On a sailboat, your sails are your engine. Most small boats don't have motors, so you're dependent on the mainsail, jib and spinnaker to get you where you want to go -- and home again! That's why it's good to know how to set your sails so they make your boat move quickly. Efficient sail shape will also make your boat easier to handle (and therefore more fun) in all conditions.
Like the wings of a plane, sails are three-dimensional airfoils. Their shape is carefully designed to impart lift and move a boat effectively on all points of sail. Unlike an airplane wing, however, the shape of a sail can be adjusted while sailing in order to get the most out of existing wind conditions.
Visualizing Sail Shape
You don't have to be an aeronautical engineer to make your sails perform well, but there are a few things that are helpful to know. First, we usually describe the shape of a sail in two dimensions by viewing a cross-section from a bird's eye perspective. To picture this, pretend you are sitting on top of your mast, and we cut your sail in half horizontally.
A boat's performance seems to be affected primarily by two components of a sail's shape: the amount of draft in the sail, and the position of that draft.
Amount of draft -- Draft is the amount of fullness, or curvature, in a sail. If a sail was flat as a board, then its cross-section would be a straight line and it would have no draft. You can measure the amount of draft in a sail in the following way:
1) Draw a line (CD) from luff to leech of the sail.
2) Find the point (E) where the sail is farthest away from this line. This is the position of maximum draft. From this point draw a perpendicular (EF) to your original line.
3) If you divide EF by CD, you get the amount of draft in the sail (expressed as a percentage of the sail's width). Most sails typically have a depth of about 15%. A sail with more draft than this would be called a "full" or "deep" sail; less draft produces a "flat" sail.
Position of draft -- Besides the amount of draft, we want to know where in the sail that draft is located. In other words, how far aft is the position of maximum draft (E) located? To find this, we divide CF by CD. A typical mainsail will have its draft located about 50% back in the sail. The draft in a normal jib might be about 40% back.
Since it's not a great idea to cut your sail in half to see its shape, the above discussion is more helpful in theory than in practice. However, one thing racing sailors do to help them visualize sail shape while sailing is to put "draft stripes" on their sail. Draft stripes are black lines that run across the mainsail or jib from luff to leech. These show the sail's shape as if you had cut the sail in half at that point. Since it's usually tough to get to the top of the mast, most sailors usually look at a sail from under the boom.
How Your Sails Should Look
Now that you know how to describe the shape of a sail, you're ready to start optimizing your sail shape for various conditions. We've already given some rough numbers for your average mainsail and jib shapes, but these vary with the amount of wind and the type of boat.
Catamarans and iceboats, for example, are always going fast with their apparent wind very far ahead, so they have fully battened mains that are very flat. In fact, their sails are quite like an airplane wing. (Plane wings don't have to change shape because they are always going about the same speed with the wind hitting them at the same angle.) In fact, some fast boats, like the Stars & Stripes catamaran, use "wing" sails, which are "hard."
Slower sailboats (including almost everything from Lasers to 12-Meters) are more sensitive to the shape of their air foils. So to get top performance, you have to adjust sail shape according to conditions. Here are some general guides.
Light Air -- When the wind is light, a boat needs lots of power from its sails. It needs to catch every bit of wind possible to push its hull through the water. The sails, therefore, should be quite full.
Heavy Air -- Most boats get "overpowered" when the wind reaches about 15 knots. At this point, they have all the wind power they can use. More power only heels the boat over more, which makes it slower and harder to control. In this situation, you have to start "depowering." The sails should be flat so the wind spills out of them easily. The windier it is, the flatter your sails need to be.
Waves -- When the water is rough, your boat is constantly being slowed as the bow plows into waves. What you need is a sail shape that's good for acceleration, so the boat will get going again after it hits each wave. The best shape for acceleration has the draft fairly far forward.
Upwind -- When a boat is sailing into the wind, you want sails that are relatively flat. Flatter sails reduce drag when sailing upwind and also allow you to point a little closer to the wind.
Downwind -- As soon as you stop sailing upwind and turn downwind, the ideal sail shape is much fuller. This is because the wind is now starting to push the boat, and having more draft in your sails means you catch more wind.
Controlling Sail Shape
Now that you have a rough idea of what sail shape is good for different conditions, how can you make your sails look that way? Like an airline pilot, the skipper and crew of a sailboat have a lot of controls at their fingertips:
Backstay -- A number of larger one-designs (and almost all bigger boats) have adjustable backstays. The harder you pull on the backstay, the more you bend the mast, and the flatter your mainsail gets. Pulling on the backstay also tightens your forestay, which flattens the jib. When you ease the backstay off again, you instantly add fullness and power to the sails.
