Influence of the weight distribution in a boat proposed means of control

LJ

C,)

Ç)

CL

C)

Ct:

CL

ç Ok

Q

Q

ú.

O9

P.

ci,)

LU

in the permanent Committee's opinion, the distribution of weight should be controlied in a une design class.

IYLI 196r Amendrnnnts to the rules of the internationo]. Finn Class".

I N I R O D U C T i O N

ZCtrC

This memorandum comprises four parts.

In the first, I have tried to show that weight distribution in a boat plays a

-dominant part in the losses of its propulsive power. Will the reader, please, forgive me for resorting to mathematical demonstrations; if I did not make myself

clear, he can go straight over to my conclusjcn

In the second part, I explain the measuring method we h3ve adopted after many trials.

The third one gives the results of a few experiments, among which a demonstration

made before the I Y R U Technical Committee. As a matter cf fact, nothing better

than a demonstration can show how simple this system is, notwithster.ding the

lengthy and confused justifications given below. We resorted to a chronometer that was automatically started so as to avoid any influence from thi operator, and could thus ascertain the precision of the method;moreover, it became clear that the

measurement could be made with an ordinary chronometer recording time ìn of seconds; the results obtained were thus precise to a few mm.

In the fourth part, as a conclusion to this work, _{I formulate a set of rules which} govern the weight distribution and the position of the centre of gravity.

I F A TECHNICAL COMMITTEE

Gilbert LAMBOLEY Chairman

ST CYR AU MONT D'OR

_{- 69}

FRANCE

Mim

PART OJE

EFFECTS OF THE DI5IRH3UTION OF WEIGHT - =

DISTRII3UTIDN 1W MA55

cl.

fi9ire I

In Fig. i if' G _{is the centre of gravity, the boat can be considered as a large}

%

number of small units

_{of mass}

rn ; if d is their distance

from

the transverse

axis passing through te centre of' _{gravity G , one may call:}

2'

Ihe moment cf inrtia _{around this axis, the sum of all the}

m d

terss, i.e.

m

¿Z

The radius of' gyration _{around the same axis, a lengthsuch} that =

, _{the total mass of the boat,}

f

A

f

MOVEMENT OF THE E3OAT RELATIVE TO A SMOOTH SEA

fure

Z Ihis motion can be split into three:

- Sideways drift (distance run = forward motion + drift) along the natural

load water line AA of' the boat; the champion tries to get the best forwerd motion

he can. To do so he takes the maximum f energy from the wind to give it to the boat which in turn has it stolen from it by the water supporting it, through the

displacement of' this water and the friction of the wetted surface. There is a maxirniin speed (otherwise it would go on increasing) when the water takes all the

supplied energy; then the boat's energy remains constant and equal to _j- MV2

M = boat's mass, V = speed).

- Heeling and counter heeling, of course to be avoided.

- Also to be avoided, pitching motion around the transverse axis, or changes in the boat's trim, to which we shall revert later. ihese movements take enorty as in the first paragraph and this energy which is taken from the wind with great trouble to overcome the water resistance should not be wasted but should be used fully to make the-boat move forward.

In figure 2 in order not to draw the boat twice, which would make the illustration

less clear, we have shown two extreme wet erlir'tes EE and 1F. T0 understand what the

f

f

A

A

boat is doing, please turn the paper back and forth so that the waterline remains

horizontal in both cases.

t> eveLoad ,etentrtc

f

Ct,rv. A

Note: The respective positions AA, CE, G and C do not represent what actually happen

fiire 3

J

L0t us draw whet happens. in Figure 3 we have the normal water line AP' and another

line LL forming

angle c'( with AA, G the centre of qravity, CA and CE the centres of

buoyancy (where ArchimedeB thrust is applied) or each one of the two waterlinas; W is the boat's weicht equal to the buoyancy.

Lines

CM remain tangential to a curve called the developed metecentric. The curve along which C moves is the buoyncy curve and CM remains at right angles to it.

The displaced vole of water V contained between the water plane and the wetted surface of the hull remains constant, its weight being W

For small oscillations, the points ME remain near the point M called the metacentre (thi8 metacentre is also the instantaneoug rotation centre), C being the longitudinal metacentric radius R Since M i above G , the torque given by the Archimedes

thrust and the weight tonds to bring the boat upriht, that is to bring back linc AA towards line EL. This torque, for small pitching oscillations of maximum amplitude is

The pitching energy is moreover equal. to this torque.

