America’s Cup. Notes on designing Alan Bond’s new twelve

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America Cup Update

By Johan Vaentijn

Since the use of 12-Metre racing

sloops tor the Americas Cup series in

1958, millions of dollars have been

spent ori research and develcpment. In the programs of various designers,

many "breakthrough" shapes and

forms have been invented. Most of the

odd shapes actually built have been

unsuccessful partly because of

unpre-dictable scale effects with the use of

small models and partly because of the many restrict ions of the rule itself. Tank tasting simple shapes hasproven to be more successful.

When commissioned to design the nC'N West Australian America's Cup challenger by Alan Bond, Bob Miller and I were given about seven months

to come up with a fast boat. This short

period demanded proper planning

and a systematic approach. In the fol-owing our basic approach is outlined although some of the parameters and

nLimbers shown are changed 1rori the values for the neviAustra/ia (Fig. 1).

To minimise the scale effects we

de-cided to t'y for the largest possible

model size. This brought us to Deift

University. Holland. Here the tack can

hancle 12-Metre models at a scale of

1:9 which gives a LWL of about 165m

with an overall model length of about

225m. This larger model size avoids a

lot of scale effects that appear on

smaller models. The models 'Nere

towed by a special carriage which car-ried the operator and the instruments. These instruments are specia!iy

devel-oped and built by the University for

yacht testing and this facility ¡s among the best available in the world.

First, data was compiled of the wind

conditions off Newport, Rhode Island. During July the wind is anywhere

be-tween six to 15 knots 70 percent of the time.

During August, the range of

about six

to

10 knots is favoured.

In September the America's CUP races are sailed in winds ranging between 11

and 15 knots 43 percent of the time

with 27 percent below this and the rest

above. Consequently, we would need

a hoal that has her best ¿hi-round per. formance in winds between 11 arid 15 140

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knots, better performance below 11

knots than the present 12-Metres, and

similar windward performance

in winds over 15 knots.

Analysis was also made cl the spe-cific sea conditions at Newport in the

area ot the race course. In the 10 to 15

knots true wind range, the average

wave height is two to three feet. A

measured spectrUm corresponding t

this range was used in calculating the

rough-water characteristics of the

models.

At tirst an intensive study was made of aU ot the successful and a number of less successful 12-Metre yachts of the

past. Comparisons were made on

paper to try to determine what rriakes a

winning Twelve. Analysing all of the

The rw bc.at h oniy

faer up to thout

20 not ofbeze.

if you couki b

cer-tdr. t woud ;ow,

Southn Cross

vud

th boat

to take to Newport

1974 races and allowing for tactical

er-rors, we decided that Southern Cross

was about 25 seconds slower than

Courageous on a 4.5-mile windward

leg and 15 seconds slower downwind in 12 knots of breeze.

From all the comparisons two

mod-els were developed primarily to try

lighter boats, but which also had a

number of basic different features in

length, beam, longitudinal centre of

buoyancy (1.CB), prismatic coefticient

(PC), etc, Thuse models were

corn-pared against a model of Southern

Cross, 1974. Model two was five per-cent lighter than Southern Cross, and model three, 10 parcent lighter than

Southern Cross. All three models were

then tank tested in smooth water

and computer tested in rough wafer. In addition, flow studies were made. The

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principal result ot these tests was that

model three was slightly

laster all

round than model one, Southern

Cross, while model two was similar to

model one in most conditions. After reviewing the firstthree models

in various ways, we decided that we

should design a very light 12-Metre. A

light displacement would be favoured in light air for quicker tacking, better acceleration and better rough-water

performance. lt would also have more sail area per pound displacement and

per square foot of wetted area,

all speed-making factors.

Light displacement however, gets heavily penalised n the rule with the

chain girth difference. Yet testing a

very tight model with a large chain girth

penalty would give some more insiaht as to what the intluence is. The fourth

model was tobe 20 percent liglr ter than Southern Cross and would incorporate

the good features troni the previous

models.

The big problem with light-displacemnu'nt boats is thou stabilit'j Stability iS the most important tactorfor

windward performance. Thus, when reducing the displacement by 20

per-cent overSou'themn Cross without alter-ng beam, we would lose some 20 per-cent stability. This decrease would kill the windward performance as was

al-ready proven with the various studies

ot the first three models.

To geta handle on stability, a special

study was made. The most important

factors For stability are: Displacement

BWL and BWt./Tc (BWL = Beam

waterline; Te

Canoe body draft

below the waterline)

o) Vertical centre of gravity location

With a computer program, stability

curves like those shown in Figure 2

were calculated for various models.

