardware, stowage, etc.

In this study, we will look at some of the considerations used in evolving the sail inventory for the 'J' c1ass yacht "Velsheda", which demands some of these special and unusual consi-derations. Similarly we will investigate some of the demands on sail design for the ultra-modern, 'push-button sailing' superyacht.

Owners today expect sails and rig to be easily handled with minimum crew. Most of the crew work on today's superyacht is c1eaning and maintenance, so having 20 or so crew to hand to bend on and off sails is unrealistic. Consequently, SallS are expected to be bent on easily with the aid of halliard and deck winches, fit into turtles engineered for single point slinging~

stay on the boat for the whole season and not take up valuable space below or on deck. Each megayacht poses special demands and with consideration to the size of the sail budget, each sail inventory is customised to suit the owner, yacht and crew.

Sailmakers for larger yachts could be more appropriately called sail engineers as engineering disciplines prevail more today. However sailmaking for megayachts still demands that critical 'eye' combining the art with the science of the industry. Bigboat sailmakers listen weil to the feedback from the experienced captains who know weIl the demands on the product out at sea. For example a megayacht roilaway mainsail may be engineered to perfection to cover sailing in all wind conditions, but many captains will set the mainsail for motorsailing in zero wind conditions to inhibit rolling and this can impose slatting loads on the foot fabric weil beyond those nonnally experienced sailing!

The subject matter covered in this symposium presentation is not meant to be a defmitive study or manual on megayacht sail specifications - more an insight into some of the aspects involved and solutions evolved.

Sail development sequence - "Velsheda"

- Computer modelling

The computers help to narrow the range of sail options for different conditions and specify sheeting angles and expected loads. This inforination can be verified in the wind tunnel tests.

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- Wind tunnel testing

Models are then be made of the relevant sails and tested in the wind tunnel at the Wolfson Unit at Southampton University. These tests provide hard data to verify or refute the computers analysis and help to fmally determine the exact inverttory . Once the exact inventory had been decided, each sail undergoes a Relax analysis to detennine the fabric engineering requirements andthe sail engineering specifics.

- Fabrie engineering

U sing the Relax data we are able to detennine the denier count necessary in each area of each sail and manufacture the fabric accordingly. This data will be communicated to the sailmaker to be input into their own engineering parameters.

- Sail engineering

Taking the relax data and the fabric analysis, and combining it with the load analysis from the computer modelling, we engineer and build each sail accordingly.

- Initial inventory

We proposed building the main Passage main sail, Quads and a single spinnaker fITst, and then doing the initial sail trails. Time constraints demanded the full stonn, cruising and passage inventories, with the racing sail to follow. Technology advances so fast, the racing sails would he obsolete if built a year ahead of time, so wait for the advance and evolution of the new, high modulus vectran, or the no-creep spectra, etc.

- Sail trials

The best analysis is done by sailing the boat and collecting hard data such as the boats perfonnance numbers and actual sailing loads on the sails and rig. This infonnation will then he used to build the racing inventory.

- Racing inventory

The racing mainsail and additional spinnakers would he built after all the data has been assembIed and analysed and the fmal inventory agreed upon by all parties. There will be a loop back between various segments of the development sequence.

Computer modelling

This is studied in four areas: 1 Flow Analysis

Using the Crane program, sophisticated software for saillift and drag analysis, we can study the pressure distribution on the complete boat, rig and sails in visual 3 -0 or tabulated form. Also the sails are accurately 'wrapped' around the rig according to the geometric feasibility and constraints of spreaders, jumpers, rigging base, tracks, etc. A working interface with the Relax program incorporated in this software allows optimisation of sail shapes, outline sÎZes and lays the foundation for fabric selection.

2 Relax

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outline of the sail. 1t may he disp1ayed as a contoured stress map (see illustrations), or as a vector mosaic. Critically important fabric distortion will show the difference between design shape and flying shape, so pertinent allowances may be made in sail design for target/optimum apparent wind speeds.

3VPP

Velocity Prediction Program customises the hydrodynamic and aerodynamic force models to produce a variety of meaningful outputs for evaluation. Of particular interest to the sailmakers is the ability to input our own sail coefficients derived from wind tunnel tests and computer modelling. Outputs are generated as line by line data or more familiarly in polar form. Many mode1s of the same boat may he run side by side fOf comparative pUIposes.

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4 Sail Design

Each 10ft will have its' own design software and will generally be used to incorporate the fmalised shape into three-dimensional fonn using a laser cutter to cut twodimensional fabric. A 3D visual and tabulated fonn will show camber, camber position and station, entry/exit angle, twist, etc. This software will also deal with panel orientation, seam widths, hollows, rounds, corner allowances and how to present this infonnation to the laser cutter itself.

Fabric analysis (Velsheda)

The fabric selection and engineering process is one of the most important aspects of both the racing and passage inventories. With this in mind we identified four important criteria to address before designing any inventory of sails:

peiformance aesthetics reliability

ease of handling

- Performance

With "Velsheda", perfonnance is a key consideration both when cruising and racing and to properly address this component we start by frrst selecting the appropriate fibers and then designing the fabrics to specifically meet the engineering requirements of the sail plan. The final aspect is to then design the sails to match the fabric. A good perfonnance fabric for racing meets three important criteri~:

minimal initial and long term stretch

reducing the weight of the sail for an equal fabric strength improved durability

The same criteria apply to the fabric proposed for the passage sails. Both fabrics meet all three criteria.

- Aesthetics

All Kevlar, Spectra and Vectran sails are constructed in a radial fashion where the panels are aligned along the load lines in the sail. The aesthetic look of the sails is taken into consideration when they are designed so that they enhance, rather than detract from the look of the yacht.

