Sailing with engineers is always an interesting experience. I’m watching terns dive for fish and seagulls sitting on channel markers and they’re talking theoretical boat speed. Steve and our friend Bob are both engineers, and while I’m totally engrossed when the conversation is about sail trim and technique, I glaze over when they start with the equations and square roots of waterlines. In order to get them to change the subject I gave Bob the helm and went into the cabin to calculate the max speed so there would be no question to be debated. Here’s what I learned:
Hull speed is 1.34 X the square root of LWL, or length of waterline. Zzzzzz, oh sorry, I’m awake now.
Generally, the larger the boat, the faster it can go. For a displacement boat, a heavy keel boat, like Bolero, the maximum speed the hull can attain from wind power is called “hull speed” and is largely dependent on the waterline length of the boat. Hull speed is 1.34 X the square root of LWL, or length of waterline. Zzzzzz, oh sorry, I’m awake now. Bolero has a waterline length of 28.9 feet (even though her overall length is 34 feet), so she should be able to sail 1.34 X 5.4, or 7.2 knots.
Hold the phone! She’s sailed at 8 knots. I was there. Actually it was the first day we sailed her on the way back from Stamford, CT right before the jib shredded, but that’s another story. So, exceeding max hull speed is possible, but how?
The way that a displacement boat can exceed its theoretical hull speed is by surfing. Surfing is being pushed by a wave just the way surfers ride a wave on a surfboard. In large wave conditions, when running downwind, a sailboat can get on the front side of a wave and carry it for a few seconds with a tremendous burst of speed. They say it takes good helmsmanship to be able to get on the wave just right to reap the greatest benefits from it. I dunno, I was a novice and she surfed pretty easily. Beginner’s luck I guess.
Bob did his best to exceed the theoretical boat speed that day, unfortunately while I was trying to make sandwiches in the galley. After a couple of subtle hints that we didn’t really have a destination and he could just sail with the wind (which would have been an easier point of sail and create less heeling) I finally just stuck my head out and yelled, “Will you flatten out the damned boat?” I got a sheepish grin in return. And he and Steve finally got a sandwich.
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Delightful post! Few have made me smile more..or taught me as many Interesting and Arcane Facts about navigational speed. Or sandwich-making in a state of extreme heel. My compliments, as always, to a most patient chef.
Probably not patient enough, or smart enough to have made the sandwiches at home!
D.