Adjust depth and twist with backstay tension
The backstay (masthead) and running backstay (fractional) affect depth in the middle and upper genoa sections by controlling sag. To a lesser extent, they affect twist.
When you have power-hungry conditions – light air, choppy water – you need a deep sail. Sag the headstay by easing off backstay tension. This moves the luff of the sail to leeward and aft, which adds depth to the genoa because the luff moves closer to the leech (see below). On a fractional boat, easing the running backstay will accomplish the same purpose (see below).
The added depth will be noticeable in the upper half of the sail where the sag is largerelative to the chord length. Also, sag will add depth mainly to the front of the sail,making a rounder entry and a more forgiving shape.
In light air, take care to ease the backstay enough to increase sag and fullness, especially in the lulls. Light-air backstay tension should be about 25% of maximum. You'll know it's too loose when the luff curls like a spinnaker.
To check sag visually, sight up the forestay from the tack while someone plays the backstay. You'll notice that gusts automatically add a lot of sag. This is exactly the opposite of what should happen. When a gust hits, you want to flatten the sail and de-power it.
Your backstay will need a lot of range and power simply to counteract undesirable sag, let alone lessen sag as the wind strengthens. For each of your genoas, you'll have to adjust the backstay quite a bit to change the sail's shape from the low to the high end of its wind range.
Besides adding depth, head- stay sag adds power by reducing twist. It does this by letting the luff drop slightly to leeward and aft, which rotates the leech slightly to windward (see right). This is fine for medium air and a chop, but disastrous in a breeze because it adds power where it contributes most to heeling force – at the top of the rig. In these conditions you need a tighter head- stay to open the leech and de-power the sail (see below).
» Set draft position with halyard