- Reliability

We have made numerous high tech offshore sails for the Whitbread and BOC races, and feel confident that they are rugged and durable. After 33,00 miles of hard racing, the heavy weather Spectra sails on the Uruguayan Whitbread maxi showed no signs of delamination and

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negligible stretch. The taffetas provided excellent protection for the Spectra yams and served as an effectiveripstop. "Sceta Calherson" (BOC 60) won the last BOC race using Doyle Spectra sails for the Southem Ocean legs of the race. We recently manufactured a complete inventory of Spectra sails for " Taouey ", the new Perini 190' and the sails weighed less and perfonned better than anticipated. In the last month we have manufactured Vectran sails for the Holland 108 "Gleam" and are currently building a complete Vectran inventory for the classic Pedrick yacht, "SavanTUJh ".

In England, spectra laminates have stood up to the rigorous and punishing environment of offshore multihuIl racing. Much of this is short handed, so a rugged construction and good chafe resistance are important. These laminates have survived the test of time. On the 40'

trimaran FPC (fonner MTC/Mollymawk), just setting off to smash the transatlantic 4000

mile record, we had little or no work to do on the sails after two years and 10,000 miles of

hard racing. The same story with the spectra inventory on the fastest offshore sailing boat

from Britain, 'Spirit of England', another 40' trimaran.

- Ease of hamlling

Multiple plied sails are easier to handle than single ply sails of the same denier count. They are lighter and more pliable and their overall weight is reduced by omitting the taffeta' s from between the plies. Carefully plying to increase the denier count in the high load areas reduces the overall weight of the sail therefore making them easier to handle. All plies will are laminated together (pre ply) during the manufacturing process.

Spectra

In recent years Spectra has been the most appropriate yam to use to build perfonnance cruising and racing sails because of its proven history of durability and strength, however weaving and laminating the fiber has not been easy. Unlike Dacron, it is very difficuit to achieve a tight weave with Spectra because of its high modulus. A tight weave is desirabie to reduce bias stretch. In i993 we used a woven spectra with an X-pIy for the sails for the

Ron Holland mega yacht "Juliet lt. The X -ply was added to reduce the bias stretch and at that

time it was the most advanced Spectra fabric construction available. We had tested some newer designs that incOlporated an inserted spectra laminated with the Spectra yams under tension while laminating, and the initia! test results were so vastly superior that we thought that there must be some kind of mistake. There was in fact no mistake. The fabric was significantly better and we have since manufactured numerous inventories from this type of Spectra including the new sails for "Onyx sur Mer".

The major perfonnance advancement was achieved by producing a fabric without any crimp in the yams. Woven Spectra's incur crimp as part ofthe weaving process and the yams will try and straighten out under the load. (see diagram 1). By pre-tensioning the yams and laminating them while under tension, the initial and long term creep has been minimized. There are two additiona! advantages that inserted Spectra has over their woven counterparts. Because wovens have a lot of initia! stretch as the crimp is attempting to straighten out, and then "recover " again as soon as the load is reduced, the continua! movement weakens the bond between fibers and the Mylar, adding to the possibility of delamination. The other

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a very slippery feel to it. When these Spectra fabrics are laid up they are laid in a grid fashion (see diagram 2). There are large gaps between the yams and this allows the Mylar to bond through the gaps onto the taffetas which are polyester and bond very weU. On a woven Spectra there are no gaps and therefore only Spectra to bond to.

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Vectran

The term "liquid crystal polymer" fITst appeared in the sailing press with regards to Cuben fiber, the revolutionary sail fabric developed for the America's Cup. The fiber that was being referred to was Vectran. lt was originally developed as a minimum stretch, non water absorbing fiber for towing submarine listening devices. Vectran was not immediately seized upon for general purpose sail fabric primarily due to its exorbitant price tag. lt was 40-50 %

more expensive than Kevlar or Spectra. Vectran's extremely high modules, virtually zero long term or "creep", and its nearly 100% retention of strength after flexing all made Vectran an appropriate fiber to consider for a high tech racing fabric.

In summary the properties of Vectran are remarkable:

lts stretch modulus is on a par with Spectra and Kevlar 29 It kas zero strength loss with flex

It kas zero creep (long term stretch)

lts thin ribbon type structure is relatively casy to laminate

lt is from the polyester family and as such laminates weU to polyester taffetas

It is impervious to pressure and heat and therefore its modules is not degraded in the laminating process.

The above properties all add up to make a fiber that is superior to either Spectra or Kevlar in terms oflong term and strength retention, especially in highly loaded sails. Vectran's only negative property is that it does not have great UV resistance. In this Vectran fabric that we are proposing , the Vectran fibers themselves are sandwiched between the taffetas and there

-fore not exposed to UV. They are inserted in the same manner as the Spectra samples. This fabric combines high density warp inserted Vectran that is laminated to a low stretch film

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with Polyester taffetas. lt is engineered after the very successful line of Spectra fabrics and is a logical improvement upon them. By keeping the same engineering and simply inserting a better fiber, we are able to use the vast amount of engineering data that we have already aceumulated and apply it to this inventory.

We recommended Vectran for 'Velsheda's' eruising inventory for the reasons detailed here.

