Trimming the mainsail 

Using the kicker, mainsheet and traveller to best effect.

The mainsail is primarily set and controlled using the mainsheet, kicker (vang) and traveller.

Their combined affect is to adjust the sail angle to the wind and the amount of twist in the sail.

Twist

Wind speed increases and frees a little with height. Therefore the top of the sail is trimmed freer than the base of the sail.

The amount of twist is judged by looking up the leech to see if the top of the sail falls to leeward more than the central sections. A more precise method is watching the telltales streaming from the leech.

When going upwind, the mainsheet and the traveller are used to control twist. When sailing off the wind, the kicker takes over.

Mainsheet

The mainsheet not only adjusts the angle of the sail to the wind, it also affects twist when sailing close to the wind.

The sheet exerts a large downwards force on the leech of the sail when the boom is directly above it, much more than the kicker.

Adjust the mainsheet so that the top batten is roughly parallel with the boom or the top telltale streams straight out from the leech.

Too much sheet tension and the top telltale will fall to leeward as the leech closes and twist is reduced.

Once the sail set is correct for close hauled or fine reaching, the mainsheet traveller is used to quickly depower the sail in gusts, without spoiling twist.

Traveller

When going upwind, once twist is set using the mainsheet, the traveller is used to power or depower the boat.

It is normally centralised but can be hoisted to windward in light airs or to leeward in heavy airs.

Moving the traveller to windward, easing the sheet and kicker allows the boom to stay near the centreline with the sheet eased.

The eased sheet allows the boom to rise in the light airs, creating twist.

If weather helm is detected or boat heel increases, ease the traveller to leeward. This retains sail shape and twist but reduces heeling force. The luff spills wind but the leech keeps driving.

If you are still over-pressed with the traveller to leeward; reef, centre the traveler and start the process again.

Kicker or vang

The kicker controls leech tension and twist when the boom has passed to leeward of the traveller.

Easing the kicker allows the boom to rise, opening the leech and increasing twist.

Tightening the kicker will close the leech decreasing twist.

Often the kicker needs adjusting to stop the top of the sail twisting too much when sailing off the wind and on a run.

Rod kickers

Contrary to belief, rod kickers do not exert more force downwards. Their role is to create more upwards force in light airs.

They push a heavy boom upwards using gas struts or springs allowing the leech to open instead of the weight of the boom pulling down in light airs, closing the leech.

Twist rules

Going upwind
1. Light airs: traveller to windward, sheet eased to centreline or just to leeward.
2. Medium airs: traveller centralised, main sheeted normally to achieve twist.
3. Gusts: leave the sheet alone, ease the traveller.
4. Increasing wind: reef, centralise the traveller and start as (2)

   Reaching and running

5. Use the kicker to adjust twist once the boom passes outboard of the traveller.

Written by: Simon Jinks, RYA Yachtmaster™ Instructor Examiner and Journalist Illustrations: RYA Sail Trim Handbook by Rob Gibson