With all the data obtained, graphs werd

developed thaI would allow predicting

the stability at 25 degrees heel angle of

a proposed model within one to two

porcenit(Fig. 3).

The graphs amo developed in such a

displacement, a BWL and BWL/Tc can

be selected. As the relative centre of

gravity of 12-Metres is more or less

es-tablished, the stability arm NG sin

could be read and a cross-check on stability made. If it fell within two per-cent of the required righting moment,

the midship section could be drawn up using BWL and Tc from the graphs.

In-creasing the BWL increases stability

but also increases fhe resistance. The

idea was to find the optimum stability

for the least resistance.

The overall stability of the boaf also

depends on the rig size. The larger the

sail area, the larger the heeling

mo-ment. As reducing the displacement

also increases the chain girfh penalty,

the total sail area for the fourth model

would become, therefore, similar fo the original Southern Cross.

Test data for each of the four niodols was compared to investigate the

influ-ence of various parameters on

resist-ance.

Upright resistance of the boat can be divided info three main groups:

a) Frictional resistance

b) Residuary resistance

C) Added resistance because of

waves

Frictional resistance is dependent on

the wetted surface and is easily

pre-dictable. To reduce frictional

resist-ance, the wetted area should be

minirnised where possible.

The residuary or wave-forming

re-sistance is more difficult to control. The significant parameters include BWL vs

LWL, PC, LCB, displacement,

wa-terline entrance and exit angles, and the shape of stations, waterlines, and

diagonals. The relative influence of

these parameters also depend on the speed range. As mentioned, we were

designing for a wind range up to 15

knots. The maximum downwind or

reaching speed

a 12-Metre will achieve under those conditions will be eighf fo nine knots. Up to about eight fo

nine knots, the waterline length of the

boat is not the mosf important factor as

this range still falls within maximum

hull speed.

Above this speed the waterline

length becomes increasingly

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tant. One graph developed after the fourth model was tested plots

LWL/BWL versus residuary resistance

per ton displacement (Fig. 4). In the

low speed range there

is little dif-ference between the narrower boats and the beamier boats. However, as speed increases, resistance increa-ses. The beamier the boat the greater

the increase. For a boat speed of eigi

to nine knots the narrower waterline

boat has the advantage.

The second graph developed shows

displacement vers':s resistance (Fig. 5). After plotting the first four models

we saw a tendency toward an optimum

displacement around 58,000 pounds in the curves. This graph can only be

used here as the stability falls within a small range which keeps the models n a close relationship.

Similar indications in regard to beam

and displacement came oui of the

rough-water studios. To get an indica-tion of the performance in a seaway, a

Deltt computer program for yachts in sea conditions was used. This pro-gram calculates

for' any given sea

141

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-Figure 1: Profile of the new Australia superimposed on larger profile of Southern Cross

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)5 0 10 200 30: 2(1 .1 2.2 2.3 2.4 . .. .7 Heel-angle BWL/Tc

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12

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HRS/Disphiccment tons Vs= 2.10M/sec 3.8 3.9 4 4.1 t I 4.2 4.3 4,4 LWL/81'VL

spectrum the added resistance, the

pitch motion, the heave motion, etc.

For two different sized models the radii

of gyration were calculated. This

radius seems to vary with the LOA

which in turn varies with the LWL. For comparison's sake an cverae raaius of gyration of 21.5 percent LWL was

used for all the models. In addition, two

additional radii of gyration were

cho-sen, one 10 percent larger and one 10

percent smaller. The principal result was that for a 10-percent increase in

radius of gyration a 10-percent

in-crease of added resistance was found.

The added resistance versus

dis-placement is plotted in Figure 6. From this graph we noted that the added re-sistance goes down with the displace-ment. In addition, it seems to vary with

the beam. The bearnier model has

more resistance than

the narrower

model. This ditterenco is worse at

lower speeds and diminishes more or

less in the high-speed range.

Model one seemed to have more

added resistance than the other

mod-els. The reason for this could have

been her relative fineness at stations one-halt and one, just forward of the drastic displacement increase al

sta-tion two. The fine entry provides

minimal damping of

motion and

Mo3t

of the odd

shapes actually

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seemed to be the cause of the extra

added resistance. The last model had

relatively less added resistance than

the other models. This was caused by

her having more displacement in the bow with steeper profile in this area than ay other model. In absolute sea-keeping values

it appeared that the

lightest model (model four) was the

best of all according fo the computer.

However, in the test tank, although the very light model did not quite live up to expectations in performance, it

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Upright resistance vs Displacement

Vs= 4,5MI5

54 58 62 66 '- 70

i i I i t I I i t

Displacement in pounds

showed that we were going in the right

direction. Model tour was a rather big

step towards lighter displacement.

