Sail draft

Since the jib or genoa provides a relatively large share of the driving force for your boat. it's key to optimize this sail's shape. Unlike the mainsail, the jib is not attached to a rigid spar. so you can't fine-tune its shape nearly as much. That's whv the headsail should get priority when adjusting the backstay (which affects both main and jib). However, you must be sure to set up the jib so it works in concert with the main (e.g. it doesn't close the slot and cause backwind).

Jibs and genoas are typically a little fuller than mains and have their draft farther forward. I like to start by setting the lead position so the foot and the leech both reach their perfect trim at the same time. I want the upper leech (batten) roughly parallel to the boat's centerline, slight wrinkles along the luff, tell?tales that break evenly up and down the luff, and a luff sag (fullness) to match wind velocity (see chart).
Of course, waves are a big factor in jib trim. When waves are bigger than the wind, you need a fuller, more draft-forward shape to punch through the chop. And you need less sheet tension and more twist. A common misconception is that tighter jib trim makes pointing easier. But in order to point high you first need speed, which often comes from easing the sheet.


Light-air headsail trim

In light air upwind, you want a powerful headsail with a full shape, especially if you are punching into waves. Here the back?stay is very eased to allow the headstay to sag and push fullness into the sail.The jib luff is also eased enough so you can see 'speed wrinkles' along the luff. This adds fullness and keeps the draft from moving too far forward (which could happen with this much luff sag).

The jib sheet is not trimmed too tightly - you can see that the upper leech falls off (twists) slightly to leeward and the mid-leech is not too far in on the spreader. At the same time the jib lead is relatively forward to make the sail more powerful, keep twist under control when easing the sail, and make the tell?tales break more or less evenly up and down the luff.

Medium-air headsail trim

When the wind builds, the major changes you should make to the jib or genoa are more backstay tension, more halyard (luff) tension and a tighter sheet-You can see in this photo that there is less luff sag and the sail is flatter overall.Also, the halyard has been tensioned to remove the speed wrinkles, flatten the sail and pull the draft forward (to offset the effect of reducing headstay sag).

The exit of the sail is quite straight, and the sheet has been trimmed harder so that a) the top batten is roughly paral?lel to the centerline of the boat; and b) the mid-leech trims closer to the rig (see its position on the spreader). In addition, the lead is slightly farther aft so the sail can be trimmed tight for point mode without making the leech too tight.

In heavier air this sail would need more backstay, halyard, sheet tension and twist (if you are starting to depower).