SMP

Super Modulus Polyester may well be the quiet revolution in sail fabric, bringing as important a leap forward as the transition from cotton to daeron in the 1950's. lt carries approximately twice the modulus of regular polyester, yet looks and feels like conventional dacron sails.

lt is Pentex

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by Allied Signal, a giant breakthrough in yam technology. Modern water-jet looms allow wide, tight weaving of this fibre, allowing stretch characteristics that were only available before with complex laminates. The absence of laminate glue minimises mildew problems and of course a woven fabric cannot delaminate! With classic yacht restoration and new building on classic lines, many owners prefer a loss of performance, rather than loose the aesthetics of cross-cut sails. Some feel that radially cut sails look out of place on a traditional yacht, but previously only the laminates could carry the megayacht sailloadings. Now SMP gives that option, although th ere is still a weight penalty compared with laminates. New mainsails in SMP have recently been made for the Farr 140 "Mirabella lIl" and the 'J' class "Endeavour".

A feature of SMP sail construction is to construct the corner and reef patching in box radial panels of very generous size. This is fabric laid in sections approximately the width of the laser cutting bed (I.50m) so the important higher modulus fill (weft) threads are orientated to the load line of the sail, thus creating interesting new cosmetics to the sail construction, particularly in translucent, through-light conditions.

DOYLE SAILMAKERS LOAD ANAL YSIS

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Wind tunnel tests - Wolfson Unit

Intent

To evaluate alternative sail combinations, sheeting arrangements, headstay sag, pointing and rig/ sail relationship within the time scale available

Model

A scale model to 1:25 was constructed by staff of the Wolfson Unit in accordance with drawings supplied by the designer. The model sails were built to exact scale for geometrie outline, camber, camber position, entry and exit angles, twist and sheeting angles.

The Tests

There are remote control sail winches instalIed and the model is mounted on the dynomometer unit. The tests measure Driving Force, Heeling Force, Heeling Moment and longitudinal Centre of Effort of the aerodynamic force. Rigging and sheet loads are measured by removable strain guage link. Photographs from overhead and perpendicular to the hull were taken, also digital images of the test environment. An overhead video camara aided visual trim and real time defmitions of twist and camber.

Test Days

There was a two day consecutive test period with a later two day session to return after evaluation of initial results. The latter session was used more to analyse spinnaker combinations and aquire data on downwind sail sets.

Data

Measured force data is reduced to coefficient form and presented as envelopes of driving force and heel force coefficient, plotted against apparent wind angle. The position of the sail plan Centre of Effort was mapped as apparent wind angle and sail settings were changed. The wind tunnel test data was supplied in a fonn for input to WinDesign VPP software.

Wind tunnel model testing

Model on table with Big Quad, Prestige Staysail and Cruising Main set. Tumtable shows graduations of Apparent Wind Angle, aft athwartships dynomometer rod visible also sheet position marks alogside of hull.

Technology

The Fiberbond sail

The development of the Fiberbond - MD concept began in early 1994 with three goals:

To make the corners of tri-radial saUs more efficient

To eliminatie any seam creep

To create a more durabie, light weight peiformance saU

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patches, and moulded shapes faired to the highest level achieved these goals and have

resulted in sails that have the following attributes:

they are lighter tkan any previous or competing sails for their strength resistance the saUs look physically smoother

the sails hold their shape longer

the saUs are more flexible and therefore easier to handle they are faster and stay faster longer

Wind tunnel model testing

Model on table with Big Quad Prestige Staysail and Cruising Main set. Tumtable shows

graduations of Apparant Wind Angle, aft athwartships dynamometer rod visible also sheet

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The fITst Fiberbond Genoa was built for three quarter tonner racing in the 1994 SORC. The result was an overwhelming victory and the boat went on to win the Mass Bay B overall

championsbip, as weIl as the PHRF New England championsbips. The 47' "Numbers" won

Class A in the Commodores Cup of that year.

Christophe Auguin then utilised this durable weight saving sail construction to win the BOC

'Challenge in "Sceta Calberson ", and the maxi "Nieorette" walked away with the overall

victory and line honours in the last Fastnet Race. Both these results speak to the efficacy of

tbis light sail construction.

The performance advantage was clear, however the production process remained achallenge. Today the production of these sails has been perfected and special fabric have been created to maximize their advantage. The concept has been enlarged from simply replacing corners to producing the entire head, clew and tacks utilising the Fiberbond load bearing concept. This concept allows Doyle to produce sails that have 100% efficient fiber counts in the corner in excess of moulded sails have a lighter and more efficient high pressured laminated fabrics in the middle of the sail. This method allows us to have multi-directional tbreadlines to ensure off threadline stability. This is the key to longevity.

PaneIled sails have been around a long time and the laminating process has been greatly improved, but perhaps the biggest advantage in years has been the ability to enhance the durability of shape. This has been achieved by using IDtraBond seam technology. In summary, modem sails are stronger, lighter and better shaped than ever before regardless ofhow they are made. Doyle's Fiberbond technology has proven itselfboth around the world and around the buoys.

Fiberbond vs Moulded

The late st Fiberbond sails are smoother than ever before because of a combination of Doyles' new "geodesie fairing program" that checks the fmal moulded surface in an entirely new

way. This plus the IDtraBond gluing method makes them physically one without stitching. This not only prevents seam stretch, but it keeps the sail watertight as well.

Moulded is not a bad method of constructing a sail for aspecific purpose. When given unlimited time with the fiber laying gantling so that the fibers can be laid in multiple load paths as was done with the fmal Americas Cup sails, the method is quite efficient.

The greatest difficulties with moulded sails have come from problems laminating the high fiber content of the corners, and also of handling the off-tbreadline loading. In addition, because the entire sail is laminated on the mould the lamination is not as efficient or as strong as can be acbieved with high pressure roller and precisely controlled temperatures. For these reasons moulded is certainly not the way to produce the lightest possible sail. Some direct comparisons would be:

Sail Mumm 30 Mainsail TayIor 40 Genoa Fiberbond 22 27 (Sail weight in pounds)

Moulded 27 29 Difference 5 2 % Difference 22.7 7.4

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The reason for tbis weight advantage in the panelled sails becomes clearer when one exainines the exact makeup of a moulded fabric versus fabric produced in a laminator. The folIowing chart compares fabric from the mid leech of a Mumm 36 Genoa.