Mainsheet -- On boats without backstays, pulling on the mainsheet tightens the leech of the sail and actually controls mast bend. This makes the mainsail flatter. The mainsheet is a little like the gearshift on your car. When you ease it out, the sail gets full and draft forward, and the boat accelerates. When you trim it in, you shift the mainsail into a flatter, pointing mode as the boat gets up to speed.
Outhaul -- The outhaul controls fullness in the lower part of the mainsail. It should be tight for heavy air and eased off in lighter air and when going downwind.
Cunningham -- This is actually a downhaul on the luff of the mainsail that was named after its inventor, America's Cup skipper Briggs Cunningham. When you pull on the cunningham, you flatten the mainsail and pull its draft forward. Like the outhaul, this control should be eased off all the way to make the sail full when going downwind.
Jib halyard -- This control acts just like a cunningham on the jib, except it pulls up instead of down.
Traveler -- The traveler allows you to control the athwartships position of the boom. In light air, pull the traveler above centerline so the boom can be centered without pulling too hard on the mainsheet. In heavy air, drop the traveler to spill wind and depower the sail.
Boom vang -- The boom vang is used mostly in medium or heavy air when you are reaching or running. Without the vang on, the boom will tend to lift up in the air, which hurts sail shape and can be a little scary. Keep the vang on tight enough so the boom stays pretty level when you ease the mainsheet.
Sail Trim Rules of Thumb
One objective when you're out sailing is to trim your sails efficiently so you get the best performance from your boat. To do this, it's helpful to have some quick, dependable sailtrim references. These will give you maximum performance with a minimum of effort.
Mainsail - Upwind
1) End of boom near the centerline -- Trim the main in all the way so the end of the boom will be somewhere between the boat's centerline and the leeward corner of the transom.
2) Top batten parallel to boom -- When the main is trimmed, the top batten should be roughly parallel to the boom. You can gauge this by sighting up from under the boom. If the batten hooks to windward, ease the mainsheet. If it falls off to leeward, trim the sheet.
Mainsail - Downwind
1) Ease the sail until it luffs along the mast -- You'll get optimum downwind performance if you ease your main as far as possible. Ease the sheet until you see a bubble along the luff of the main, then trim it in slightly.
2) Vang tight enough so boom is horizontal -- If it's windy, pull on your boom vang so it keeps the boom roughly horizontal and the upper batten parallel to the boom.
3) Telltale on top batten just flowing -- Another good guide for downwind trim is to use the telltale (if you have one) on the end of the top batten. You want this telltale to fly as much as possible; if it's curling around behind the sail, try easing your vang or mainsheet a little.
Jib - Upwind
1) Middle batten parallel to centerline -- Just like the main, the jib should be trimmed in all the way for sailing upwind. If you have battens along the leech of your jib, you want these roughly parallel to the boat's centerline.
2) Slot between main and jib has an even, consistent curve -- When you look at the "slot" from the stern of your boat, you should see similar curves in both the main and jib. If you don't, try moving the jib lead athwartships.
3) Telltales flutter simultaneously from top to bottom -- The fore and aft position of the jib lead is important for top performance. Ideally, the telltales along the luff of the jib should all move simultaneously. If the top telltales flutter first, move your jib lead forward, and vice versa.
Jib - Downwind
1) Don't fly jib and spinnaker simultaneously -- On most boats, you should drop the jib so it won't take wind away from the spinnaker. This is more important in light air than heavy air.
2) Ease sail until telltales start to break -- If you don't set a spinnaker, ease the jib as far as possible until the windward telltales start to flutter or the front of the jib begins luffing.
3) Move jib leads farther outboard (or "wing" the sail) -- The jib leads should be as far outboard as possible for downwind sailing. If you're on a run, you can "wing" the jib to the windward side to help keep it filled.
Trimming Your Spinnaker
Spinnakers are free-flying creatures that change shape continually. Unlike the main and jib, which are attached along one entire edge, spinnakers are fixed at only two points -- the head and tack (at the spinnaker pole). For this reason, spinnaker aerodynamics has always been a bit of a mystery, even to the best sailors and sailmakers.