This step was necessary to investigate

the much-discussed

light-displace-ment concept as well as trying to find a lower displacement limit.

From the resistance studies

'

concluded that the optimum

displace-ment should be around 58,000

pounds. Model five was of a design

having this displacement. The rating

parameters were chosen based ort

s,tudies done by computer. The sail

area was increased by four percent

over Southern Cross

for improved light-air performance. From the

vari-ous studies we decided to take a

freeboard penalty of 15 cm. Lowering the freeboard would increase the

sta-bility by towering the centre of gravity,

decreasing heeling moment decreas-ing of the heeldecreas-ing arm (for similar sail area), reduction of wind resistance of

the hull, and reducing gyradius for

improved rough-water performance.

The chain girth was kept loa minimum

while the midship section was

devel-oped using the stability parameters

graph.

At the same time we designed the sixth and last model. The rule 'length" and freeboard were kept the same as

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model five. This model was slightly

more radical in hull shape and slightly heavier. The bow was made steeper,

thus tilling the forward stations with

more displacement farther below the waterline. The reason for this was an expected rough-wafer improvement.

The rest of the hufl was much more

veed than the other models. All other parameters were developed from the

various graphs used in the design

sys-tem. Making an additional model

allowed us to study within reasonable displacemont and stability limits two

rather opposite hull shapes.

The keel has two principal jobs:

keeping the boat from going sideways;

and providing a place for the ballast

necessary for low centre of gravity.

From the first model tests model

three appeared to have the smallest

leeway angle. Model two made about 10 percent more leeway. Model one made about 20 percent more leeway

than model three over the whole

Operating range.

To get a handle on what size keel is needed, we used a calculation method based on the test results as well as

vari-ous papers and theories of

aerody-namics and fluid dyaerody-namics. This

method calculates the theoretical side force developed by both keel and

rud.., rr'

-der. The same total results for side

force were measured in the tank in a

special series of tests. From testing

model nne appeared to develop about

11 percent less total side force than

both models two and three. This

allowed for the big rudder on model one. From the calculation method the

same total difference was found.

This diflerence can be further

ana-lysed with the calculation method.

Allowing for the bigger rudder, the keel

for model one was producing 15 per-cent less side force than the keel of

model three for a constant

speed-length ratio. At the same speed for

model three as model one this

dif-ference increased to about 20 percent.

The keel appeared the main tool for

controlling side force and thus leeway.

Any deviation in

side force in

the

calculation method was therefore

as-sumed to give a similar percent devia-tion on the leeway angle.

The rudder we see only as a means

of controlling the direction of the boat

and not for controlling leeway. A large skeg and rudder combination

theoreti-cally would be more efficient with a

smaller keel to obtain the required side force. However, a combination like this does not allow a boat to turn quickly, a

very important factor

in successful

match racing. This combinatìon was therefore not considered. The larger

leeway on model one increased the in-duced drag of the model and is one of

the main reasons for the difference in windward performance between model one and three.

To aid stability one should like the biggest possible koel for a low centre

of gravity. Lengthening the keel

in-creases the induced drag while at the

same time increasing the wetted area.

The exact happy medium is difficult to predict. However, with the calculating method as a guide, we were able to

come within a few percent for this type of boat in plantorm area and leeway.

From the video tapes taken during

lank testing, we saw thatthe flat bottom

on model one is superior over the V-bottomed keels ofthe other designs. n the case of the flat bottom. hardly any crossflow occurred atthe tip, while the

crossflow on the V-bottomed keel was

very noticeable. In the calculation'

method this decreased the developed

side force by about iwo percent. An ad-vantage of the flat bottom is that it

low-ered the centré of gravity of the boat and therefore increased the stability and thus the windward performance. However, from the various tests it

ap-peared that a V-bottom seemed to give

less resistance and tended to

encour-age a vortex coming from the keel.

A comparison was made between

the most common airfoil

sections.

Based on various investigations the

keel appeared to develop up to 50 per-cent of the total side force. This would

mean that the design section lift coef-ficient 1er 12-Metre keels varies

be-tween 0.10 and 0.15, favoring the

lower end of the scale. Factors that

helped to decide what type of section

to use included tow drag, good lift

characteristics with and without trim

tab, and relative centre of gravity loca-tion.

Among the many hydrodynamic

comparisons that were made, a few of the rnost.important are: sectional area

curves, longitudinal centre of

buoy-ancy(LCB), prismatic coefficient(PC). There were many possible shapes

that were considered good. When

comparing those curves, the keel is

taken oft and all areas were taken to the

farbody of the boat After testing the

first three models, we discovered a

no-table difference regarding the starting

point of the bow wave. At low and

medium speeds, the Southern Cross model showed the bow wave starting

relatively farther aft of station O than the other models. At very high speeds this

tendency was reversed.