Moulded Medium Genoa

Glue

Mylar fIlm:2mm top and bottom Kevlar Fiber 2.0 oz 2.0 oz 0.2 oz 4.2oz 48% 48% 4% 100% Tota!

Panelled Medium Genoa

Glue

Mylar fIlm: 2mm top and bottom Kevlar Fiber 1.30z 1.50z .950z 3.750z 34.7% 40.0% 25,3% 100% Tota!

When moulded sails were frrst introduced they were in many instances lighter then the sails that they replaced. The reasons for this saving was threefold:

Moulded saUs just about eliminaties comer patching sa ving a great deal of weight

Early moulded saUs were for the most pan made from high modulus Kevlar 49 fibers

replacing saUs made from lower modulus Kevlar 29 fibers. This was a 20% sa ving

in yam content alone.

SaUlightness has taken an even higher priority over all saU durabUity and newer saUs

are in almost every case lighter than the saUs that they replaced.

SaU loading is understood better than ever before and therefore all saUs are more efficiently engineered.

The design of Doyle's latest Fiberbond takes advantage of all of the above developments. It

utilises only Kevlar 49 and are assembied using the latest in bonding technology. Efficient high pressure laminated panels are used in the centre of the sails and the fiber reinforcements in the corners are utilised for minimum weight and maximum strength.

Design shape

While most moulded sails literature is written so that one imagines the fIlm being moulded into shape, the method of the fIlm is virtually the sail that is done in producing cross cut sails. The initial fIlm is pre cut with broad seaming and then laid across the sail on the mould in order to have the yams and top fIlm laminated to it. The fIlm achieves its moulded shape by broadseaming, not by being moulded.

The key to perfonnance of a sail is selecting the right shape, and is not related to how that shape is established.

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Durability

While there are surely success stories, "moulded" sails have had their problems with regards to durability particuIarIy with delamination. The reason for tbis is two foId:

The lamination process is not controlled as it is in a laminator.

The off threadline loading can cause any laminated fabric to have bonding problems and the smaller, more specific yam content of moulded saUs makes them more vulnerable to such off threadline loading.

Construction details .•• .

the 'Organic' approach

Soft corner attachments

DoyIe pioneered soft corner attachments for the head and tacks of rollerfurling sails to remove the hard spots caused by pressing in, or webbed-on rings. The soft attachments

resuited in a smoother sail when furled. As a development on he soft head and tack, Doyle

introduced the soft clew which removed the need to have a large clew ring in the corner of the sail. The ring was dangerous to the crew and damaging to the rig and rigging. The sheet is spliced onto a series of Spectra or Vectran lines on the sail distrlbuting the load evenly into the body of the sail. We recommend that all the corner attachments be "soft".

Doyle Sailmakers Soft Clew

Fiber patching

The large st gains in sail engineering have been made in corner reinforcement and we have been able to dramatically increase the strength and load distribution in the corner of the sails while at the same time reducing the weight and the bulk. We call tbis fiber patching and it consists of unidirectional fibers radiating out from the corners of the sail along the load lines. The length of each strip is determined by the amount of load in that particular area of the

sail. Fiber patching eliminaties the need for conventional fabric build up in the corners

normally used to achieve the desired strength. The conventional patches added greatly to the overall weight of the sail without enhancing performance in any way. Fiber patching is

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lighter, stronger and more durable than any other type of reinforcement. With the weight and bulk of the sails being a major concern, fiber patching will go along way to reducing both and promotes a more "organic" approach to the sail engineering ethos.

Ultra Bond Seams

Historically seams have been a problem area in sails because they tend to creep apart under constant load. We have taken a leaf out of our racing note book and applied it to the cruising yacht industry. For more than two years we have been gluing all our race sails and using them without any stitching. The glue is applied with a special glue gun and then seams are subjected to pressure to bond them permanently. The result is that the seams are now the strongest part of the sail. For our large cruising yacht sails we do still add a security row of stitches and then cover them with a water proofmg agent to stop water absOlption between plies. The greatest benefit other than the sails not distorting due to the seams moving, is that the sail will not come apart if a seam happens to chafe through.

Sai) trials

Captured UItraBond Seam

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Fabrlc _ _ _ _ _ _ UItraBond Seam Bond

Most modem sailing megayacht designs are designed to sail reasonably weIl and not be over-compromised to the motorsailing concept. Sometimes with the complexity of these vessels, it is easy to loose sight of the fact that they are primarily sailing boats and the sails and their operating equipment should take reasonably high order on the priority scale. Sail trials will involve checking much non-related equipment, but requiring the sails to provide heel angle, winch operation, line handling, etc.

However, much of the directly related equipment such as running rigging, sheet and halliard winches, tracks, control hardware, instrumentation, etc. All this must happen in concert, often on untried hardware in a new environment.

