How to optimise sail trim

It is difficult to describe ‘correct’ sail shape, but the three most important things to consider are: sail draft (the fullness of the sail), draft position, and twist (controlled by the kicker/vang and leech tension).

Sail draft

Sail draft (chord depth) is an imaginary line from luff to leech called the chord. Chord depth can then be expressed as the ratio percentage between the maximum draft (d) and chord length (c). Draft stripes or seams in the sail can be used to estimate the depth. It is quite difficult to measure, so cruising sailors will use their eyes and experience to estimate draft.

Allowing the sail to fall more out to leeward at the top than in the lower parts is called ‘twisting the sail’. Photo: c/o Fernhurst Books

Draft position

The distance from the luff to where you find the maximum draft in the sail is called the draft position. Draft forward gives a lower lift/drag ratio, and you can’t point as high as with the draft aft. But it is a more forgiving shape, making it easier to steer and giving a wider ‘groove’. The ‘groove’ is a narrow course range determined by a combination of your sail trim, boatspeed and pointing ability. Once ‘in the groove’ your boat comes alive and travels at maximum efficiency. Draft forward is therefore more suitable in rougher conditions or for a less experienced helmsman.

Draft aft gives a better lift/drag ratio than draft forward, allowing you to point higher. The sail will, though, stall more easily if the boat is not steered correctly. Draft aft sail shape is therefore best in easier conditions (medium winds and flat seas).

The shape of the sail’s entry can be critical especially for the genoa (jib), which has no mast in front of it to affect the airflow. A round entry reduces the pointing ability but is less affected by changes in angle of incidence, making it easier to steer.

Different draft positions (shown in white), illustrating their impact on sail trim performance. Photo: c/o Fernhurst Books

A fine entry allows higher pointing but is less tolerant to changes in angle of attack which makes steering more demanding. The forestay and halyard tension also have an impact on the fullness of the sail entry.

Twist

True wind speed increases with height, so the higher above deck level we measure, the stronger it gets.

Boatspeed-generated wind remains constant with height. Adding the vectors together shows that the resultant apparent wind shifts further aft and increases in strength with height. You therefore have to trim the sail in such a way that the wind’s angle of incidence to the sail will be constant from the foot to the head of the sail.

This is done by letting the sail fall more out to leeward at the top than in the lower parts, called ‘twisting the sail’. Mainsail twist is primarily adjusted by the kicking strap (kicker or vang), the mainsheet and the position of the traveller on the track. The twist of the genoa (jib), meanwhile, is adjusted by moving the sail’s sheeting point, and is affected by sheet tension.

Vertical Sail Shape

The sail should have a little more fullness in the upper parts to be more efficient. This can be difficult to judge and is of more interest to the keen racing yachtsman. Most cruising yachtsmen and many racers don’t try to fine-tune the sails vertically beyond the curvature the sailmaker has built into the sails.

But in strong winds the upper parts of the sails ought to be flattened in order to avoid excessive heeling forces. This is a common problem for most yachtsmen and usually more twist will be employed.

Airflow is bent due to the mainsail which allows the genoa to be sheeted at a wider angle to the boat’s centreline. Photo: c/o Fernhurst Books

Genoa trim

The genoa (jib) is often considered to be the boat’s ‘engine’. The mainsail can then be seen as a kind of trim flap that gives the boat its pointing ability and controls the helm balance. This should not be taken too literally as each sail’s function overlaps with the other. Why is the genoa so important?

The genoa does not have a mast in front of it to disturb the airflow and create vortices and drag. It can be set at a wider angle to the wind than might be expected, due to the ‘upwash’.

Upwash is the change of airflow direction created ahead of a sail (or an object) before the flow has reached the sail (the object) itself. The genoa is in the main’s upwash area and benefits from a wider wind angle (gets a ‘lift’) and can be sheeted further out from the boat’s centreline.

Conversely the mainsail functions in the genoa’s ‘backwash’ and must be sheeted in harder than if it was operating alone. Trim the genoa for driving force and the main primarily to obtain a correct helm balance.

The genoas are named Genoa No.1, 2, 3, and so on, with decreasing size. Genoa 3 is effectively a standard jib. The size of the genoa is given in square feet or square metres, but it is also defined by the length of LP (luff perpendicular).

We describe genoa sizes by the degree of ‘overlap’. This is a measure of how much the sail overlaps the mast. A common value for Genoa No.1 is 150%, for Genoa No.2 130-140%, and for Genoa 3 about 100% (that is, roughly no overlap).

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