The effective waterline length that a

hull sees in the water at various speeds

depends on the shape of the area

curve in the ends and not where the

profite of the hull intersects the

wa-143

r

r,

t. Added i esistance 4/VS vs Displacement Figures 4, 5 and 6:

vs Tank- lest data for models

one Through four is plotted in these design curves for se/cc fing optimum values of beam and dis placement

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-58 62 66 70 74x10

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terifle. The foIIowng approach to

cor-rect for the fullness of the area curves in the bow seemed to give sorno proof

and insight of this commonly

misun-deistood problem. One can draw

tangent lines at the area curves where

the curve is two-and-a-half percent of

the maximum area(Fig. 7). The LWL of

Southern Cross would become about

three percent shorter while the best of

the other models would shorten the

LWL by about one percent. The

per-centages derived

in

this manner

agreed with the observations made on the various models.

The after ending of the area curves were all similar. The curves are steep

on the 12-Metre yachts in this area and no correction such as the one forward could be justified.

When studying the flow around the

afterbody we found that model one had

the cleanest flow with the least

turbu-lence.

Models two and three had

slightly more turbulence with fuller and

shallower bushes, but this turbulence

did not seem to slow them down.

Successful yachts, 12-Metres as

well as medium and large ocean racing yachts, all have similar prismatic

coef-ficients and longitudinal centers of

buoyancy. Boats with the LCB forward tend to go better on running and reach-ing legs forsimilar POs. Boatswith high POs tend to go better in the high-speed

range with less driving power. For a

12-Metre this would be heavy weather reaching and running in winds over 25

knots. A high PC will hurt light and

moderate air performance and

down-wind performance in general.

Both LCB and PC are dependent on the shape of the area curves and have to be combined with those when com-paring them. In studyingfheareacurves

the LWL for model one was actually considerably shorter. AIowing for this,

LOB went relatively farther forward and

the PC went up drastically. This actu-ally says that Southern Cross should

be a good heavy-weather boat.

The upright resistance at all

com-mon sailing speeds were the highest

for model one: except at very high

speeds she was better than all

the

models. A general tendency was that

the heavy-displacement-type boat had

more favourable resistances per fon displacement than her lighter sister.

Model one had the best

ratio for

LWL/BWL of all the models and with the heavy displacement should there-fore have better resistances per ton. Having a relatively higher PC and

far-ther forward LOB we reasoned that she therefore got her worst performance in

the regular operating speed range. The driving power as well as heeling torce of a boat is dependent on the rel-ative amount of sail. Each boat has an

ideal combination for a particular wind range.

lt was therefore important to

144

decide viheii the boat should perform

to her optimum.

For easy comparison, we plotted

ratios of sail area (SA) to wetted area (WA) and displacement against boat

speeds for various particular wind

speeds. As we have used different

scales for each model, small speed

curves for wind conditions were made by plotung boat speed agmnst relative

sail area. Downwind it was simple as each model went more quickly when

the sail area was increased.

lt was

clear that the lightest boat with the least

wetted area would be the fastest in

winds up to about 20 knots. Thereafter,

the length would start to count and

make the bigger boat faster.

Close-hauled, the problem was

more complicated as the stability

became an important factor. Bolow five

knots of true wind, stability won't do

much but in winds of 12 to 14 knots, it is

critical. Decreasing the size of a boat by one percent allows an increase of

about two percent in sail area.

The resulting change in the absolute

values of the hull will be: First, a de-crease in stability because of loss of

We decided that

j9

cFispiacement

o1d be fvred

in liht ar

or çukher thckin.,

ber aCCerØ

ad better

rough-wi:er p&krmince

beam, loss of displacement, loss of

ballast with a rise in the centre of grav-ity of the boat. Second, a stabilgrav-ity loss occurs because the sail area increases

thus increasing the heeling moment

(sail area x heeling arm). With the vari-ous scale models the speeds could be

plotted against sail area ratios for a

wind condition. t clearly showed at a

chosen wind speed what the optimum size for the boat should be.

Reviewing the test results with dif-ferent scale models, we decided that

the chain girth was a 'killing" factor.

When we even arbitrarily tilled in the

gamboards, the sail area could be

in-creased drastically improving

per-formance. This minimum chain girth

had to be taken lar any chosen

dis-placement. As the stability had to be

increased for the

lighter boat, the

freeboard penalty could be taken to

our advantage. Theoretically, in the

tank performance improved without

counting on surTh factors as windage, reduction of pitching radius, etc.

