by Mark Wallis, Doyle Sails

Maximum draft should be positioned between 35 and 45% of sail width from the leading edge, but this can vary according to conditions.

This shape variation is related to tension on the luff, leech and foot. The draft position varies with the wind
speed.

• When the wind strengthens the draft tends to move aft.
• When the wind decreases the draft tends to move forward.

Adjust tensions on the sail in relation to the wind strength to maintain the draft at the correct position.
A straight trailing edge allows for a freer air outflow and influences the power of the sail. A better wind angle
can be provided with a curved trailing edge, but this will reduce the air outflow (i.e. cause drag). You will
learn how to obtain the best performance from your sails according to sea and wind conditions.

In general, a sail is always fuller at the base than at the top and will have a fuller leech in the lower part and a straighter leech in the head. This is known as twist. A balanced helm should mean good sail trim and good boat speed. Headsails and mainsails are constantly interacting and a bad air outflow from the genoa will  have a negative effect on the main.

Light wind - 0/12 knots apparent
The headsail luff must be flatter with a fairly tight leech in flat water. In sloppy sea conditions the draft should be forward with the genoa car perhaps 1 or 2 holes forward of normal with the sheet eased very slightly to open the leech. Pointing in these conditions is less important than boatspeed. Speed will help prevent the boat being knocked sideways by waves and helps the keel to work better.

Medium wind -12/20 knots apparent
The mainsail should be flattened, Cunningham should be coming on to flatten the bottom of the main and pull the draft forward to around 33% and the leech relatively tight, easing the mainsheet bit by bit to open the twist as the wind builds. The mainsheet traveller is being gradually eased down the track to ease pressure on the helm. The genoa should have more halyard and the car should be coming back to flatten the foot and open the upper leech to spill the wind to prevent backwinding the mainsail.

Strong wind - over 20 knots apparent
A balanced helm is of prime importance here!

• A mainsail which is too large or full will increase weather helm.
• A mainsail which is too small compared to the headsail may cause the yacht to get lee helm which makes the yacht slip away to leeward and could be dangerous.

Generally the mainsail should be flat with maximum outhaul, Cunningham on, the leech twisted (mainsheet eased slightly) and the traveller dropped away to leeward according to helm pressure.
Best performance is gained with the jib halyard cranked up more with maximum draft around 33% from the luff, the foot flat and the leech opened up to spill the wind.
The genoa car can even be shifted aft from normal 1 or 2 holes in extreme conditions. Remember it is always a good idea to try heavy weather settings before you need them so the skipper and crew
are familiar with the procedure!
Correct interaction of main and jib will make sailing smoother. The power ratio fore and aft of the boat’s centre of lateral resistance determines the balance and this is directly related to the sail shape.

These tips are a general guide but will suit most yachts. If you would like to discuss your yacht in detail contact me here at Doyle Sails NZ Ltd, phone 09 307 0799, 025-226-1196 or email me at: wally@doylesails.co.nz