Ihe pitching period is:

T = 2'l

K W (R - a)

lt is also called the period of free oscillation.

Let us remember that 'G is the moment of inertia around an horizontal axis passing through the centre of gravity.

The shorter the period, the largor must be the accelerating forces to make the hoot pitch and the more stable the boat will remain and the nearer will its waterline

remain to AA.

Therefore it is of advantage to reduce 'G and to increase R - a.

In the case of the Finn where the hull shape is carefully controlled (!!!) R is fixed and one can only try to reduce l and'a that is to gather together the material of the hull nearer the centre of gravity and to lower this centre of

gravity.

For a Finn, R is of the order of 25 metres.

W have

G

Mf2 (M

being the boat's mass and

'f'the radius of gyration). Thc rinns

I was able to inspect had a radius of gyration'9 between 1,12 m and 1,30 m and distances" a "of between B and 17

In our example the term (R - a) has a negligible effect. On the other hand, the value 1G has an important role since it can alter the pitching period between

x 1.122

V 145 X g x

0,51 X 1,12 = 0,57 sec.

and T = 0,51 X 1,30 = 0,66 sec.

i.e. a differente of more than 6%.

Thus, even in calm water, the weight distribution is important, the boat being more resistant to pitching motion ehen the weight is gathered near the centre

of gravity.

\AVL MOTiON

îhis motion is still poorly understood. Iherefore we will select the simplest

theory formulated by GLR5IN[R. (Fi.t)

P t> Lret1OY)

o

fLeure 4.

fhe liquid particles P describe circles of radius OP and return to their starting point. Ihey take a time 2 T to go around the full circle, this time

being the wave period. Their speed of rotation is W = . If the wave moves

towards Ox , they rotate clockwise. Let us aosurne that the wave is stopped

at a

time

t , the

position of e3ch particle P at the same level at rest will be obtiined by moving the

circle

with centre O horizontally and making

it rotate. (Ihe concentric circle of radius Op

síu

have, in this movement, to

roll

without slip under the horizontal line passing through p' ).

This shows the speed of the particles at each paint. The particular instance

of

the

surface

is an interesting one; we see immediately that in the wave crest,

the particles always move in the direction of the waves.

On the lee side of the wave crest, the movement is upwards. On the windward side

it is downwards.

emember that the period 2 T and the wave length 2 L are connected by T2

(tgt

is the acceleration due to gravity). The speed of wave propagation 'c' is

connected to the period by e

L.-This, numerically, gives:

ftgure s::,

(a)

When the waves begin to break it is because the crest

particle

speed reaches the

value ic'.

Therefore watch out for the distance 2 L between crests, have confidrìce in GER5TNER and 'yJu will surely know already

how advantageous

it is to remain

on the wave crests on a reach and on the run!

*

*

THE [FFC1 OF THE WAVES ON THE COAT

We are going to examine the boat in its more difficult behaviour, i.e. sailing

against

the waves and the wind.

ZL

6 Period 2 T (in seconds) Total lenrith 2 L (in metres) Speeci 'c'

(in metres per sec.)

1,7

4,S°m

2,6 (Finn length) 2 6 3,1 4 . 25 6,2 6 56 9,4 a loo 12,5 9 12G 14,1

sz:rTrrnr'

L

L

Fig. 5 (a) - The boat climbs the wave, the water particles lift it and help it to move to level LL , then they brake it more and more up to the crest. The transom

is the last part tu be lifted by the particles. During all that time, the lifting motion converts the kinetic energy of the boat into potential energy and slows

it down further.

fture

' (b)

Fig. 5 (b) - Thc boat has climbed the crest; it has bean slowed down fully and the water particles will draw it to the bottom of the swell at an increasing rate; the stem is first drawn down. The potential energy decreases and the speed increases.

L

fLgure

(c)

Once it has passed line _{LL the boat streightens up, its speed reaches a maximum} at the bottom of the wove, it being helped further by the water particles.

In brief, the swell gives a pitching motion to the boat, thispitching being made up of a periodic motion up end down nd a periodic rotation around a transverse axis.

The water particles gently help the boat to retain its trim in relation to the water surface. Therefore there is little loss of energy due to motion in relation to the water. Ihe energy of the motion given to the boat is supplied by the swell and is not taken from the propulsive power.