Speed Made Good (VMG) is one of

the iiiost important numbers when

test-ing. This VMG is the speed, including

leeway, yaw, etc. , that a boat actually

makes going to the weather mark. In

America's Cup racing 60 percent of the

race is to windward. If the opposition can be beaten to windward, one can

stay ahead on the other legs. The VMG was calculated by the computer at 10,

20 and 30 degrees of heel by balanc-ing all the torces and seekbalanc-ing the ap-timum. The VMG curve was obtained

by fairing a curve through the three

known points.

The flow around the aft part of the

hull in the busme and the keel is impar-tant. To make a detailed study, the first

three models were painted white, and small tufts about one inch long were

glued to the hull. A special underwater

video camera was set up. During the

runs we could see what was happening en a TV screen: and also recorded on a

video tape for later re-play and direct

comparisons between the various

models.

A number of selected runs were

made with the cameras in various

places. The runs included upright an-gles attive different speeds and under

heel at 10, 20 and 30 degrees atthe

ex-pected boat sped with corresponding

leeway angle for

those conditions,

covering in this way all realistic sailing possibilities.

The atterbodies of both two and

three were pulled

in to different degrees, because al the rudder stock being far forward. Model one, on the

other hand, had the rudder well alt and

clean flowing buttock

lines. In the

upright conditions two and three had

considerable turbulence around the

rudder stock, and at speeds of seven to

eight knots this turbulence would

ex-tend about four to five feet aft in full

size. The part of the rudder extended

below the skeg had little turbulence

and seemed to be efficient as a control surface.

Model one,

in contrast, had little

turbulence in the rudder-stock area at

high speeds as well as at low speeds in

upright condihons. However, at the

middle and upper speed range, turbu-lence started to occur close to the

rud-der tip. This appeared again unrud-der

heeled conditions, apparently from a

'/ortex coming from the keel.

This turbulence did not appear on the other models because of their shallower rud-der. On all models there seemed to be

a strong crossflow going under the

counter directly aft of the rudders under heeled conditions.

Another noticeable difference was at

the keel tips. A tip crossf low was

ap-parent on model three, and tufts

started to change direction as high as two feet above the knuckle line in the

torward part of the keel. On model one,

on the other hand, the crosstlow did

not start until about one toot above the

condi-e;

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8

2

lions.. Model two had crossflow some-where in between. For similar draft this would mean that 15 to 20 percent of the draft of model three was not working as

efficiently as the keel of model one.

From this if could be concluded that

the flat-bottom ti would be better than the V-bottom tip. However, other

fac-tors were also involved that do not

make this decision this simple.

The final model (model five) was

some 8,000 pounds lighter in

displace-ment and with about 70 square feet

more sail area than Southern Cross. In Figure 8the VMD curves are plotted for

both Southern Cross and Australia in both smooth water and rough water. From the graph note that a large gain has been made in rough-water

per-formance in light airs. In a simulated

T7iïi>

100%LWL.

America's Cup race of, say, 12 knots

(first race of 1974 series) Australia

would be 0.06 m/sec. faster than

Southern Cross, 50 seconds on a 42-mile windward leg. Downwind in the same breeze, Australia would be 37

seconds faster than Southern Cross on

a 4k-mile leg.

In Figure 1 Australia s

superim-posed on Southern Cross to show the basic dìfferences. The new boat is a lot

smaller than Southern Cross.

Howev-er, it should be pointed ouf that the new boat is only faster up to about 20 knots of breeze.

If you could be certain

it

would be blowing, Southern Cross

would be the boat to take to Newport.

In conclusion, we would like to say

that we do have a potentially fast West

Australian challenger. However, the

Figure 7: Construction of langent line to section area curve

used to predict apparent shortening of waterline length

Figure 8:

Speed made-good

curves showing the Australia superior to Southern Cross sai/in g to windward in true wind speeds

opto lOm/sec

(79.42 knots)

Dutch designer Johan Va/cnt/ja worked for Sparkman arid Stephens in New York before join/n g in partnership

with Australian designer Bob Miller.

The new West Australian 12-Meter

Australia is their first major

project-Ed. 145

L

_waor Smooth

'ï!

,

-Rough waler coss Australa

opposition will also have worked hard to have a faster defender and mign make the final competitors similar n boat speed.

To win the Arrierica's Cup the next

step is to match sails, organisation.

crew work, etc., with the deten dens and

to out-perform them. The opposition should never be underestimated and especially in such a prestigious series

as the America's Cup.

V

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