First Fit

The fITst check is dimensional accuracy - do the sails fit the boat and have the stretch allowances been correct? Even the high modulus laminates will stretch over the long luff and leech lengths of a megayacht, bothduring fITst fIT sail trials and also over time and use. Visual checks on set backs, slide/car attachments, rigging clearance, boom height, sheeting angles, etc. all provide some intense interest during the fITst set of the sails. There will be close scrutiny for cleanliness of shape and as the sail trials evolve, the ability to change trim will be investigated. In order to record the results and cross check the performance, certain tools are at the loft's disposal:

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Performance

The race course is the norina! proving ground, but the megayacht will usually not have a sparing partner against which perfonnance may he compared, but the computed model works just as weIl. Referral to the VPP polars will made to make comparison between the yacht's actual performance and the predicted range. Unusual nodes plus or minus will cause close investigation to the reasons and the yachts instruments . and/ or VPP input checked more thoroughly.

Quantifying the results

Under sailing conditions, the sail designer will use a digital camera to take shots undemeath the saillooking up from the mid-foot at the draft stripes. From this position he will record true lengths in order to check camber and position, luff and leech angles and twist under optimum apparent wind angles.

With an onboard laptop computer, he may refer to the original design shapes, rotated to the viewing angle from the deck. Using the camera's software, these shapes may be super-imposed onto the laptop sereen to compare theactual flying shape to the desired design shape.

Furthennore, the captain may he given a me of these sail shapes to give a visualisation on optimum sail trim for various sails combinations under different conditions. This me is easily presented as hardcopy or in software form and may he remotely upgraded at any time to the boat from the 10ft by modem as most megayachts possess this communications capability.

The generaIly fast and copious feedback of information to the 10ft from the yacht has lead to quick evolution in the development of superyacht sails. The increasingly higher technical understanding of the today's superyacht crew also has pushed tbis process and the days are long gone when the sailloft supplied sails from a mysterieus and little understooclprofession. Indeed today a continuous and healthy dialogue throughout the whole refit/new build project between owner, designer, captain and crew, rigger, spamiaker, ensures minimum roOm for error in that most dangerous of areas hetween professions: the interface. e.g. slides and cars fitting spars, luff tapes fitting hiff sections, upgrades of sailplans · or changing of specifications and so on.

The megayacht sailmaker cannot work alone and must possess a good understanding and empathy of the other disciplines involved, as weIl as an adaptability and ingenuity to move with tbis fast changing market.

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Passenger Comfort on board Motor Yachts

Ir. R.P. Dallinga, MARIN, Wageningen

Ir. H.M. van Wieringen, F. DE VaaGT International Ship Design and Engineering,

Aerdenhout

1 INTRODUCTION

In the course of 1996 a systematic investigation was conducted to identify measures which improve the passenger comfort on board large motor yachts. The work, which was based on information from literature and in-house model test results, numerical predictions and dedicated model tests, lead to a number of general observations and conclusions regarding the hydrodynamics and seakeeping of motor yachts, which are described in this paper.

A relevant measure for the performance of a given design is the fraction of time that is

available for a specific purpose. From a

passenger's point of view the "purpose" of the ship could vary from providing a sta bie platform for leisure and business purposes to providing

safe and comfortabie transport on specific routes.

The performance in the above sense is governed by the hydrodynamic characteristics of the vessel,

"mission" related criteria and the prevailing wave

climate. These three performance aspects wil! be

discussed separately, after which specific design issues wil! be addressed.

2 HYDRODYNAMIC CHARACTERISTICS OF MOTOR YACHTS

Linear Theory

Within the concepts of linear seakeeping theory,

the problem of quantifying the motions of the vessel among waves allows separate treatment of the wave induced forces and the motion induced reaction forces. This greatly enhances the understanding of the nature of the motion characteristics.

Wave Induced Excitation

In general the wave induced forces acting on the captive huil among waves are the sum of the

pressure forces acting on the huil and

contributions from the appendages, like rudders, bilge keels and stabilizer fins (Fig. 1). In the current investigation the impact of huil form and transverse stability on the roll excitation received particular attention; calculations by means of a strip-theory code were used to quantify these

effects.

Fig. 2 shows a typical example of the nature of

the roll excitation for a ship as a function of wave frequency and wave direction.

_ 0

Fig. 1 Huil contour with appendages

Fig. 2 Wave induced roll excitation

It shows that, contrary to common belief, the highest excitation levels are not found in beam seas; bow and stern waves just forward or aft of

beam seas, introduce considerably higher levels.

In the case with the higher transverse stability these peaks in quartering waves are hardly

affected, an increasing stability magnifies in

particular the excitation levels in beam seas. The results of varying the huil form (quite drastically) within a given set of main dimensions

showed peak level variations around 20%.

Hydrodynamic Reaction Forces

The potential theory underlying the calculation of the wave induced forces and hydrodynamic

reaction forces accounts for wave making effects.

This approach has proven to be fairly reliable in

many aspects except for the damping in roll. The

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are relatively smalI, implying that other contributions become important. The investigation focussed in detail on the results of roll decay tests of nearly 30 vacht like huil forms. Fig. 3 shows the principle of a roll decay test.

Fig. 3 Roll decay test

The decay tests shared the common characteristic that the roll damping showed a considerable roll amplitude dependency. See Fig.

4. ~ (I) "0 (I) c. E z =. Cl c: "Cl. E '" D ä a:

Zero-Speed Roll Damping

, _I

I

'

~

. ...•.... .. -~-

-i:

.

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•

, . -' •.. ""

,

.. -:.:~ .. ....

:

'

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j • _i

Roll Velocity Amplitude [degls}

Fig. 4 Zero speed roll damping

....

i

The linear part of the damping (or the damping at small roll amplitude) proved to be about twice the value predicted by potential theory; it showed a clear tendency to increase with increasing natural frequency (a "stiff' ship), see Fig. 5.

Because the damping from the appendages is associated with drag farces the increase of the roll damping was associated with the rail velocity amplitude. Neglecting the contribution of the huil the test results we re compared with a theoretical estimate based on the drag from a rail motion around the centre of gravity of the vessel. Contributions from a centre skeg, rudders, bilge keels and passive fins were considered.