1l1 this seems to go against what experience teaches us. In fact we have lost sight of a fundamental factor which is the variation of the wave lengths around

their mean. fhis variation can cause a violent motion in relation to the water;

from

which we get shocks that absorb energy in a devastating way since they not only disperse the bonus energy given to the boat by the waves, but also take part of the propulsive power gained from the wind.

PostLve

pLtchLn3

Tkgtcve pLtckirt

fotiows

fpire 6

B

Fig. 6 illustrates what happens to a boat in a chop and shows that the enforced

pitching due to the swell has a vory serious effect. Iheory says that the pitching motion of a

boat is

of course split up into e free oscillation of period _L in

f relation to the water, and a forced oscillation due to the swell, having period TH , i.e

T2

2CTTt H

2Ttt

sin

T H 2 2

L

_THTL

K

Ihe period TL is small in

relation to

that of the swell (0,6 sec. as against 2 sec.

for a

severe chop). Also

the amplitude -?

L of the free oscillations remains small

and one has in practice

T2

H 2'fl' t

2 2

THT

L

is much greater than it would seem thatc'( remains near 2

2t

H (9 H H since T2 2 FI T L

Ihorefore the boat quietly follows the swell. However two annoying things can happen:

- TH is a multiple of TL; there is resonance and the amplitude grows out of all

proportion; if the swell is regular, it is sufficient to change the heading slightly to damp out the resonance, but from time to time one can be surprised by variations in TH making a multiple of

L

and causing a sudden lurch.

- The variations of T11 are approximately equal to TL and the helmsman needs all his skill to avoid the impacts developing into continuous slamming. In this summary of wave motion we have seen that

2

TN

_L

4 q

therefore a variation diH corresponds to a variation dL in accordance with the

relation

dT

2ftdL

H

for a wave length of 6 metres we have seen that

T 2 sec.

dIH could be equal to the boat free period rL#o,o sec.

g

if 2 dL = dI4 c d TH = 1,85 m

2 dL is the variation in the wave crest spacing. It is normal to see a wave length

varying by 307o since the more choppy the sea, the closer are the waves.

1or a wave length of 25 metres we would have

2 dL 6,2 X 0,6 3,72 m

this corresponding to a variatkon of 15% but there the helmsman has the time to anticipate the impact and change the boat's heading thus varying the wave

length ori which the boat travels.

*

EFFECT OF WEIGHT 31srRIBufIrJN

-Weight distribution has a relationship with the radius of gyration'of the boat. We have seen already that it had an effuct ont the boat'.s free oscillation period

TL. In the case of irregular forced oscillations it has an even greater importance. If'W'js the maximum angular speed of a forced oscillation, the energy stored by the boat in this pitching motion is

I ,

2 o2

If the oscillatory motion is altered suddenly, its energy undergoes a sudden variation. Any sudden variation entails a sudden loss. Ihe loss of energy is absorbed by an impact with the waves; this impact also absorbs part of the boat's propulsive power, slowing it down rapidly. In order to reduce the impact as much as possible

, 2 2

hu) f.)

should remain as low os püssible.

The angular speed'w'is derived from the swell: M is fixed by the rules; on the

other hand one can alter'f in some degree. This radius of gyration should be ao low as possible and therefore all possible weight of' materials should be gathered near the centre of gravity.

Between two extreme cases in the Finn class I was able to obsexve

F 1,12 n

and (1,30 n

the energy stored at a constant angular by

1,122

34%

1,122

This is a very great deal I!

EFFECT DF T1L CENTRE OF GRAVITY POSITION H

z

I

\

ftgre 7

On the crest of a wave the boat moyos around a point 'H' thon the centre of rotation moves away towards infinity whon the boat passes point P, l.a. the rotation passes

through an extreme

end is on the

paint of changing

direction. hen the boat travcls

over distance PBQ, the

centre of its forced rotational

movement returne from infinity

up to point 1B and then goes back again towards infinity (Fig.?). The energy supplied by the swell to the boat is in fact

Point 'B because of the

shape of the swell,

is much farther from the boat than

'H 6)

When two waves move further

apart or nearer

each other, the height of point J varies

relatively little. On the other hand

can undergo

large variations between crests

and it is in fact from point Q (start of a crest) onwards that interesting things

happen.

The distance IHG therefore should, it seems, be reduced as much as possible especially as the wave crests become steeper. At breaking point, if the boat were reduad

to a single point,' I'would actually be on the crest. In fact it is lower since the boat is supported over some of its length. Fig. 9.

fLure

CONCLUS ION

The boat should move as lightly.as

possible in a swell. T

do this one

_{tríes to} gather as much as possible of the

weight of

_{material near the centre of qravity and} to lower that c-tre of gravity.

When one is carrying boats on shore, and we have all done this at some time, we sometimes feel that

for

tha same given weight some boats are lighter and more easily handled than others. It is simply because the seemingly heavy ones h3ve too much weight in the ends and that it is easier to handle concentrated weight than distributed weight.

Th waves supporting your craft have the same feeling and will consequently bear the boat lightly or heavily. This heaviness is characterised by _{the expression:}

1

2 p2

t

f

t

s--where the radius of gyration repressnting

the weight distribution is expressed

es o square whereas the total wcight M only cornes in linearly.

Morver M

appears in the propulsive power expression

In

order

that this energy. be

retained in spite

of' the waves, it is best to koep

ari appreciable value for M 0n the Finns this value is quite high and e1los

thorn to go through quite e steep chop. These two main reasons will

encourage us to

PART TWO

=

CONTROL 0F THE WEIGHT DISTRIBUTION

== a z = = = rrrfl

THE NEED FOR SOtE CONTROL

I believe that I have demonstrated that the

_{desire of}

_{helmsmen to lighten the ends} of their boats is not a passing fashion.

The weight distribution of e boat seems to be even more important thon the actual weight. To control this distribution there is no available instrument and for a

long time people have tried to find a mean of control in as simple e manner as one

cari control' the weight i.e. with a scale.

CONTROL AT PRESENT CARRIED OUT IN THE FINN _CLASS

Finn rule n°] says that the wuight distribution should be as close as possible to that of a wooden boat with a hull of constant thickness throughout.

The shifting of the centre of _{gravity by noving material is further}

restricted by

a certain number of other rules such _as:

17) - Laminated wood and plastic construction together is not allowed.

- Tho hull material must not contain trapped air cells. Hollow _reinforcing pieces must b _{left open at their ends. Ihe hull thickness should always be} greater than J ma. Hulls may have to be drilled in order that checks can be

made.

- Wooden hulls should be at least 9 sm thick. Mouldei _wood

_should

_{be made up cf} constant density layers.

4 _{47 49) - Extremely accurate definition of the floor boards. The}

_material

density is leid down.

65) - Height of mast centre of gravity. 19) - Restriction of openings

in

transom.

98) -. Forbidding the carrying of' items which might seiva as ballast. The inclusion of light or heavy materials in the conatrttion is not allowed.

We have always known that these rules left some latitude to the builder. Furthernore they are often imprecise and advantage is

sometimes taken of this lack of precicion

to hide some trickery (eles it has been known to happen!!)

The owners'associations have been trying for e long time to find easy ways to measure the distribution of weight in many classes. In fact several methods have been

known for a long time, but to put them into practice would involve calculations that would frighten the measurers and be quite unuseoble in regatta conditions.

Now the computer has allowed us to

develop the following procedure which has been found particularly simple.

MEASUREMENT SYSTEM

We adopted the

_{Pendulum " method}

_{discovered by HUYGHENS.}

Let us consider an object 5 _{oscillating around}

an axis O

- with a radius of gyration _{(I have shown in the} first section that this radius of gyration was a characteristic of the weight distribution)

- and G as centre of gravity

- the distance 0G being " a " (Fig. 1)

We are dealing with a

_composite

_{pendulum, the oscillotion period of which around}

the axis O is

2 + 1'

fure

Z

o.

CALCULATION of a1, a2 andf

Th main item is the drawing of a graph allowing the graphical solution of the

above

equations. The accurate

drawing of this graph has recently been made

possible

by the use of electronic drawing machines controlled by a computer. The position of each point on the graph paper is drawn within 0,1 mm in relation to

the ruled

lines of the paper.

For my first trials, _{i had established a graph for curves}

_{10 (a,)}

_end

_{f (a.,)}

_for values of and

2 increasing in steps

cf

0,05 sec.: the

measurmnt of 11 and

_'2

immediately gave me the values of a1 and

The experiments described in part three

of

this memorandum were carried out

in this way.

flut he.e, I must thank Mr Mc KINNON for his very interesting contribution. He suggested a graph which could be used either for a precise measurement or for a simple control.

(We

should note

in passing that these oscillations have nothing to do with those of a boat on the water).

