How a sail works
Sails are nothing more than a wing. Just as a gull can twitch and flex the leading and trailing edges of a wing to maximize the effect of a passing zephyr, so can you as a sailor squeeze the most from a breeze and translate that into boat speed.
But at its core, a sail operates on two principles, lift and push.
The lift is what makes planes fly and eagles soar. The wind is diverted into two paths around a sail on the forward side and on the back side of the sail. The difference in speed and inversely pressures of those two streams is converted into force through a principle first described by Swiss Mathematician Daniel Bernoulli.
That force is communicated to the boat through the mast and that energy is transferred to the hull in a lateral direction that is redirected into forward motion through the lateral resistance of the keel on the boat which slips forward like a wet watermelon seed squeezed between your fingers.. Sounds complicated right? It is.
All you really need to know is that for a main sail to work, it needs to have a nice fluid promoting shape to it like a smooth airplane wing set on its edge and pointing to the heavens. That’s why sails need to be kind of stiff and if you pull it in too far, it slows you down and if you let it out too much, it also slows you down.
With a jib it also needs to have a nice wing shape to it, but in that case it needs to be pulled in concert with the mainsail because it enhances the flow of wind on the backside of the main. This is all under the lift principle which is mostly what is happening when you are upwind sailing.
When you are downwind sailing it's all push and no lift.
Downwind sailing and the Pirates
When I describe this subject to kids they could care less about the physics of it all and would most times rather go swimming than learn about the science of sailing.
So to catch their attention I differentiate upwind sailing or lift, by comparing downwind sailing to something they can get jazzed by, pirates. Down wind sailing or running is all push and no lift and that’s what powered the pirate boats.
Back in the days of Blackbeard and Kidd, boats were square rigged and their sails were rigged perpendicular to the boat so that they could catch the wind and the westerly trade winds could push their boat up the coast to the New World. They did not have a great ability to sail upwind because Bernoulli hadn’t quite explained to them at that point that a boat can be lifted upwind instead pushed down wind and Marconi, another guy who revolutionized sailing, hadn’t created the triangular sail yet.
When we are sailing downwind, the sails are catching wind and pushing the boat downwind. Accordingly when you are sailing upwind you are italian (due to Marconi’s invention of the triangular sail that lifts the boat) and when you are downwind sailing you are a Pirate.
The Shape of Wind
Now that we know the difference between upwind sailing and downwind sailing, we must talk about the shape of the sail. By understanding the shape of the sail, we can best adjust our sails to catch the shape of the wind. There is a shape to the wind.
On the upwind leg, the wind streams across our sails, while on the downwind leg, the wind fills our sails like a parachute. If you were to think about the shape of the wind on each leg, the upwind would be shaped like a stream of water coming from a garden hose while the downwind would be shaped like a bucket covering a much wider surface area.
And when you think about how to catch wind of each shape, you need a wider surface area to catch all the wind on the downwind leg like a parachute and a more narrow or slot shape to enhance the flow of the wind on the upwind leg, just like the nozzle of the hose.
Accordingly when we trim our sails to downwind, we go with the phrase, “if in doubt, let it out”. To catch all the wind you can on a wide surface area to push you downwind, you want as much sail out as you can get to catch the wind.
On the upwind leg, we don't have a sexy little phrase to remind you, because we just pull the sail all the way in. However, we pull the jib and the main in so that they will enhance each other and create a slot between the two to enhance the flow of the wind between them.
I hear you though, what about the points of sail in between???
On the reaches,(IE close, beam and broad), you are combining the principles of lift and push at varying degrees with more lift as you head upwind and more push as you head downwind. The trim of your sails will be in proportion to either force of lift or push, with a close reach mostly lift and a broad reach composed mostly of pushing force.
When you have the angle or trim of your sails working for you, then we worry about the finer details like shape.
Going back to that wing shape we talked about, once your sail is trimmed in the right place, we begin to look at the finer controls that might be available to us on the sail to make that perfect smooth wing like that of the seagull.
Birds have muscles in their wing that alter the angle of the feathers on the leading and trailing edges of their wing so they can fly up and down. Planes have flaps that can articulate to add lift to fly high and to slow the speed of the plane when landing. Sails have similar controls to change the angle and curve of the sail that can speed up or slow down the boat.
The curve of a sail is referred to as a camber of the sail. The more Camber a sail has, the deeper the pocket in the sail to catch more wind.
When we adjust the shape of the sail, we are deepening or making more shallow the camber of the sail. We can also change the location of the deepest pocket in the sail by moving forward or aft. Finally we can change the twist of the sail creating more of a pocket aloft or down lower depending on our needs.
To understand where these controls are on the sail, we must now talk about the sides and corners of the sail.
All Marconi rigged boats, or triangular sails, have three sides and three corners. The three sides of a sail are the leech, the luff and the foot, with the three corners referred to as the head (the top corner), the tack (the front corner) and clew (the back corner).
The leech runs from head to the clew or top corner to the back corner. The foot runs from the tack to the clew and the luff runs from the head to the tack. All the controls on the sail change the length of each of the sides to increase or decrease the depth and location of the camber or pocket of the sail.
The lines used to change the shape of the sail have all kinds of names like leech line and halyard tension and outhaul. Every sail is a little different so I don't want you to get bogged down for now on the names of the lines, but rather focus on the theory behind the shape of the sail.
Where to put the camber
The camber or pocket of your sail will be determined by the conditions of the day and the comfort of your crew. Choppy days, heavy wind days and days with no wind at all, mandate that the shape of your sail ought to change.
The higher your camber on your sail, means the higher the center of effort is on your boat where you will catch more wind aloft. Big waves or heavy wind with a high center of effort leads to a more tippy or unstable vessel. On choppy or heavy wind days you will want to flatten your sail with a lower and more shallow pocket that is closer to the deck.
On lightwind days with smoother seas, you can ease your sail out and go with fatter pockets that are higher aloft. The best sailors can use all the controls they have available to them to take advantage of the conditions to effect the greatest speed out of their vessel or the most comfortable ride for their crew depending on their priority.
One more thing on sail shape- while you can feel and see the greatest effect on shape up wind, there are just many ways you change the shape of your sail on the downwind leg. Think about where you want the center of effort on your boat and shape your sails to your conditions.
This is Not a Primer for Racing Boats
I decided a long time ago that racing was a great way to ruin a good day of sailing. While I am no longer a fan of racing, that doesn’t mean that I can’t appreciate your interests in the subject.
All of these tips on sail shape and trim are designed to help people who have questions about sail shape, but are intimidated by the “authorities” on sailing. Admittedly, there is a ton of info out there about how to make your boat go fast that I have not included here, that just goes over the heads of aspiring sailors. It is my hope that this has broken down the subject in a way that helps you better understand the theory of sail shape and trim.
If you are interested in using these tips to make your boat go faster to win races, then go for it. Try out a few different shapes on your boat and see what makes your boat go fastest. But for everyone else who isn’t interested in winning the Wednesday night beer can series, please know that this stuff applies to you as well.
Cruising in rough weather can be made so much more comfortable with a change of sail shape and the longevity of your sails will be improved if you trim your sails properly to the conditions. Cruising sailors would do well to try out the controls on their boat and see what happens when you ease the outhaul and tighten the leech line.
Either way, remember, do good, have fun and sail far. Thanks for reading.