Sailing with friends and clients the most often ask question I get is "how do I trim my jib". That's not surprising because there are more variables in setting it correctly than there are with the mainsail, but armed with a few quick basics you can quickly take the mystery out of it. Whether racing or cruising you will enjoy sailing more with a well trimmed sail and it is important; both for performance and for the longevity of your sail.
To start you need to understand that any good sail designer has to work with the rig parameters of your boat. The spreader locations, lengths, and sheeting angle are fixed while the headstay sag is somewhat adjustable, so knowing these the designer will develop a sail shape that will work with your boat. With this in mind your starting point for medium air is setting the sail so that it sheets fairly close to your rig, just missing the upper spreader and the chainplate at the deck. Start by sheeting the sail in until either the leech comes within 3" or 4" of the spreader or the foot comes within 2" to 3" of the chainplate, whichever comes first. If the leech gets close to the spreader first then move the jib lead aft until you reach the point where both leech and foot trim correctly at the same time as the sail is sheeted in. If the foot comes up against the chainplate before the leech is into the spreader adjust the lead forward until both are correct at the same time. This location should be marked as your "base" setting.
To understand when and why you change from the base location you need to keep in mind a few simple ideas on what difference various adjustments will make to performance. In general all adjustments fall into two categories; things that will make you go faster, and things that will make you point higher. Things that make you go faster include easing the jib sheet slightly, moving the jib lead forward slightly, inducing more headstay sag, and easing the main traveler and or sheet a little. Things that make you point higher are sheeting the jib in a little tighter, moving the lead aft a bit, decreasing headstay sag, and tightening the leech on the main. As you are sailing you should always be aware of how your boat is sailing compared with boats around you; are you pointing well but not going fast enough, or are you going fast but not pointing? Once you make that determination you can then make changes that will improve you performance in the needed direction.
In lighter air or bumpier conditions you start by moving your jib lead forward one hole at a time until you reach the point so that when the sail is sheeted with the leech 4" to 6" off the spreader the foot will be 6" to 8" away from the chainplate. This makes the whole sail fuller which will increase your power. If you need to increase your pointing move the lead back a little so that the foot is sheeted tighter to the boat making it flatter while the leech remains 2" to 3" off the spreader. As the wind increases and you start to get overpowered move the lead back even further so that the foot is sheeted right in hard against the chainplate making it as flat as possible while the leech is twisted further away from the spreader to open the slot and let more air spill out of the top of the sail.
Headstay sag also affects how full your headsail is; the more the headstay sags aft the more material is pushed back into the sail making it fuller, the straighter you make you headstay the more material will be pulled forward out of the sail making it flatter. Controlling it can be hard but in general for masthead boats it comes from backstay tension, for fractional boats with inline shrouds it comes mostly from runner tension, and from boats with swept aft shrouds it comes both from shroud tension and backstay tension. For more on how these work together see my article on Controlling Headstay Sag because it can be quite involved and requires more in depth study than this article.
Here is a short list of basic settings you can use as a guide to get you started. All boats and all sails are slightly different but these guides will at least give you a good reference point and an idea of the type of changes you make for different conditions.
· Medium air 7-10 knots: Headstay sag should be around 2”-3” (for properly designed medium headsail). Jib lead is set so that when foot is within 2”-3” of chainplate the leech is 3”-4” off upper spreader. Luff should have hint of wrinkles
· Light Air: Let headstay sag back a little more, luff should have some wrinkles. Adjust lead so that foot 4”-8” off chainplate when leech is 4”-6” off upper spreader.
· 11-14 knots: increase headstay tension as wind increases to try and keep sag as low as possible. Tighten luff to just remove wrinkles. Adjust lead so that foot is up against chainplate when leech is 1”-2” of spreader.
· Heavy air: Change sail if possible. Get headstay as tight as possible. No wrinkles in luff. Move lead aft so that foot is stretched tight around chainplate and leech is 3”-4” of spreader, more as wind increases.