If the position of

the centre of gravity

is known,

a " is known and from

the

measurement of T one immediately derives

r In foct " a ' is difficult to assess and actually one has two unknowns: end e If the object under

examination is

being made to oscillate in succession crourd

o two axes 01 ₂

separated by a known distance

b,' one can measure twQ periods T and around these two axes and crie has two equations to calculate r and "a'(Fig. 2). I 2 n2 T

2TrV-_-

a1 g

with a1 - a2

b 2 2

a2 +

a2 g

i

In a system with axis (T2, T1) we draw the curves representing the relation between T w2 first for given velues of a, then for given values of ¡ . Thus

we obtain two series of curves having parameters " " arid

"

f ".

The point of coordinates

2' T) is situated bath on a curve " a1 constant " and on a curve

" e constant "; which gives without any possible ambiguity the unknown valuas of " a1 and "

f

The type of graph which is obtained can be seen on the cover of this booklet. More curves than needed were intetionnally drawn so es to find the area which is to be enlarged for practical applications. It may seen complicated because of the large number of curves but in fact it does all the dull and tiresome work for you in advance and allows you immediately to find the result. Experience has shown that everyone is capable of workin this system, as you will he able to see.

Now i have completed a program which enables nie to draw in less than one hour any sort of graph which could be needed for centreboard boats as well as for keel boats and according to the way they can be hung up. All the diagrams in this booklet were drawn with the help of this program.

POSITION OF THE CENTRL UF GFAVIfY

Moreover the method provides the position of the centre of gravity " G ".

As " L ' is in line with the U and axis, its fore and aft position may he

located by the measurement of " 1 " (Fig. 3). Ihen we wiLL measure the distance d " between O and the underneath of the hull (it so happens that 01 is always situated above the centreplate box and with a rule down the inside of this box one can easily measure " d ") (Fiq.3) The vertical position of the centre of

gravity " G tt will thus be known from dimension " h " such that

h d - a1

fi9ure 3

r-

-

18

5LTT1'UG UP I;Kt.5 U, ANI) U IN IRACFICI

{hís is what took m th

1orit

Fin1iy I found that the simplest way was to support the boat by the rubbingstrakes. Iherefore I made two

brackets as illustrated in Fiq. 4 on which the Loot could he hung.

80

lo'

LBAR 02 T- B 4 R fiu.re Lt

rhese items are cut out of a single 6 mm thick metal sheet and can Le made by any sheet ñetal worker. They weigh a little more than 1 Kg each and combine with the boat's mass in its pitching motion but being very close to the centre cf gravity

(ROSS

(1ìDN kA

ftiire 6

In Figure 5 you can see how to seit up the apparatus.

There are two trestles one metre high (or raised on blocks so that the top crossbar ìs one metre above ground level).

_brcRet

-Oz A (T 2a r) We ht

V77,

figu.re .

Two pivots are mounted on the top crossbars, by

SectiorL SteeL

means of bolts, clamps or simply twisted wire.

T 33

_{These pivots are made m'to a knife edge by}

using a I bar (Fig.6).

Two counterweights are attached on either side of the trestles (unless the letter ere fixed to the fluor, which allows us to retain them in position at all times with the pivots well

lined up).

All you have to do is to bring the boat between the trestles and hang it on the pivots, either at 0 or at and to shift it slightly until it is approximately

horizontal when et rest.

In a sheltered position the oscillations are danped in approximately 100 periods, making a perfect pendulum.

i have tried to offset the pivots by combining the rotation around the axis °1

or ₀₂ with a transverse rocking motion. We took measurements at CA5CAIS in the open and in a strong wind (this is actually not recommended). I have always found that the lengthwise pitching period remained constant within a few huridreths of

MEAStJREMENT 0F PERIOD T

Ihis is the operation requiring the most care.

One can use a hand operated stopwatch. This is satisfactory for the measurer who has to chcck only one boat at a time and cr take his time.

The starting of the stopwatch should take place when the boat passes the rest position since it travels fastest there and the error on the stopwatch starting

position is the smallest.

Mr. STUART in 5OUTH AFRICA Obtained very good results by setting two rods opposite each other when at rest, one on the boat and the other mounted to a fixed past

(see Fig.7 ). Two timekeepers on either side measure the time of the pitching period. If there is no difference in their time keeping of ten pitching periods, one is assured of a good accuracy. îhe accuracy should be as high a one liundreth

of a second.

lcr important meetings I would nevertheless recommend using an eutomatically operated stopwatch since I would pity the unfortunate time keepers having to measure the

110 boats of' o Gold Cup by counting 170 X 20 periods! Therefore we have designed

the arrangement shown in Figure 8.

