Sail dynamics

Well-trimmed sails

I am very interested in the science of how our boats interact with natural forces, both wind and water.  I sail partially for the connection with nature and feel more connected when I understand it.  I have found a person, Paul Exner out of Tortola, British Virgin Islands, who can explain these concepts at a high level while putting them into practice on a boat. These are my thoughts on an afternoon we spent together.

The properly set sails of a boat cut through the wind like a blade.  The rounded shape of this blade bends the wind just like a river bends around a rock, we just don’t see the wind bend. And the forces created are different when a gas is bent.  Bending the wind creates relatively lower air pressure on the convex side of the sail which pulls the sail forward.  To harness this force we are able to shape and position the sail.  This triangular piece of fabric has curve built into it which we can shape by varying the tension on each of its three edges.   Likewise,  we can change the angle at which we present the sail to the wind.  The shape we create and where we position the sail vary based on the force and direction of the wind.  Ideally we position the leading edge of the sail so that it evenly splits flow of wind to each side and as the force of the wind gets stronger, we decrease the curve in the sail. As the boat moves, the speed and direction of the wind relative to the sail changes.  Getting it all right is as much art as science. 

The elements of sail shape on which we focused were draft and twist.  Draft is the depth of the curve in the sail and we can change both the amount of depth and its location. Deeper draft bends the wind more, maximizing the lower air pressure that pulls the sail forward. But there are limits to the benefits we can get with draft because the wind needs to flow smoothly across the back of the sail, bending with the sail, to create the low pressure which causes the suction. In light air the wind cannot follow the sharp curve associated with deep draft and it will spin off, forming eddies like on the side of a river, effectively unwinding its energy into the eddy rather than drawing the sail forward. In higher winds, the sideways forces associated with a deep draft can be too much and will cause the boat to lean over to the point that the hull does not move well through the water.

Apparent wind is the sum of the wind due to
boat speed plus the actual wind.  Higher
winds make the apparent wind
come more from the side

The twist of a sail is often unrecognized. To appreciate twist, see it in the picture which accompanies this post.  At the base of the sail the trailing edge points nearly straight back while at the top it is released off to the side.  The sail is twisted from bottom to top along a vertical axis. The angle of the sail relative to the boat is wider at the top than it is at the bottom. This feature is useful in light air because the wind will blow harder at the top of the mast than at the surface.  This occurs because the wind near the water’s surface is slowed due to friction.  The apparent wind (see diagram) at the top of the sail is more to the side so that section of the sail can be set wider to match the wind at that elevation. We can allow more or less twist so as to match this deviation of the apparent wind and keep the sail working through its entire height.  Of course while all this theory is good, sometimes sailing with less twist so that the sail is trimmed theoretically too tight at the top results in faster boat speed, possibly due to hydrodynamic considerations. 

In higher winds there is less change in wind speed caused by friction with the water.  Twist can still be used instrumentally because we might not want the entire sail to be working and we can twist off the top of the sail where it would have the most leverage to create heel. We can sometimes get the top of the sail to luff while it is working lower down.

Twist can do funny things when a boat is going downwind with the sail all the way  out. If we allow twist with the sail in this position, the trailing edge of the upper part of the sail can actually be pointed forward. A strong puff in this setting will heel the boat to windward — the boat rolls away from the side that the boom is on, opposite what one normally expects.  This can induce uncomfortable and potentially dangerous rolling.

Wind bends before it
meets the sail

The two sails of a boat like mine, a sloop, work together rather than as individual units.  Each sail changes the airflow over the other.  A cool thing to understand is that the wind begins to bend even before it reaches the sail in the same way that a river begins bending upstream of a rock.  The wrinkle in the wind created by a sail begins upwind of the sail! They figured this out observing the flow of smoke over a sail. Now consider the wind that is about to go over the mainsail. It is changing its course before it meets the mast, coming now slightly more from the side. This local wind shift is occurring right where the jib is operating and helps he jib work better. It creates a local apparent wind which is at a wider angle to the boat than the true wind so the jib experiences a lift -- the wind is slightly less opposed to our direction of travel. Unfortunately the jib does not return the favor to the mainsail. The jib causes the wind that flows over the mainsail to be more forward of the sail, so the main experiences the opposite of a lift, a header.  The advantage to the jib outweighs the disadvantage to he main.

