Towards a better sail

Article for Gu8rdtan upp1ement

on IMuatrial Tetj1ea

TITLE.

TOWMD8 A BETTER SAIL

HORs

M.R. Dixon

Id

FIBRFI LIpiD:

HARROGATE

"TO.ARt$ A TTIR SAIL"

The function of a yacht sail is to use t6e wind's energy to drive the yacht throu.h the water, When the y.cht is sailing to windward the sail

func.tiona as an aerofoil relying on a pressure difference between its two

aides to induce lift, the forward drivinç force of the Bujls is a component

of lift. If the sail is to function eLficiently as an aerofoil, it must be

carefully tailored to the optimum 8h:ipe. This requires a hit decree of

skill on the part of the sail maker. For the sail to niaint;iin it sh&pe, a. resistance to stretch under high wind forces is essential as even a very small stretch cr alter the curvature of the aail appreciably.

Before one can look for methgd of altering the stretch properties

of a sail cloth to give better sails with better perfornince it is essential

to hve

an

accurate knowlee of what does happen to a sail in use and how much stretch occurs and how this stretch is distributed. At the present

time it is probably true to say that no-one baa mesaured all these stretches

with any decree of accuracy. This leads us to the

uestionz-How is it possible to measure this stretch when the sail is in use? Various people, including ourselves, have tried using a ruler but

this method is not very satisfactory. You cannot measure what is happening

at different

points

in the sail at the same time. Also the wind flow over the aail is disturbed by the presence of someone holding a ruler end, of

course, some parts of the sril are virtually inaccessible. To overcome these

problems, a method. is beir dveloped which pill allow stretch and stretch

distribution to be

measured at all points on the sail simultaneously.

The method is based on photoçruiwietrys _{the technique used to make}

survey maps from ariel photorapha. Experiments are carried out on a land

based Draon

mast, An anemometer, wind direction indicater and strain gauge

load cell are set up so that wind speed and direction anti the force

exerted

through the aail

sheet can be continuously measured.

The puotographic technique employs two similar cameras mounted at opposite ends of a boom, they are set at accurately defined angles to, and

distances from,

one another. The shutters are connected up so that they are

triggered off simultaneously and at the same time mark the recorder chart so the wind speed and direction and the sail sheet load are known

for the instant

the photographs are taken.

The

pairs of photoraphe from each exposure are analysed using a etereo-comarator; this is a device which gives the

Cartesion

oo-ordinetea in dimensions of all the points of

interest on the

abject

photo-graphed, So that a complete picture can be obtained, the whole sail is marked

the oo-ordintes

of each of those Crosses

_{C.qn be deterxdned}

_t

n instant

of time when the

sail. is in use. _{A computer ha}

been _{prormied to compare}

'the dietancos

_{between pairs}

of points on the sail before _{use and in us.}

under known Wind _conditions.

ThiS gives the _{amount of stretch}

occu.rring at

various points on the _{sail and aeo the}

direction in which this stretch

occurs. _{So th.t the}

distribution of this stretch about _{any one point Can}

be aeily

_appreciated

it is plotted

_{on a polar diagr}

thio is a method

of ehowj _{visually the magnitude of}

Stretch

_{in different directions.} To

give the full picture of Stretch _{.in the sail}

a

series of these polar

d.iarans

is draws on a plan of the sail.

_{The aoonprj}

fi&uro Showa an example of this. _{In this instance}

the Stretch at only

a few of the points has been

plotted for

_{the Sake of}

simplicity. _{Phe s.i1 used}

to _{ive thece results wss}

a

_{8Ofl mainsail made from}

a

ood quality cloth _{of around 6 Oz./8..yd.}

On thia fiiu'e the scale gives _{an indication of the}

maiitude of

the stretch;

the warp and

_{weft directions}

of the cloth _{are also Shown}

80

that it is pos..

Bible to see how the direction of _{stretch in the}

clotl(at different points

of the sail,

_varies.

_I

_{is interestj}

_to

note that in _{moot instances there}

is a contraction _{at 90}

to the stretch of _{the cloth.}

A further point _of

note is that the

stretch increases with wind speed. _{There is}

an

apparent

anomaly for the 6 _{m.p.h. results;,}

the' _rep..8o

is that the 12, 17 and 27 m.p.ti. figures were _{taken duxjzi}

the first

_{hour or so}

of sailin _{and the}

6g' m.p.h. figure _{was taken after}

several hours ha _elapsed

a1gestjxg that

a Sail

tends to 'grow' _{during the course}

of a day's sai1j, _{After the sail,}

had been removed

from the mast it was measured, _{it was I ound}

that it had almost reooverj _{to its}

_{oriini4 size,}

the lar,gest

reoorjed stretch be&ng

l, but in _{enera1, recovery}

wos complete. _{¶heae are only}

early results from the work but _{it .ieema likely,}

that in the hands of an expert _{on sail cloth} ConstrUction and on _{sailxnakiz, developments}

could be in.ttjatc _{to improve}

still further the

_{psrfornjance of sails}

_{made from TIRYLJY}

polyeøteg5jj

cloth.

LE. iion

-.'----:

27mph.

(7mph

(2mph

IN

_{DRAGON MAIN SAIL}

27mph

(7mph

'F

,.

---I.

I..

--I_--._ I

F

'-

,

I. ,1 F 27mph 17mph 12mph

50f EXTENSION.

SCALE

5%CONTRACflON

27mph

27mph

_/

I

6 /2 mph

7mph

17mph

12mph

6'/2 mph

17mph

6' /2rnph

27mph

12 mph

6'!2 mph

\

17mph