overdue at Sans Souci
Join Date: Nov 2007
Location: Midland Ontario
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"Hull speed" is always a great bar-talk subject for sailors. I wrote about it with Steve Killing in our book "Yacht Design Explained." I mention this not to slyly sell more copies because it's currently out of print, although we're planning a second edition (some day...).
The standard square-root calc as stated is of course correct. It is a good rule of thumb because it ties speed to the wavemaking ability of the hull. The idea is that the length of one wave (bow to stern) is as much wave as a displacing hull can generate, and the length of a wave is tied to its speed (and that of the hull creating it). As the boat tries to go faster, the stern wave has to move aft into the wake, denying buoyant support to the stern. The stern drops, and in that bow-up stance it takes a lot more power than a sailboat can normally generate to go any faster without surfing or planing.
But it's just a rule of thumb. One of the factors that can increase a hull's theoretical speed is volume in the stern. Boats with wider sections aft have more buoyant volume and can stretch out the wavelength more. Another big one is what the effective waterline length is when a boat heels. Meter class designs have long overhangs for a good reason. When the boat is upright, the measured waterline is short and the rating rule thinks the boat is slow. When the boat heels, a whole lot of unmeasured waterline is put to work in the overhangs. Most every keelboat has "more" waterline when heeled than when vertical.
There are a whole bunch of other factors here as well, including the massive increase in wavemaking drag as a hull approaches hull speed that places increased demands on power to overcome the theoretical limit, and an increase in form drag as the stern drops. But in the end, most every keelboat seems capable of exceeding its theoretical hull speed in the right conditions, for example on a reach.
For what it's worth, I sail a C&C 27 mk 1, which has an LWL of 21 ft and very little effective overhang at the stern. (It's an earlier generation of C&C than the 24. The hulls are not really comparable.) Square root is 4.58, which gives a theoretical speed of 6.13. That's a pretty fair assessment of my redline speed going to windward. But I can routinely hit 6.8 on a white-sail reach.
To keep measured waterline length's influence in perspective, later versions of the 27 had the stern stretched, increasing LWL 8 inches, which would seem to be a lot in a small keelboat. But all 27s race level in class competition, which tells you there's a lot more to performance over a range of conditions than a raw number like LWL.
And yes, it's important to use GPS speed. I also have an impellor speedo, and it routinely disagrees with my GPS by half a knot.
C&C 27 Mk 1
Midland Bay Sailing Club
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It is better to be vaguely correct than specifically wrong.