The space between the sails is referred to as the slot. I do not find consensus about what is going on within the slot. There does seem to be agreement that he slot should be even, so that the sail shape and position of he two sails should be similar.  The slot would not be pinched or become wider further aft.

The shape of a sail is modified via the tension on each of its three edges — the leech (trailing edge), the luff (forward edge) and the foot — and, for the mainsail, our sail trim devices allow us to tension each edge individually. Tension of the leech by pulling the back corner of the sail down removes twist.  Leech tension, with the boom in close, is created by the mainsheet because with the boom over the deck the mainsheet pulls more down than in.  People tend to think of the mainsheet as moving the boom in and out, but it pulls down, too. Once the boom is far enough out that the mainsheet pulls more in than down, then leech tension is achieved with the boom vang. 

Tension on the foot reduces draft in the bottom of the sail while tension is the luff also reduces draft at the same time that it pulls the draft forward.  Luff tension is typically created by a downhaul, or Cunningham, however my boat is not set up with one.   We found that I could create good luff tension with the halyard, probably because the Harken Battcar system eliminates friction between the mast and the sail so the tension from the halyard goes evenly to the luff.  We also found halyard tension would change the twist.  This makes sense because pulling the head of the sail pulls only slightly less on the leech than the luff. 

When the sail is trimmed close the the boat and the mainsail is pulling more down than in, the traveler is the best tool to position the sail.  Some guy walking the docks told me I was wasting my time redesigning my traveler, but I find it an essential tool when close-hauled. Trying to use the mainsheet to bring the main in tighter at this point does more to change its shape than its position. The traveler, which controls where the mainsheet pulls from, will change the angle of the sail to the wind without changing its shape. As the sail moves further outboard the mainsheet takes on its commonly perceived role of positioning the sail in and out.

In contrast to the main, the jib only has one point, the clew, from which it is typically controlled.  We can control luff tension, and therefore draft, with the halyard but with furling gear and headstay foils, not many people are going to do that. We can, however, pull the clew in different directions to affect jib shape. Most boats have a track by which the jib fairlead (the pulley that the jib sheet goes through before it is led back to a winch) can be moved foreword or aft. Moving the fairlead foreword causes the jib sheet to pull the clew more down than back so the leech is is tensioned relatively more than the foot. Move the fairlead back and the foot gets tensioned more than the leech.  The center position for the fairlead is found empirically and is that point at which the entire length of the jib’s luff breaks at once (or all the tell tales lift together) as you head into the wind. Partially rolling up the jib changes that center. 

The jib sheet can also be redirected so that it pulls more from a position further outboard through the use of a barber hauler. This device can simply be a block attached to the rail through which the jib sheet passes before it is led to the fairlead. Paul figured out that on my Morris Justine, when sailing on beam and broad reaches, the sail had a more powerful shape with the clew further outboard.  We began with the straight forward system of a block (pulley) attached at the widest part of the boat. Over the course of the afternoon we invented a system in which we could vary the height of that block. We did this by putting the block on a line that could be controlled by a winch.  If you enlarge the picture attached to this post you can see this setup.  The jib sheet is led via the aft corner to its winch and the barber hauler is going through the midship hawse pipe, through the genoa fairlead, then to a winch

An important aspect of any headsail trim is holding the clew steady. If the clew is allowed to move around, the flow of wind across the sail can detach and the suction is broken.  The more stable the clew the better and thus the benefit of fixing it with a pole or building rigid sails like the America's Cup boats.

I have deliberately not discussed the hydrodynamic issues which include the forces generated as the keel moves through water.  The keel acts like a foil, creating forces which help the boat go upwind, similar to the sail.  There is interplay between the aerodynamic and the hydrodynamic forces.  I will write about that as I understand it better.

We had the picture of our handiwork taken by a guy standing in an inflatable, one hand on a throttle extension and the other holding a camera.  He is the boating equivalent of the guys who position themselves on popular ski runs and try to sell you a picture. He came along just as we were getting things dialed in. Good timing to sell me a picture. An excellent souvenir for a sailing dork’s vacation.

References:

Sail Power by Wallace Ross
Seaworthiness, The Forgotten Factor by C.J. Marchaj