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Boat design: question about increasing hull length

Kevin Murray

New Member
Enlarging a plans built sailboat question.
I’m going to build be starting on a plans built sailboat in spring 2019 and want it a little longer than the plans call for. I’m thinking about an 11 or 12 1/2 % increase in Length. The question is what is best to do with the beam ? Also a likewise increase of 11 to 12 1/2% ?
I don’t know anything about sailboat design, woodworking I can do. My GUESS is increase everything in like proportions.


Active Member
I'm not a sailboat designer, but I noticed a trend while looking at boats from the 70's and 80's.
The ones I looked at seem to have a max width for stability that doesn't increase with length as much as I expected. I saw 14' boats that were stable with a 6' beam and I saw boats at 17' and 19' long that were even more stable with the same 6' beam. I'm sure that there is a formula for the increase in flotation volume, but it's not just increasing the beam at the same percentage.

It's probable that increasing the length at water line to give a higher hull speed gets killed with too much beam.