__Trouble with the bubble________________________

Sails gain in efficiency as foils, when as much wind flows over the surface as possible.
As I stated often, jibs are more efficient because wind doesn’t have to travel around a thick mast, only a thin cable or line.

As wind travels around a mast, a disturbed air bubble (vortex) is formed just aft of the mast. Air has to travel over this area before it can reach the sail surface.

The illustration shows a cross section of sail with the bubble just behind the mast. Below shows where the mast and the curvature of the sail causes the attached wind flow to separate from the sail.

Because there is no attached flow over the bubble (and no power generated) the actual CE (centre of effort) of the sail becomes difficult to accurately pinpoint. For simplicity, the sails CE is reckoned to be at its geometric centre, in actuality, it is just a little further aft, and also a touch higher aloft.

There are ways to reduce the size of the bubble, and gain more power out of the sail area.

Because there is no attached flow over the bubble (and no power generated) the actual CE (centre of effort) of the sail becomes difficult to accurately pinpoint. For simplicity, the sails CE is reckoned to be at its geometric centre, in actuality, it is just a little further aft, and also a touch higher aloft.

There are ways to reduce the size of the bubble, and gain more power out of the sail area.

Tweak the sail - Some methods of attaching the sail are easy to adjust temporarily, good if you are sailing along a stretch of beach on the same tack. With the bread bag clips, just twist the clip so that the luff of the sail is over to the leeward side of the mast.
The same goes with the split straw, just give it that quarter turn. You can then turn it back whenever you change course.

Note: There are actually two mast separation bubbles formed, one on the windward side and one on the leeward side of the sail. Both have a negative affect the sails performance, the above method increases the bubble on the windward side, however you will notice a definite windward improvement when the bubble is reduced on the lee side.

If you are going to lace the sail on, make sure that it is not laced tight to the mast. Lace it loose enough that the sail (along the luff edge) will move (billow) over to the leeward side of the mast.

Modify the mast- The mast itself can be shaped to have a finer entrance, more like an oval rather than round. This works when the mast is exposed or when the sail wraps around the mast, the plastic will cling to the wood and conform to its shape when wet.

The best way to do this is to draw a pencil line down the front of the mast and shave each side with a block plane, you can shape it with sand paper too, if you have the patience. The pencil line helps to guide you, and can be erased later. Don’t shape the foot of the mast of course, leave about two inches untouched. Obviously shaping the mast will weaken it, you will probably need to add stays if you are going to use a headsail.

__The sleeve sail - One type of sail design that addresses the problem of mast separation bubbles is the sleeve sail. The sail has a pocket at the luff with the mast inside. Here are two ways to make a sleeve sail. Remember its always a good idea to work from a paper pattern.

Taping- The luff is folded over and taped to the other side. This is easy and works, but it will eventually come apart.

If you are up to a challenge, try this method that is more durable. Set the edge of the sail half way across a strip of tape. Then place the sail along the luff onto a broom handle or a similar sized dowel or tube.

Roll the edge over the tube (sticky side up) and carefully let the sticky edge grip onto the sail surface. Smooth the contact area, creasing the tape down the middle, finish by running another strip of tape along the seam.

Fusing- The luff of the sail is folded over twice as seen above.
Use a soldering pen and a metal ruler. Once folded, the outer edge gets trimmed away with the hot tip, fusing the edges together as it cuts.

(See multi-panel sails for fusing methods)

The pocket sail will be self supporting, but you need to anchor it down at the ‘tack’ either to the hull or to the boom to keep the sail from sliding up the mast. A single clip around the bottom of the mast will do, or with a strip of tape around the boom, a bit of tape is placed in the middle part with the sticky sides faced together so that the sail does not stick to the boom.

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