hull speed CD 33

[Ed Haley]

"Hull Speed" as it continues to be misinterpreted is only one of a great many factors that contribute to a limitation, or accentuation, of the speed potential of a vessel.

Of course, Andy is correct about boat speed. There are many times a boat can exceed "hull speed" and in many different references. Even though my hull speed is about 6.6 knots (based on her waterline length), I have had her over 10.0 knots (gps), 8.6 knots (thru the water) and 5.4 knots (toward a waypoint) all at the same time while surfing down the face of a wave.

The "hull speed" of a boat is simply, by definition, the theoretical speed of a boat based on her waterline length . It's not a limit nor is it a minimum. It's just a reference number to compare boats of different length. One of many.

I'll bet that if we were to list all the possible factors that determined how fast a sailboat goes through the water we would have close to 50 items, maybe even 100. Number 1 would be the skipper.

[Andy Denmark]

What I'm trying to say is that some high-performance boats routinely sail in this are between pure displacement speed and planing, not just in momentary surfing. This transitional domain, > theoretical "hull speed" and < planing speed, is a normal operating regime for the "hot" boats. Some pure displacement hulls also function in this area provided there is sufficient wind, some other factors line up, and the skipper knows how how to drive the boat that hard.

Having sailed a number of high performance racing boats and owning and racing a Moore 24 for 20+ years, it is readily apparent that the 1.34 X sq. root of LWL simply doesn't apply to them. As you say, Ed, it is simply a mathematical way of comparing boats.

The Moore 24, with only 20 ft of LWL and in just the right conditions, would sometimes plane upwind -- just crack off about 5-10 degrees in a sustained puff, get the crew weight out and aft, and up she'd come, getting sometimes 7.5+ kts throught the water in this mode. Of course, that is planing and not what we're talking about here. In lighter conditions with wind abaft the beam the M-24 would attain speeds that weren't planing but well beyond the theoretical "hull speed" derivation and sustain this rate for entire legs of a course (read, miles long) without surfing.

As for engine torque curves, yes, optimizing cruising rpm's to the peak of the curve is the way to go -- more efficient and the engine is happiest at this speed (by design). For most engines this point is usually 80% of max rpms and the biggest reason (I think, anyway) for having the prop correctly pitched (and, as Cathy correctly states, keeping it clean and free of marine growth). On long deliveries under power, I will almost always verify that the prop is clean as both performance and fuel economy will be seriously compromised.

FWIW
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HERBALAIRE VAPORIZER

[Dean Abramson]

I agree with you, Andy, but I do think that hull speed is a useful figure to reference when considering the maximum speed of full-keel Cape Dorys motoring in calm water. At least, that's what my personal experience would seem to indicate.

Dean

[The Patriot]

... "hull speed" is a way of quantifying wave generation based on the single parameter of waterline length, what it is not is a determinant of maximum attainable speed. Boats routinely exceed the speed determined by this equation without getting into a planing regime ... Personally, I think that "hull speed" is a gross misnomer for no other reason than it confuses a lot of people.

I quite agree. This term is one that allows sailing instructors to appear to be in control, but it offers little more.

For example I can increase the waterline on my CD 28 considerably by simply loading up with gear and provisions (and I have done this to the extent of having raised the boot twice over the years). In my warped opinion this makes my boat look rather cool, and she also seems somewhat longer than she actually is. Yet I seriously doubt that the increased waterline length is available as increased speed under power.

I stress "under power" since when sailing, all bets are off. Going uphill at about 15 or so degrees heel lets me lengthen the waterline, and I suspect I can get a little more speed on that basis, provided the seas and wind waves don't beat me up too much. Once or twice while "getting out of the way of the wind" I saw the first number on the speedo turn to a "7" over the last 25 plus years. However I can assure you that the following wind and rolling seas had more to do with it than waterline length.