ÇIeIe

"od.

roCr ep

citact micro SwiFC1 IIlOO5CC. 51opwJtC1 soenoci cpCrated

V's

hiire

7

fturc

8

"

'

_'AS

20

The roller axis should be placed in line with the transas when at rest and so that the micro switch is just closing. During the pitching motion the roller rolls smoothly either on the transom or on the hull.

For the measurements involved in a Gold Cup regatta we might have a bank of three supports or more, the operator moving his feeler from boat to boat (F1g.10)

5ince each measurement proper takes less than one minute, there would be needed approximately 170 X 2 = 3e0 minutes i.e. approximately six hours to check all

the boats

in a G0ld Cup. What other measurement is at present carried out as quickly? A boat with too small a would be corrected immediately by

fitting

lead in the bow and in

the stern.

figure 9

f,

ACCURACY OF RE5ULT5

We believe that we have designed an accurate and practical way cf detecting the weight distribution in a boat.

Ihe general graph which can be seen on the cover of this booklet clearly shows that the value of " a1 " plays a prominent part in the acurecy of results. Ihe smaller the value of " ti the more accurate the results. This means that the boat must be hung up as close as possible to the centre of gravity. Ihere we have to agree to a compromise between the simplicity of the bearing apparatus and the value of " ci1

The apparatus I propose, unables us to bring the values of a1 " down to about 0,45 m.

For such a value, Figure 10 illustrates the consequences cf ± 0,01 s and ± 0,02 s errors in the measurement cf and T2. An accuracy of 1 0,01 s upon T and T2 leads to an accuracy of ± 1 cm upon () and 0,5 cm upon'a11.

The comparison between automatic measurement and hand measurement enabled me to find that a human time keeper could time with an accuracy of ± 0,05 s. (I was not

expecting such an accuracy). Over ten periods he will, therefore, _{make a total} error of i 0,1 s, i.e. 0,01 s for one period; and " a " can then be gauged with the precision indicated above.

ÂT,

maurQ-d ia'u, o T

1-Scak t

4.1 Cfl

=OOk

:1

-

:±

-H

I

-

L

-

--- I

I I

41

-0.0 s_____

t'

t2 >

I

E

0.o

_1 <ç,

ur1O

Errors ina1 and

p rn

rQ.\aon

¶o

O 0k

and

O.srrrsJ:n

and

..Exarnpk 31v2.n for aQ.L.6rn

ADVANTAGES BROUGHT BY THE METHOD

The suggested method allows, in the case of the Finn, the deletion or the single

control, of rules 3,YT,19,20,45,49,65,79,98. \iJhat is even better, it allows us to

find the fore and aft position of the centre of gravity and its vertical position.

These two measurements

form

at preaent the subject of rulos 66 and 87 (Fig.11).

Rule 87 is most vague sincè the contot point of the rubbirrqstrake with the ground can vary by 35 mm, the height of this rubbir-strake can vary by ± 10 mm in relation to the normal hull template. The contact point can also vary lengthwise. A boat of which the centre of gravity is 5 cm lower than the other could thus come artificially within the rules.

W0 should remember that each one of the measurements under rules 66 or 87 is at least as lengthy as the proposed single oethod of controlling all these rules and this single operation gives the means of checking what now forms the subject of

twelve rules.

But the main advantage is prheps the means of freeing the construction of the Finn and probably that of many other classes. The means of being able to check everything would in fact prevent a lot of troubles. It is, in particular, cf interest to note that the present rules do not allow a rational construction of glass fibre boats, thus explaining to some extent their high price which also allowed the wodcn boat builders to raise their price.

RiJIt 86

RIJL[ 81

2IK9s

5t 3

1970 CDLI) CUP AI CASCAIS

PART THREE

EXPERIMENTS MAUL UP TU JUANA1Y 1571

2

-

= = r

These niasurements were achieved by Dr. John CLARKE (LANADA) _{in very bad conditions,} being operated outside with much wind.

