As with the mainsail there are three main ways that the genoa trimmer can ensure that their sail is optimised. The trimmer will add power by trimming the sail in or reduce power by easing the sail. In addition they will control the draft and the twist of the sail.
Overall Genoa Shape
The aim of setting the genoa is to create a smooth shape which is parallel to the mainsail. This will create a lovely slot which will speed the airflow all the way across the leeward side of the mainsail to its leech. If the mainsail backwinds slightly that is acceptable. In light winds we will have the sheet eased slightly (the leech will be quite far from the top speader) and as the wind builds we will trim it in until the leech is virtually touching the top spreader.
Using the Genoa Leads
The genoa leads should also be set so that the genoa has an even shape. This should result in the top telltales breaking first and then the rest of the telltales breaking in turn towards the foot of the sail. When we want to add power to the genoa (e.g. to cut through waves) we can move the leads forwards so that the foot of the sail is given more draft. When the lead is all the way forwards we will see the telltales all breaking simultaneously. When the water is smoother we create a flatter sail by moving the leads aft. The benefit of this is that we can now trim the sheets in harder to allow the boat to point higher.
We can also move the lead aft in heavy winds to add twist, spilling wind from the top of the genoa.
The trimmer adjusts the shape of the sail using the sheet and the lead position. The use of these controls allows either a deeper draft (for more power) or a flatter draft (for less drag).
The trimmer controls the amount of twist in the sail by using the sheet. As the sheet is trimmed in the angle of attack is changed but then when the it is hard in clew of the sail is pulled down, changing the amount of twist. The lead position position can also be used to change the amount of twist. A closed leech will keep the power in the sail whereas twist will spill power from the sail.
A good crew will know which mainsail will be required before it is requested. This knowledge comes about by regular practice on the boat and the creation of a graph to show which headsail is suitable for various windspeeds and sea states.