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Technical Manual

Trimming The Main

by Trevor Gore

It has become apparent that many helms have difficulty deciding where, precisely, to sheet the main sail. In a single sailed boat like the O.K. this is particularly important as it has a major bearing on the boats performance.

Let's first consider the mast's position in the boat and its rake. The mast should be raked such that at maximum depower (i.e. when the mast is bent as much as you are ever going to bend it) the boom should clear the aft deck by about 50mm.

With this amount of rake the mast should be positioned such that when the boat is level (I mean really level!) and sailing to windward at full bore without the luff of the sail lifting, the boat will sail a straight course with the tiller on the centreline. In more direct terms, that means the centre of the heel of the mast should be positioned about 500-520mm forward of station 3, and at rest, depending on the cut of your sail, the boom should be between 300 and 400mm above the aft deck.

Now, imagine you are sailing to windward in a force 3. Most helms should be nicely sat out and holding full power. The boom end should be set directly above the gunwhale, this position being controlled by the traveller, which should be directly beneath the boom, some two-thirds of the way out. Assuming your traveller system works, and the mainsheet does not pull the traveller to the centre of the boat, pulling the mainsheet should move the boom in the vertical plane only. The angle the boom makes to the centreline of the boat should not change. However, changing the mainsheet tension will alter the boat's pointing ability. This is because as the mast straightens and bends with mainsheet tension the luff of the sail becomes fuller or flatter. A flatter sail points higher.

OK, so where do you cleat the sheet? That depends mainly on the water conditions. In flat water (i.e. inland) the sail can be flattened quite markedly without loss of speed, and with improved pointing ability. In a chop, the sail should be allowed to become fuller, and the boat sailed freer so that power is maintained to punch through the chop, and after each pitch the airflow over the sail recovors more quickly.
The trick in both cases is to increase the mainsheet tension and point higher until the boat begins to slow down, then release it a notch.

Except in really exceptional circumstances it never pays to sheet the boom closer to the boat's centreline than having the boom out above the gunwhale.

As the wind increases the traveller should be positioned progressively further outboard and the mainsheet tension increased, resulting in the boom being farther out, the sail being flatter and the pointing ability much the same. Remember, as you depower do not allow the luff to lift. Flatten the sail using mast bend and clew outhaul, and trip the leech by pulling down the cunningham. For details see "Windward Workout".

The boom can be sheeted outboard advantageously until the boom end is some 600mm (yes! two feet) over the gunwhale. This is how it is possible to sail the O.K. to windward in windstrengths of Force 7 and 8. However, most boats will have run out of traveller well before 6OOmm, so to keep the sail flat kicking strap tension musbe used. Hence the necessity for very powerful kicking straps. Just remember to release it before you tack!

If you've recently changed to the class from a crewed boat, this sort of sheeting may go a bit against the grain, but it's the only way to do it.

In light winds, things get a little trickier. First it is important to assess the amount of wind shear, i.e. Is the wind direction at the top of the sail the same as at the bottom? If it is, the boat can be coaxed into sailing quite rapidly with a flattened sail (most conveniently achieved using kicker tension) whilst pointing very high. However, usually there is significant wind shear, and in order to achieve a correctly sheeted upper part of the sail, the boom must be allowed to rise to permit the top of the sail to twist off such that it is not oversheeted and stalled out. The object is to attempt to keep all the sail's tell tales (one on each panel up the luff, set about ½ of the way back, and one on each batten) pocket flying backwards on both sides at all times.

In stronger winds, windshear tends to be compensated for automatically due to the mast bending sideways which has a similar effect to allowing the sail to twist.

In the lighter winds some compromise is usually necessary as there tend to be other influences, like the incescant pincher who you may have difficulty passing, but once through you'd leave for dead!


Trevor Gore was British national champion five times in the 1980's, and a Worlds bronze medallist. He is a highly qualified engineer, and applies his engineering theory and practice to his sailing. He was a regular contributor to the 'Newsletter'.