Jib and Genoa Trim from Dave Dellenbaugh's Speed & Smarts
Upwash is the bend that a sail induces in the approaching air flow. For example, the wind begins to curve around a mainsail well before it actually touches the sail. Sitting in this upwash region, the genoa thinks it is in a lift, and consequently can be trimmed farther off the centerline of the boat than the main. This makes the genoa more efficient by rotating its forces (perpendicular to the chord line) more forward and less sideways than the main.
If your main is the sailplan's rudder, then the genoa is its motor. Of course, their functions overlap, but in general you should trim your genoa for drive and your main for helm balance.
Overlap (%) = LP divided by J
On IOR boats, the largest headsails usually have a 150% overlap; No. 2s have a 130% overlap; No. 3s have a 98% overlap, and so on. Most PHRF boats are allowed 155% genoas without penalty.
The sails that are less than maximum size are used in heavier winds once the maximum amount of force for a given boat has been reached. Beyond this point, maximum sail area simply overburdens the boat, and it's better to reduce drag by changing down to a smaller jib. This will improve the lift-to-drag ratio (the best indicator of upwind performance) by holding lift at a minimum [sic maximum?] while lowering drag.
Note that smaller headsails are almost always shorter on the foot, but still nearly full hoist. The reason for this is that a high-aspect-ratio sail is more efficient. The sailmaker preserves the maximum wingspan of the boat's foils, but shortenis their width to the limits of construction technology.
Since your genoa determines your ultimate sailpower and the total heeling force, heel angle is a very important indicator when choosing a sail. As a rule, if your heel exceeds about 25 degrees, change down to a smaller genoa. Long and narrow boats may be able to maintain speed with a bit more heel than this, but modern fractional rigs must be sailed considerably flatter.
Helm balance is another consideration. If you have too much helm, changing to a smaller genoa might be a good idea. This relieves windward helm by reducing the angle of heel, removing sail area from the back of the genoa, and permitting the traveler to be eased further, which opens the main's leech.
Don't forget that each of your headsails is designed for a maximum wind velocity. This number (specified as true or apparent wind) should be written clearly on the genoa clew, so you'll be sure to change before exceeding that limit. Finally, if you can flog your main and maintain the same speed, it's time to depower.
A narrow sheeting angle works best for high-efficiency conditions when the hull is easily driven. Narrowing the sheeting angle rotates the sail's forces to the side, cutting down on drive and increasing heel, but letting you point higher. Though this makes the genoa more efficient, the sail is also very critical -- more prone to stall and less able to accelerate.
Sheet inboard when you have these conditions:
The trimmer's secondary responsibility is to help the helmsman steer the boat. For example, he should ease the sheet for big waves or sudden lifts, and trim for flat spots and headers. Then, as the helmsman brings the boat back up to speed and on the wind, the trimmer must slowly re-trim the sheet. All this requires constant communication to make it work properly.
Trimming the sheet affects the genoa in several ways. It reduces twist, reduces depth and narrows the sheeting angle all at the same time. These changes combine to let you point higher. Easing the sheet has the opposite effect -- more speed and less pointing ability.
As a guide for proper sheet tension, observe how far the genoa is from the upper spreader and from the chainplates. We cannot prescribe these exact distances without knowing more about your boat and the conditions you're sailing in. These are measurements you'll have to get (and add to your sail data) through trial and error.
When sail twist matches wind twist, the genoa is perfectly trimmed from top to bottom. Now the sail should luff simultaneously up and down the luff when you head up slowly past close-hauled. Set your lead position by luffing up slowly and watching your tell-tales. The windward telltales should 'break' evenly from top to bottom at the same time.
If the top telltales flutter before the bottom, the sail is twisted too much. Move the lead forward to pull down on the clew, increase leech tension and reduce twist. If the bottom telltales luff first, the sail needs more twist. Move the lead aft to relax tension on the leech, allowing the clew to rise and the sail to twist.
Moving the genoa lead position also affects foot depth, much as the outhaul controls foot depth on a mainsail. To add depth, move the lead forward. This shortens the distance from clew to tack, and moves the foot of the sail farther away from the chainplates. (The upper two-thirds of the genoa will keep about the same shape.)
Use your Sailscope to measure the depths of your sail at each of the three draft stripes (the middle one is most important). The table in Figure 12 gives approximate target depths for the various genoas. Note that the depths do not vary too much from sail to sail. The acceptable range of genoa depths is actually quite narrow. (If your boat has an unusual sailplan, hull shape or sheeting angle, these suggested depths may not work for you.)
When you have power-hungry conditions (light air, choppy water) you need a deep sail. Sag the headstay by easing off backstay tension. This adds depth to the genoa as the luff moves closer to the leech. On a fractional boat, ease the running backstay for the same purpose.
The added depth will be noticeable in the upper half of the sail where the sag is large relative 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 actually 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 snaps and 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 depower 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.
The nice thing about fractionally rigged boats is that the runners are so easy to play. Consequently, they should be adjusted continuously, in concert with the genoa trimmer and helmsman, to keep the boat sailing as fast as possible.
Twist -- Besides adding depth, headstay sag adds power by reducing twist. It does this by letting the luff drop slightly to leeward, which rotates the leech slightly to windward. 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 headstay to open the leech and depower the sail.
Use your Sailscope to locate the position of maximum draft on each of your draft stripes. For No. 1 genoas, draft position should be roughly 45-47%. Refer back to Figure 12 for some rough draft position targets for each headsail.
A draft forward shape (40%-45%) is more forgiving than a draft aft shape (47%-50%). Move the draft forward when you need a wider groove, such as in a chop or with an inexperienced helmsman. Move draft aft in ideal conditions (i.e. smooth water and medium air) for maximum pointing ability.
The "groove" -- Let's examine the importance of draft position a little more closely. What do we mean when we say a draft-forward sail is more forgiving and has a wider groove?
The groove is that optimal combination of sail trim, boatspeed and pointing ability at which your boat comes alive. We're always searching for the groove when we sail upwind (and downwind).
We can make the groove easier to find by increasing halyard tension or headstay sag to make the genoa more draft forward. A draft-forward shape is more forgiving because it's harder to stall. In other words, the helmsperson can make wider course changes and still keep flow attached on the leeward telltales.
The disadvantage to widening the groove is that it harms your flat-water pointing ability. So the groove should only be wide enough in each condition for the helmsman to control the telltales with the helm.
It's important for the trimmer to help the helmsman respond to changes in the wind. The trimmer can react faster than the helmsman, especially in light air when the boat turns slowly. If the helmsman tries to hurry by jamming the tiller hard over, the rudder will brake the boat. He has to let the sails turn the boat. If the sheet is eased first, it will help the helmsman head up slowly, and the jib can be re-trimmed in concert. This maintains the best speed.
Here are the problems whenever your apparent wind is in the 35 to 65 degree range:
First of all, it's important to understand that, compared to a new sail, an older genoa is more draft aft; further away from the rig; flatter in the forward sections; fuller overall; and tighter leeched. When you put all this together you have a slow sail, unless you take a few counter-measures: