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Ask Sail: Do Square-Top Mains Make a Difference?

Square-top mains are better for multiple reasons

Square-top mains are better for multiple reasons

Q: I’ve been watching the most recent America’s Cup down in Auckland, which has got me thinking about technology and sails. My question is, how much difference do square-top mains, like those used aboard the AC75 mononulls, really make? In the America’s Cup, obviously every fraction of boat speed counts. But what about on a cruising monohull or Wednesday night beer-can racer? Would that little bit of extra sail area up high be even noticeable? — Mike Stella, 

Brian Hancock Replies

The square top makes a huge difference not only because of the additional sail area, but because of the overall geometry of the sail. You don’t see rudders shaped like triangles, because that would be very inefficient. Same thing with keels and mains. The top third of most mainsails is quite inefficient because of the short chord length, i.e., the distance from the luff to the leech. The resulting lack of power is especially problematic because there is more wind aloft than there is at deck level. Water creates drag on any passing breeze, so the farther away from the water the wind is the faster it is. This in turn means not having a square-top main results in failing to take full advantage of whatever breeze you may have. The problem is the backstay. You can’t have a true square-top main if you have a single backstay. Multihulls can get away without backstays because they can spread their shrouds out wide to support the mast. Not so with a monohull. So you usually end up with a conventional mast, a pesky single backstay and a triangular-shaped mainsail.

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