"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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William R. Watt wrote:
"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.
In context, I did note that the 1.3 number was for displacement sailboat
hulls. From what I observe, there probably is still a similar number at
which the hockey stick of power kicks in.
One of the more interesting facets of canoe design is that you have
another dimension - depth of water - to deal with. I can say with
absolute certainty that two boats with the same basic and loaded
displacement, same length, roughly the same wettted area, but slightly
different lines, can behave very differently when you move into suck
water or the shallows. At progressively higher speeds, it gets harder
and harder to paddle uphill (onto the bow wave), until you break over -
which usually happens in 12-18" of water. Want to watch a canoe fly,
follow a well paddled pro boat as they get into shallow water and hit
the gas.
OTOH, most boats won't get to that point - you simply can't get enough
power out of two mortals to get a typical touring boat 'popped up'.
[snip] William makes great points about design programs and the problems
of computing drag and displacement.

Marsh Jones
Minnesota




w

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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).
[...]
In context, I did note that the 1.3 number was for displacement sailboat
hulls. From what I observe, there probably is still a similar number at
which the hockey stick of power kicks in.

Well it was a bit misleading - I don't see too many 16' displacement sailboats*.
Hence I assumed you meant a canoe.

Mike
*I saw one from the train yesterday - an old micro-America's-cup hull
that someone's working on in their backyard.