In spite of this, there are a few guidelines for trimming your spinnaker so it pulls the boat as quickly as possible. Some of these were covered in the last chapter, but let's review them quickly. First, pull your spinnaker pole aft until it is roughly perpendicular to the true wind. Second, adjust the height of the pole (with the topping lift) so the two clews of the spinnaker are level. Third, always ease the spinnaker sheet until the luff of the sail begins to curl slightly.
If it gets windy, most spinnakers can turn into a handful, especially if you're inexperienced. Here are a few things you can do to keep the chute (and your boat) under control.
- Move the spinnaker sheet lead forward. This "chokes down" the spinnaker and keeps it from oscillating back and forth.
- Over-square the pole. In other words, trim the guy more than you would normally. This makes the bottom part of the spinnaker flatter and keeps it steady.
- Move your crew weight aft. This keeps the spinnaker from pulling the bow down into the water, which would make the boat squirrelly.
- Head up a little higher. This works very well when you are going dead downwind and the boat starts to roll back and forth.
Solving Common Trimming Problems
Sometimes when you're sailing along you look up at the sails and you know that something isn't right. You may not be sure exactly what, but you know that there's probably something you could do to make your sails better. Let's look at some potential problems:
- Bubble in the front of the main -- A bubble in the main is caused by backwind from the jib. You will always have a little bubble when sailing upwind, especially in heavy air. If the backwind seems excessive, however, it means that your jib is overtrimmed or your main is not trimmed in enough.
- Lots of wrinkles -- Wrinkles in a sail aren't necessarily bad. They don't look too great, but a few wrinkles here and there won't significantly affect boat performance. If your sail has unsightly wrinkles, however, there are several things you can do. First, check to be sure your sail is pulled all the way to the top of the mast. Second, straighten out your mast by easing the backstay and/or vang. And third, try pulling on the cunningham.
- Leech flutters -- Sometimes the leech of your main or jib will flap obnoxiously in the breeze, sounding something like an engine. This is not good for the sail because it quickly deteriorates the cloth. However, this condition is common on older sails, and there is often not a lot you can do to prevent it. If you're sailing a bigger boat, check to see if your sail has a leechline running up inside the leech. If it does, tighten this line.
- Leech hooks -- Another potential problem with the leech of your main is that it may hook to windward. This is another common problem with older sails. The only thing you can do is refrain from trimming the sheet too hard. If your sail has a leechline, see if this line needs to be eased.
- Spinnaker collapses -- A collapsing spinnaker is a sign that your sail is being trimmed incorrectly. Tell your trimmer to pull the sheet a little harder and not to let the luff of the spinnaker curl so much. You may also want to ease the guy forward a little.
Another sailtrim concern is what to do when it's windy and things start getting a little out of control. Learning to sail in a breeze usually requires time in the boat, but you can shape your sails to make life a little easier. First of all, make your sails as flat as possible. Pull the backstay on very tight. Then tighten the outhaul, cunningham and jib halyard quite a bit. Let your traveler slide to leeward, and move your jib leads outboard and aft. Put the vang on just tight enough to keep the boom from flying up in the air.
Be sure to keep the mainsheet and jib sheets in hand at all times. Don't cleat them! Keep these sheets eased enough so your boat doesn't heel too much, and be ready to ease them further if you get a puff. Depowering the sailplan like this will make your boat a lot easier and more fun to sail when it's windy. If the breeze really comes up, don't hesitate to drop your jib and sail on main alone. Just watch out because it's a little easier to get caught in irons this way.
Taking Care of Your Sails
Sails play an important role on your boat and they're relatively fragile, so it's important to take special care with them. This will help prevent damage and extend the life of the sail (thereby saving you money).
When you're out sailing, avoid flogging your sails whenever possible. In other words, unless you have a good reason, don't just let your sails flap in the breeze. This is one of the fastest ways to break down the sail material.
Be sure that you take good care of your sails whenever they aren't on the boat as well. The best way to put sails away is to roll them. This avoids creasing the sail, which eventually breaks down sailcloth. Many sailmakers supply "sausage" bags with their sails for rolling. If you don't roll your sails, the next best alternative is folding.Every few times you go sailing, wash your sails with fresh water and let them dry completely before you put them away. This procedure should definitely be followed when putting the sail away for the winter. Make sure, when storing your sail, that you protect it from the sun. If you leave your mainsail on the boom, use a cover. Ultraviolet degradation is another quick way to break down sailcloth.