The general format of the roll damping estimate was: 1500 r---.,..--~! ~---,----, ... _ .... , ... ; ... i._ ... · ... . · ...

-1

...

.

... , ... ; ... 1 ... . : : Ol c: 'ö. ... ' , ... _ ... _ .. _ .... : .... t ... E lil 0 (5 500 cr: c: :.J

..

~!.

.

.• : ... :.

' : . ! : ... .... ' .... _ ...

:

... -~ ... ...

:.

. i ! • I : ! 0.5 1 1.5

Natural Freq. [rad/sj

I.Measured - Pot.Theory

I

Fig. 5 Roll damping, linear part

in which the product of the oscillation frequency

ro and the roll amplitude cp represents the angular velocity amplitude, Arel a reference area and rrel the arm to the centre of gravity of the vessel. The unknown drag coefficients were varied to obtain the best fit with the results of the available decay tests of all vessels; the result is shown in Fig. 6 . ~ ""0 ~ ::::-~ '"

i

10000 z =. '" Ol 0>

'"

""0 c: Ol a. ~ 0> c: 1000 'ë. E

'"

D Ö a:: "ti ~ Il.

Measured Damping [kNm/(rad/s) I(radls))

Fig. 6 Roll damping, increase per rad/s roll velocity

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The variations showed that at zero speed the bilge keels provide by far the largest damping contribution; a drag coefficient around 6 provided a satisfactory fit with the test data. The magnitude agrees with information from literature. The contribution of the passive fins and rudders is surprisingly smalI, mostly because of the relatively low associated drag coefficient, which ranges around 2. For the skeg a drag coeffieient of 1 was used.During a series of dedicated model tests the above empirical model was verified by measuring the actual forces acting on the fins and bilge keels during tests in irregular waves.

Fig. 7 shows a typical measurement set-up.

Evaluating the energy dissipation assoeiated with rolling allowed an estimate on the effeetive drag coefficient of the appendages. The effective linearized damping B$$ followed from:

T

fM~

.

cÎ>dt

1 ·2 0

2

Bcjlcjlcp T

in which McjI represents the time history of the moment around the centre of gravity, cp the time

history of the roll velocity and T the test duration.

Fig. 7 Liftldrag transducer

Fig. 8 illustrates the convergence of the dissipated power in the course of time for a test in irregular waves. The convergence is quite fast; actually this procedure showed surprisingly consistent results for various test conditions, even decay tests in calm water. Taking the significant single angular velocity (twice the rms) as a measure for the roll amplitudes in an irregular seaway drag coefficients between 4 and 10 were obtained for the bilge keels.

Fig. 9 shows an example of a cross-plot of the

roll veloeity and the bilge keel forces. The effe ets

of the wave forces on the windward side of the vessel can be clearly recognized on top of a trend opposing the roll velocity.

Ir---~I---~--__. 0.9 J - - - --:-j' - - - ' - - - - { Pbk' 0.8 ~ 1 0.7 F.-,---t---+--__i ~ P 0.611 ~ fini' 0 .. < 1 - ' 1 • • t---..---+---==-=----:!::-=...---1 .l \

---Jr--~-- ~~I~~-+~-+_~rc---+-__i ~ P 0.4 11 ... I ~ arti 0.3 I-'----....O>...!"l--"'---+---j 0.2 ~----+---+----j _: ~----0.1 __ ' - __________ • o -0.1 ' - - - ' - - - ' - - - ' o 200 400 t_j Time [5] Fig. 8 Dissipated power

20r----.---r----ïr----~--~ 10 FThkSBi FTbkPSj o -10 L-__ --L _ _ _ - ' -_ _ ..J... ____ ..l....-_ _ ---l -4

Fig. 9 Cross plot

Despite the obvious disturbances on top of the velocity introduced by rOll, the resulting roll damping was quite similar to the values in calm water, confirming that decay tests provides relevant information on the non-linear roll damping of a ship rolling in irregular waves. See

Fig. 10. 2000 ...---..,..--,---, """':- -,--' ... __ .•....•. ;._. __ ... -_ ... -. ! ' ! :g 1500 .. : •..

i

.

1 ..

;

.. I

..

,

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i. ~ "';ooi +' j'--(f' ~ 1000 .. --... ;...oo>-. . .. --~ ... + ... ; ... .

f~=~

~

ct

J

i

..

i :

ft

"

-500 '----'--'--'---'---'---' o 1 2 3 4

Sign.S.Arrc>1. Reil Vel. [deg's]

.B.Keels A Fins

- B.Keels cD=6

... FinscD=2

-Cwtr Decay

Fig. 10 Roll damping, zero speed

Fig. 11 summarizes the roll damping contributions at zero speed, it iIIustrates the relatively large contribution of the bilge keels.

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1000,---.,..---;; CU .!:: ~Wave

i

mBK Ol EB Skeg I: .~ 111 Rudders cu BRns Cl '0 IBTotal a:

Fig. 11 Zero speed roll damping contributions Anti-Roll Tanks

A basic disadvantage of drag-based rail damping is the fact that the damping contribution becomes

. only significant if the vessel rails. For this reason

the merits ot a U-type rail stabilizing tank, Fig. 12, were investigated.

Fig. 12 U-type rail stabilizing tank

The theoretical analysis, which considers the reaction farces of the tank due to pure rail, was performed according to work done by by Stigter

[Ref. 1 J. Apart from selecting the main

dimensions such that favourable tuning is obtained with the natural trequency of rail of the ship, the internal damping of the tank is a design issue. Ta obtain a high tank response and related high rail damping at sm all rail amplitudes a geometry with minimal internal drag was adopted.