KC 70 69 RAUDASCHL _{h 0,259 J. 2,11 m}

_Pr

_{1,29 in} K 334 69 ELVSTROM h 0,324 1 = 2,13 in P = 1,125 in B 93 70 VLEIGER _{h = 0,232 1 2,12 rn (r 1,29} in 1 588 67 RAUDASEHL _{h 0,27 1 2,18 m} P 1,155 m 7 10 LLVSTROM _h 0,24 1 2,05 o

Cr

1,25 in SA 399 69 ELVSTHOM _{h 0,260 1 = 2,09 o}

fr

1,175 in 5 368 63 NEWPORT _{h r 0,233 1 = 2,13 o}

_Pr

1,115 in H 404 _{69 RAUDA5CHL h r 0,302} J.r 2,17 o ('r 1,15 n

The KG TB is a Raudaschì Finn whose transom was repaired and made 5 Kg heavier. This is an interesting case.

The centre of gravity longitudinal position of the other two " Raudaschi " is

given by

j2,1Bm and l2,17m

That of the KG 78 is given by

i 2,11 m

The normal weight of a boat minus spars and centreboard is approximately M 120 Kg.

It can easily be ascertained that the measure 2,11 m found for the KG 70 is

explained by the weight added when repairin.j the boat.lndoed there is little

difference between

120 Kg X 2,10 n 263,60 Kg/rn and 125 Kg X 2,11 ni = 263,75 Kg/rn

The radius of gyration of the other two " flaudaschl " is P = 1,15 m; that

of

the KG 78 has become _(D 1,29 n.

W find the following moments of inertia round the centre of gravity G

- Finn KG 7Ei after repairs :

2

X M (1,29 n2) X 125 Kg 206,01

- Normal "Raudaschi to whose transom is added 5 Kg

'G (1,15

)2

X 120 Ky + (1,29 m - 1,15 rn)2 X 120 Kg +(2,11 )2 5 Kg = 204,48

The very slight differences (1,75 %) between the two valuen calculated for 'G proves

two things:

- first, that the measure is correct,

- secondly, that an extra weight

of 5

Kg ₍ 4,1%) can have an important effect on the radius of gyration ( 12).

If the Finn KG 78 had received it additional weight in G, its radius of gyration would have become _f) so that

F 2 X 125 Kg = X 120

f) = 1,125 m

These two different ways of distributing the extra 5K0 alter the radius of gyration

by 16,5 cm

One should not wonder why the owner of the KE 78 complained (befare the radius of gyration had been measured) that his boat had bucome a dog.

EXPERIMENTS MADE IN SOUTH AFRICA

M1. STUART measured the following boats. An excellent idea of his was to set two snail rods so that they faced each other when at rest: one was fixed on the boat, the other was fastened to a fixed post.

NEWPORT 1965 LLV5TROM 19G5 -H.V.M. (Holland) 1964 -h 0,195 h 0,211 h O,1Th 1 = 2,12 n 1 2,08 n 1 2,10 n

a boat built according to normal lines 'h' should not be inferior to 207 mm. The centres of gravity of the NEWPORT and Ii.V.M. measured by STUART are too low. If they got regular measurement certificates, this means that the current measuring system was unable to detect this irregularity.

Another Finn obtained a regular messuromant certificate and was examined several times for several Gold Cupe. However, her deck was so thin that, one day, it broke apart while she was planing.

PRACTICAL DEMONSTRATION MADE BEFORE THE I Y R ti DINGHY TECHNICAL C01I rilE ( ROYAL

THAMES YACHT CLUB - 15.1.1971)

On 11 NOVEMBER 1970, at the annual meeting of the I Y R U, Richard CREAGH OSBORNE

and Gilbert LAMBOLEY explained ho the method of pendulum had successfully been used to measure the radius of gyration of a number of dinghies of the Finn type; these radiuses of gyration are charecteritic of the distribution of weight.

On 15 JUANARY 1971, the dinghy lechnical Committee agree to have a practical demonstration made before them.

our President Vernon STRATTON obligingly brings his " Mickey Finn VI

'

end two

trestles to the garage of the R.T.Y.C.

Gilbert LAMBOLEY, the new chairman of the Technical Committee for the Finn class had flown from LYON with two brackets and a chronometer that could be sterted either by hand or electrically. These apparatus went unnoticed through the airports and

customs.

The object of our derronstration was to prove:

10) - That, thanks to the system of supports chosen, the periods of oscillation are const3nt and precise.

2') That a period of o°cillation can be measured with an ordinary chronometer

with a precision of 0,01 s. provided 10 periods are counted. 20) _{- That this method is a} simple onc.

