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Wing Sails (Part 1) (DennisD)
On a completely different tack I found, as usual, the piece by John on wing masts very informative. I have not as yet had the opportunity to check out the original on the internet, but wonder if there is more info out there somewhere on soft wing sails like the one I have fitted to my trimaran. Regular readers will know that this is basically a 10' x 4' pocket with an airfoil-shaped top panel that pulls over a frame consisting of half a dozen low speed foils cut from thin ply, which in turn slide over a square section mast. The whole lot rotates freely in a mast step. It appears to generate a lot of power, although it would be nice if there were another fitted to a craft with a known speed, to see how it compares with a more conventional rig.
When in use the wing sail develops its power by curving in excess of the foil shape on the windward side and becoming convex between the foils on the leeward side. The main problem in trimming the sail is that it is not possible to see the windward side of the sail so the helm cannot see if the foil is stalling - which it does if the angle to the wind is wrong.
This type of wing sail has a lot going for it, not least the fact that it is easy and cheap to build, and has very low sheet loads because it is partially balanced, i.e. part of the sail is forward of the mast. It also has no rigging; in my, admittedly limited, reading of microlight affairs I came across an article which pointed out that cables, as used for rigging, created 18 times more drag than faired tubing of the same thickness taken across the narrow dimension of the tube. This means that ⅛" wire can be replaced by 2.25" thick streamlined tubing! If true, that is. It makes one realise why the big racers go in for streamlined rod rigging. It also appears, to judge from the development of just one microlight aircraft, that wing tips are better if cut off square and left with the chord thickness rather than 'thinned down' at the end. My wing sail is like this, but just for the convenience of having a foil at the top of the mast - nice to know it is probably also efficient.
Wing mast and wing sail development is going to be an interesting feature of sailing in the future.
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