the physics

Yachting Monthly - - EXPERT ON BOARD -

When we are sail­ing to wind­ward, the sail acts like the wing of an air­craft. Air pass­ing over it pro­duces a low pres­sure area on the lee side and a high pres­sure area on the wind­ward side, which cre­ates lift.

This lift from the sails is coun­tered by the lat­eral re­sis­tance of the keel. With­out a keel, a sail­ing boat would sim­ply go side­ways. But, as long as the to­tal force – the cen­tre of ef­fort on the sails

– is coun­tered by the lat­eral re­sis­tance of the keel – what’s known as hy­dro­dy­namic force – and th­ese forces are in bal­ance and act­ing in line with each other, the boat will sail for­wards.

On the physics front, we also need to con­sider wind shear. The fric­tion of the wa­ter on the gra­di­ent wind causes it to back and slow down the closer it comes to the sur­face of the wa­ter and so there is a dif­fer­ence in wind speed and wind an­gle be­tween the top of the mast and the deck. The higher up, the greater the wind speed and the more the wind an­gle moves aft. The lower down, the lower the wind speed and the more the an­gle moves for­ward (backed).

So we need to be able to ad­just the twist in our sails to take ad­van­tage of this.

The head­sail acts as the for­ward driv­ing force, and the main, af­fected by the side­ways force of the wind, is more like a rud­der, turn­ing the bow to wind­ward as it pow­ers up. So we need to know what sail shape we should be mak­ing to get the most out of the wind we are given. In light airs, we want a fuller sail shape – a more pow­er­ful shape – and in heavy airs a flat­ter, less pow­er­ful shape. There are sev­eral el­e­ments that we can ad­just for the head­sail; twist, depth and draught po­si­tion. And for the main; twist, depth, draught po­si­tion and helm bal­ance.

All of th­ese can be ad­justed eas­ily with the ex­cep­tion of draught po­si­tion, which re­quires us to al­ter the hal­yard ten­sion. And as I don’t think it’s re­al­is­tic for us to be run­ning up and down tweak­ing hal­yards for ev­ery point of sail and wind strength, I’m leav­ing draught po­si­tion alone. The con­trols for ad­just­ing the set of the head­sail are:

■ Forestay ten­sion

■ Hal­yard ten­sion

■ Po­si­tion of the sheet lead

■ Car on the genoa track

■ Sheet ten­sion

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