Ask SAIL: In-mast or Full-batten Main

Don Glynn of Westlake Village, California asks:

In-mast furling: convenient, but there's a price to pay

In-mast furling: convenient, but there’s a price to pay

My sailing buddy and I are thinking about moving to a bigger boat. I now have a 31ft Pearson with a full-batten main. It is a reasonably high-performance boat, and I love to sail it. My buddy wants to go to an in-mast furling mainsail. I would like some information on how much performance is lost when switching from a full-batten to in-mast main.



Win Fowler replies:
There would be a significant performance penalty, for two reasons. First, the mast and furling gear are substantially heavier than a conventional mast. Weight aloft adds to heeling and pitching moment, which will cause your boat to become more tender and more prone to hobby-horsing, which degrades performance. 

Second, an in-mast sail without battens has a hollow leech and narrow head angle that makes the sail a much less efficient airfoil. The net deficit in performance for an in-mast furling main, according to PHRF New England, is six seconds a mile. This is a fairly arbitrary number that I think underestimates the actual penalty for most boats. It’s true the hollow-leech problem can be ameliorated through the use of vertical battens, but we frequently hear stories of in-mast sails getting stuck partially furled, and this problem is much more common in sails with vertical battens.

Still, there is no question that in-mast furling, when working properly, takes much of the effort out of handling a mainsail. If you are willing to give up some performance, it may be for you.

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