*

Three quarters of an hour after they had met President ALLON5O ALLENDE and his Committee, Vernon 5TRAITLJN, boy POTIER GODINHO ( a Portuguese member of the I.F.A.

TECI-IN1CAL COMMITTEE) and Gilbert LAMBOLEY had set up the trestles and brackets,

lifted " Mickey Finn VI " from her traiter and placed her on the oscillating

brackets. Ihey also placed the automatic chronometer ready for measuring the periods. The Dinghy bochnical Committee could then form an opinion on the method we had

proposed.

We made three measurements:

- Hull with floor and centreboard:

we measured : J. = 2130 mm d1 0,765 m T1 = 3,55 s T2 4,35 s we read on

the graph:

P=

1,145 m a1 = 0,498 m

so that h = d1 - o = 0,267 m

- 7 pounds (3,2 Kgs more) added in the centre and bottom of the boat

we measured : 1 2130 mm d1 = 0,765 mm T1 = 3,49 s T2 = 4,23 s

we read on

the graph: (0= 1,133 ni a1 0,511 m

so that h d1 - a1 0,254 m

C) - Lead at both ends (3,5 pounds 2 m in front and behind G)

:e measured i 2130 mm d1 0,765 m 11 = 3,65 s 12 4,52 s

We read on

the graph 1,174 m a1 0,487 n

so that h d1 - a 0,278 m

The following calculations made to check these measures were handed over to the

Committee.

Weight of the boat : M 121,2 Kg Casa A Moment of inertia round G

I M

2

121,2 X 1,1452 159

Case B : The moment of inertia remains the some

'G 159

Weight of the boat and lead

M 121,2 + 3,2 124,4 Kg Calculation of' the radius of gyration

l24,

í°=

113O m

Inferred position of' G the lead was pieced 0,22 below G which should

lower G by:

2 159

0 22 X

-0,006 n

1 24 4

Case C Moment of inertia round G

'G 159 + 3,2 Kg

X (2,00 rn)2 = 171,8 Calculation of the radius of' gyration

(02

f4_-> j°=

rn

Inferred position of G : the lead was placed 0,35 above G hich should raise G by

- 0,009 m

CONCLUS ION

We may consider that measurements and calculations gave the seme results as regards the value of and the inferred position of G . This proves tho precision of

the method.

*

*

The Committee thanked the 1inn class for the work they had done because it concerns every class of boat.

Ihey asked us to go on with our measurernent8 before limits should be fixed for the

value of and the position of G

It was agreed that the best thing wes

to

measure ali the boats entering the next European Championship and Gold Cup, but the finsi decision as Left to the class, because the I Y R U does not wish to interfere with our internal policy.

*

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t-PART FOUR

=

PROPOSED SET 0F RULES

After the experiments we have already made and those we shall conduct et the European races and Gold Cup, inferior end eventuelly

superior

limits will be fixed to the position of the centre of gravity and the value of the radiu3 of gyration (this being characteristic of the

distribution of weight). Iherefore,

it will no longer be necessary to calculate accurately.

Here

fully

appears the interest

of

r Mc KINNON'5 suggestion. It enables to

propose the following rule, in place of rules 86 end Bi.

LLNTrIL i

G''I Y ANO HULL

JIGHT iiI5HII1UTIOrJ

IO ET - Th hull shell

e ccntrfld in

khr' 'cIìvin conditions:

with surplus buojanc equipment and permanently attached fittings and screwed down or bolted on hatches included, and with rudder, tiller, centreboard, running rigginq and other ropes, removable fittings, loose hatches, paddle, bailer, mast boom and sail, and all other loose gear removed (except for floorboard, toe straos and lowest main sheet block).

[he hull shall be successively supported b two transverse axis O and C2 so that

the deck line is level. shall be the upper axis; the vertical distance between and U2 shall he 200 mm.

* 1t Jecember 1971

3U boats only could be measured at the 71 CUÍUPEAN CHAMPIÜN5HIP None was measured at the 71 GOLD CLIP

-Th distance from

section L

to

the

Centre

of Gravity shall be less

than

. . *

and mure than ..

Th distance from the underneath of the hull, keel bands excluded, to the centre of gravity shall not be

less

than . .

-1 _{being the oscillating period around axis (J2, the point of coordinates (T2,}

1i

shall be situated inside the graph on next page. If not, lead correctors shall be fastened to the underside of the decks to bring the former coordinates within the

20

80

180

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