The theoretical analysis indicates that the contribution of a tank with a contents of 2-3% of the ships displacement could magnify the damping four fold.

The performance of the tank was verified by mounting a model of the anti-rail tank in the ship model and measuring the farces exerted by the tank on the ship. The resulting damping was derived inthe same way as from the fins and bilge keels by evaluating the dissipated energy

a~d. comparing this value with the damping dlsslpated by a linear damping.

The damping from the passive fins, the bilge

keels and the anti-rail tank. are shown in Fig. 13.

The results contirmed the benefits of the tank

which yielded a nearly threefold reduction i~

irregular seas. Fig. 13 also demonstrates another

interesting feature of a U-type tank: the fact that

the internal damping of the tank is related to quadratic drag forces imp lies that the internal damping is relatively sm all at small rail amplitudes. This means that the tank works best at small rail angles; at larger rail angles the same

mechanism prevents high-velocity impacts in the

U-tank wing tank top.

2500 '---:---:-

,

-1""', ---',- -, - -, -...---, ... .J ... -.-.! •.. ----j ... ~. __ ._~ ... +-_ ... l ... 1... .. .

;; 2000

····:-

· :·

..

·

· ....

·:

· .

...

· ...

,

..

...

.

..

L.. . . ; .. .

i

1500

:~

:·i·

~

:

:!

:

