"Michael Daly" ) writes:
On 9-Jun-2004, Marsh Jones wrote:

knots =(1.3*((SQRT)LWL)) which means that a 16' boat can go about
5.2Knots regardless of how much power you put to it. (Unless you can
pop it over the bow wave and get it planing).

Not really. Olympic class paddlers, as one example, routinely take their
canoes/kayaks well past hull speed without any evidence of planing. Hull
speed isn't a speed limit, it's a point where paddling gets tougher, faster.
It seems to have more relevance to big fat vessels (like keel boats) than
to long, skinny paddle craft.

the "hull speed" of 1.34 times the square root of the waterline length
only applies to deep displacment hulls. canoes and kayaks are such light
displacement narrow boats that they can go faster with low power. long
narrow hulls like catamarans can also "sail through" their displacement
wave and exceed the 1.34 number with low power.

we were discussing power requirement in another thread. if you multiply
the total resistance in pounds times the speed in knots times 0.003072 you
get the power requirement in horsepower to sustain that speed. a knots is
1.15 times a mile per hour. the problem is finding out what the total
resistance is. there might be sophisticated hull design programs which
will predict more than the wetted surface and wave-making resistances but
I am not aware of any. you will also find differences in computed wetted
surface and volume for the same boat among different hull design programs.
I don't know how precise you can expect to predict teh power requirement
for any boat. Perhaps if you do the calculations for two boats using the
same hull desing software you will get a pretty good comparison. So
comparisons of calculated numbers from the same manufacutuere might be
pretty close. If there are big differences in teh numbers for the boats
you are interested in then there is probably a basis for making a choice.
But if the differences are small I would not trust the nubmers.

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