by GreenLake » Mon Feb 06, 2017 11:36 pm
Well, I thought I'd finally managed to get the DS on a plane, but perhaps not so.
We had a mid-winter race when a front came through,the wind whipped up, with gusts up to 27 knots (30+mph), enough to kick up waves higher than a foot on our lake (fetch about a mile). We were going wing on wing downwind, very nicely surfing down the waves, and the GPS read 8.5. When reviewing the track, which shows speeds averaged over 200+ foot segments, the highest speed was still 7.9 knots.
Was it planing? If you use the standard formula of 1.34 the square root of the waterline length, you get 5.3 knots for the hull speed. This formula simply accounts for the wave length of the wave train (bow wave) at any speed, with the "hull speed" taken to be the wave length at which the trough is at the stern and the boat is continuously "climbing" the bow wave.
Turns out there's a variation to that formula due to David Gerr, which replaces the geometrical constant 1.34 by something depending on the displacement to length ratio. The idea being that lighter hulls can go faster without necessarily planing.
The physics behind that formula might be that a lighter hull can move partially up the bow wave, that is, have the stern forward of the deepest part of the trough of the bow wave.
Assuming 1120lbs for a fully crewed DaySailer, and using that formula, you get a factor of 1.85 instead of 1.34, and the top 'hull" speed would increase to 7.4 kt.
This is pretty close to the maximum we observed, so did we reach a plane? Perhaps for a moment when we saw the momentary speed go to 8.5, but most of the time it didn't look like the bow was really lifting out of the water. (We were distracted a bit, a friend sailing a laser ahead of us had just capsized in our path, so we weren't focused on doing things like getting our weight aft).
Anyway, it was significantly faster than the usual (barely over 6 knots) that we seem to reach quite easily. Because in most conditions we top out at 6.1, 6.2, perhaps briefly 6.3 knots, I had assumed that those speeds represent the effective hull speed for the DS. But looking at Gerr's formula, it would appear the value is actually higher, and true planing comes even later.
~ green ~ lake ~ ~