~

~I

;

~~~~

:=~:f

:~~~*:~

]

~

:::~

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11000

:::i::~

r

:~:

..

~

r:~:~~L

=+~~~j~:

..

.

~ 5OO·T~ ·;·

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!..·~ ···~ ·i ... L .. L.. 1... l.I..

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0.5

1

1.5

2

rms RoU Vel. [degls]

Fig. 13 Rail damping zero speed

• B.Keels

• Fins CART

The theoretical analysis of the tank is based on the rail motion. If the tank is effective the rail motion becomes rather smalI, implying that the forces due to local sway accelerations become important as weil. This may explain why the actual theoretical prediction of the rail angle was relatively paar.

Fin Stabilizers

The hydrodynamic reaction farces trom rail stabilizers are governed by the effective lift generated by the fins, fin-ta-huil and huil-ta-fin interactions and the mechanical reaction ot the

fins to the moving ship. The basis tor the

prediction is provided by the knowledge

developed in the course of the MARIN

Cooperative Research Ships (CRS, [Ref. 2]).

The prediction of the generated lift is based on an empirical expression which accounts for the effe cts of fin thickness, aspect ratio, the presence of flaps, the bilge rádius at the fin and the local

thickness of the huil boundary layer. The

magnification of the effective damping by a wave making effe cts with a frequency dependent character are added ta this result.

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the incoming flow at the location of the fins by wave and diffraction effe cts (which add fin induced roll excitation) as weil as the ship motions.

Fig. 14 shows the magnitude of the various corrections to obtain the lift slope of typical non-retractable fins. 50 ~ . :1

rm

l ₀ 6 ::, _i_- f.;j;_ ...

,

-

~ )i-i ~ ~~,

~

.

S

·50 _~{~ ...

Basic 20 Asp.Rat. Subm ... g. Ship B.L. Tota! Thidq1ess Flap Bige Rad. Fil-tc>-Hul

NallJ'a ei LS Degaamm

I_

Activa Fin 0 Passiva Fin

I

Fig. 14 Lift slope degradation

It shows that the relatively low aspect ratio of non-retractable fins (as often applied on motor yachts) is the most important factor in the lift

slope, which is around 1.5 rad-1• It should be

realized that this limitation can be circumvented by applying retractable high-aspect ratio fins with

a flap. In this case a lift slope of around 6 rad-1

may be obtained (or a similar lower fin area).

Fig. 15 shows the related lift slope corrections.

Fig. 16 summarizes the roll damping contributions

in transit conditions. ~100 c:

:g

50 ~

8

0 1--...,..-... - - -... • 50 L..-...J...._.l..----L_...J...._.l..----L_...J...._.l..-...J

Basic 20 Asp.Rat Submerg. Ship B.L. Thickness Flap Bilge Rad. Total

Nature of LS Degradation

I-Individual Corrections

I

Fig. 15 Related lift slope corrections

The wake generated by the fins introduces angles of attack on down flowelements like bilge keels and rudders. These may yield forces which

magnify or counteract the damping from the fins.

Here the requirement for high damping at low speed, which requires high bilge keels, may conflict with the presence of non-retractable (Iow

span) fins.

10

STAB.P. RUO.P. SK.PR. H.FRIC H.U' STAB.C. RUOA. BK>Hll H.EOOY H.WAVE

Fig. 16 Roll damping contributions in transit conditions

The sparse amount of information suggests that the frequency dependency can be understood by assuming that the wake of the fin may be considered as quasi-stationary and that the forces on the bilge keels are generated close to the leading edge. In this case the phase angle between the farces acting on the fin and the bilge

keel is simply given by E = wx/U in which w is

the oscillation frequency, x the fin-bilge keel

separation and U the forward speed. In practice

the separation is relatively smalI, in this case the lift of the bilge keel from the downwash of the fin opposes the fin lift. The degradation can be as large as 20% of the fin lift.

Motion Characterisfics

The motion characteristics of ships, in terms of

the response per unit wave, depends on the

wave frequency (which for a given water depth governs the wave length), the heading and the

forward speed. Fig. 17 indicates typical rail

characteristics at zero speed. It shows, at zero speed, the roll peaks in conditions where the wave frequency equals the natural frequency of roll. The presence of the anti-roll tank yields a nearly four fold reduction of the roll amplitudes .

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At non-zero speed the speed affects the frequency of wave encounter. In stern quartering waves unfavourable tuning with the natural

frequency of roll can. be combined with the

relatively high roll excitation levels in short quartering waves. See Fig. 2 and the roll response in Fig. 18. The roll response is considerably lower than at zero speed because of the high roll damping trom the tin stabilizers. The transverse acceleration levels contain

components trom roll, sway and yaw. Because ot

the angular motions they depend on the location on board. Fig. 19 indicates the character of the response in transit conditions.

Fig. 18 Roll at 14 knots speed

Fig. 19 Response in transit conditions at 14 knots speed

The contributions of the roll in stern quartering waves and pure sway in beam seas can be recognized. The tact that the sway in beam seas dominates the result means that the stabilizers

perform satistactory. The eftect ot height is

shown in Fig. 20; it iIIustrates the benefits of a

low position of crucial areas on the ship.

Fig. 20 Crucial areas w.r.t. transverse

accelerations

The vertical accelerations are governed in particular by heave and pitch motions. Fig. 21

indicates the character tor a point at one-quarter

ship length trom the stern at zero speed. The

peak in beam seas is related to the heave response of the vessel.

Fig. 21 Vertical acceleration midships

The magnitude of the vertical accelerations depends strongly on the location on board, the characteristic is governed by the separation of the

centre ot flotation and centre ot buoyancy. Fig. 22

shows a typical result, iIIustrating the benetits of a suitable position of crucial areas. The vertical accelerations at a point at three-quarter ship length are about twice the levels at the lowest point.

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~ ~ ~ ~ ~ ~ 0 5 W U ~ ~ ~

Longl1Jdinal coordhalo (mi

Fig. 22 Vertical acceleration 3. Comfort Criteria

The way a ship is experienced by the passengers seems to be governed by the nature of their activity (sleeping, standing, the amount of distraction), habituation and expectations and their motivation on one side and on the other side the magnitude of the motion levels. The latter depend highlyon the location on board, on the prevailing wave climate and the way the ship is managed by the crew.

Commonly used criteria in comfort studies (CoiweIl, [Ref. 4], NORDFORSK [Ret. 5]) consider seasickness and human mobility.

Vertical acceleration levels are used as an indicator of seasickness incidence. Apart from the intensity of the accelerations (described by the rms) the mean frequency plays a role.

The local transverse accelerations are mostly used as a measure for the ability to move comfortably about the ship. A slightly more complicated parameter, which accounts for the combined effect of vertical and transverse accelerations, is the angle of the effective gravity vector with the vertical.

In practice the above criteria are used in isolation. However, there is some evidence that the occurrence of seasickness is also related to a "disorientation" related to combined transverse and vertical motions. Unfortunately there is little quantitative information available.

Simulator Study

Uncertainties regarding the criteria motivated a study on board the motion simulator at TNO Zeist. Objectives were to get a ·feel" for the differences between an existing vessel and the new design and to obtain a justitication tor the investment in stabilizers and a roll stabilizing tank. The analysis of a questionnaire among the participants confirmed the benefits of the new design.

4. Wave Climate

Wind statisties

The oldest and simplest way to characterize an offshore environment is to characterize the wind climate, for in stance in terms of the frequency of occurrence of various Beaufort numbers. These wind classes are related to area dependent "ave-rage" wave conditions. The table on page 9 and the adjacent figure summarize some commonly used relations.

Although often used in ship operations this approach fails to recognize the fact that one wind speed can come with a wide range of wave heights and periods, strongly depending on the fetch and duration (or more general the history) of the wind. Since wind speed and direction are highly variabie it means that in practice the waves are never in equilibrium with the wind.

In the situation of a relatively high wind speed (a "young", growing wave) most of the energy input will take place at the high-frequency tail of the wave spectrum. The result is a short, steep wave. If the wind speed drops relatively quickly the continuing non-linear transfer of wave energy from shorter to longer wave components as weil as frequency segregation (long waves travel faster than short waves) yields swell type wave conditions. Because the damping of wave energy is usually quite smalI, wave energy from distant wave fields is often present in exposed locations.

T.he fact th~t one wave height can show highly different penod characteristics has serious conse-quences because, as was shown in the foregoing, the motion characteristics of ships strongly depend on the wave period. This is in particular the case for roll where, if the right tuning with the natural period of roll is obtained relatively large roll angles may occur. '

The ~bove implies that wave height statistics only

provIde a rather narrow basis for design.

Wave seatter diagrams

In the offshore industry the availability of wave measurements has led to the introduction of

14th International Symposium on "Yacht design and yacht construction"

14th International Symposium on

"Yacht Design and Vacht Construction"

Amsterdam, 11 November 1996

PROCEEDINGS

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14th International Symposium on

"Yacht Design and Yacht Construction "

Amsterdam, 11 November 1996

PROCEEDINGS

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110

14th International Symposium on

"Yacht Design and Yacht Construction "

Amsterdam, 11 November 1996

PROCEEDINGS

Edited

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Table of contents

Comfon analysis for a 62-meter motor yacht.

The recreation of the classic boat.

msw

Sailmaking for the Superyacht

by W.H.I. Bullimore

Introduction

Sail development sequence - "Velsheda"

Computer modelling

SAIL DEVFLOPMENT SEQUENCE

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IntHallnventOly

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SeaTrIaIs

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Fabric analysis (Velsheda)

Spectra

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SMP

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DOYLE SAILMAKERS LOAD ANAL YSIS

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