Sail Shaping - How Sails Work
Sails work 2 different ways. The first is that they work like a kite (like kids fly), in that the sail will deflect the wind. As the wind pushes against the windward side of the sail, your sailboat goes forward.
The other way a sail works is it generates lift, just like an airplane wing does.
You adjust your sail so that the wind can blow around both sides.
The leeward side will generate lift.
The way it does this is:
- as the wind splits at your mast
- air on the windward side is compressed, making it higher pressure
- air on leeward side produces a lower pressure area
- since the sail is between the different air pressures, it is pulled to compensate
- we call this lift.
Now here is the catch, a sail that is cut flat does not do this very well, so you need to make your sail so that it will have shape to it. The exact shape you make depends on what kind of sail you have, how fast the wind is blowing, whether you want it to perform better at low speeds or high speeds, and other factors.
Rule Of Thumb for Locating A Sail's Belly
Various expert sources quote specifications on optimum sail shapes. Most of those sources have specifications that differ from each other, plus each shape has different lift capabilities for speed of the apparent wind and angle of attack. So, I don't know what is perfect for a duck, and really don't think there is going to be one perfect shape for all wind conditions.
As a general rule of thumb, a maximum of 10% draft (depth of shape), with the maximum depth located 30% aft of the luff. This is the sail shape I use, and it seems to work good.
Sail Shaping Techniques
Most ducks use polytarp sails, and it sure is nice to preserve one of the tarp edges that has grommets on it, so that edge can be laced to the mast. A simple method for creating a sail's shape, is to add "darts" to the sail.
A dart is simple: along the edge of the sail there is a small fold. The other end of that fold is a point somewhere inside the sail. The edges of the fold are sewn down flat so not to disturb the air as it flows across the sail. When complete, the dart has a V shape. It sort of resembles pleats, such as in a pleated skirt.
The point of the dart usually ends up in the middle of the sails maximum draft. Or sometimes people make a series of smaller darts around the perimeter.
3 sided sails usually have only one dart
4 sided sails often have 2 darts
Typically for duck sails:
A dimension would be 30% foot length,
B dimension would be 30% luff length,
Dart Fold is 3"
Other Dart Patterns
Here are just a few of the other dart patterns I have seen. The size of darts and placement really is an art form of creating good sails, so I encourage you to experiment to see what works the best for you. You can make scale model sails from paper to test out various dart patterns before making a full size sail.
Curved Luff & Foot
Another simple to add shape to your sail is to curve the luff and foot.
When the sail is laced to the mast and spars, the curves push extra material towards the middle of the sail creating draft.
Many commercial sails are made this way.
The A and B dimensions of a pdracer sail would typically be around 3".
When making sails from strips of dacron, sail makers often form the sail's shape by curving individual panels of the sail before attaching them together.
3D Laminates -- "3DL"
The most complicated and expensive method to create sails is to make 3D Laminates. A full size form is created in the exact shape the sail. Then layers of threads and laminating material are placed on the mold by men in hang glider flying harnesses. After all the layers are placed, a heating element is flown over the surface to fuse them together creating a perfect sail.