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hoover
03-16-2002, 05:06 PM
I've heard from a couple different people that the top speed of a ship or boat is dictated by the length of the craft, and has little to do with the size of the engine (ie, longer boats go faster) Does anyone know if this is true, and why?

kanicbird
03-16-2002, 06:03 PM
I have never heard of this (that doesn't mean it's not true). This is what I know about boat speed:

At low speeds a boat will 'push' it way through the water. Some boats with enough speed will be able to plane off, actually rising up and skimming over the water. Since you are not pushing through so much water anymore your speed increases a lot faster with added power compaired to non plaining.
At planeing speeds you boat is effected by waves/wakes much more then at non-planeing speeds and a longer boat will be more stable at those speeds.

I have a small boat with a very large engine (porportionatly). And it can go suprisingly fast (no spedometer so I can't tell you but I'm faster the 70% of the boats out there). But add some chop and you can't run it at full speed without risking flipping or pounding the \$#!+ out of it.

sailor
03-16-2002, 06:08 PM
well, it is not so simple but I'll try. It is called hull speed and is a function *mainly* of hull length but also of other factors. This only applies to displacement hulls and not to planing hulls.

A a hull moves forward through the water displacing it, it creates a bow wave which, in turn, creates a wave a bit farther back. As speed increases the bow wave becomes higher and the second wave moves aft. Then you get to a point where the boat is sitting between the two waves. It is pusing against the bow wave but get a little push as the stern is pushed by the stern wave. At this point the power needed to increase speed increases exponentially because the boat is now climbing up the entire slope of the wave as the trough is left behind. The speed at which the hull is sitting between the bow and stern waves is the hull speed. The longer the hull, the greater the speed (roughly).

A boat with anough power can climb over the bow wave and then starts planing. It is no longer displacing the water but skimming over it, After it has overcome the bow wave it needs less power to keep going. That initial overcoming of the wave is the most difficult part.

kanicbird
03-16-2002, 06:17 PM
sailor I'd thought you'd be comming along to this thread sooner or later. That was a great explanation.

SCSimmons
03-16-2002, 06:56 PM
For future reference, this is called "Froude's Law". Gives you a good term to do a 'net search for if you want more into, anyway ...
-Christian

sailor
03-16-2002, 07:34 PM
Hull Speed = 1.34 * SQRT(LWL)
where LWL is the hull length at the waterline in feet. This length changes as the (sailing) boat heels and so heeling is also a consideration.

A Google search for hull speed disclosed some pages:

http://www.cs.brown.edu/people/jfh/personal_other/boats/FAQ/node19.html
http://www.sailingusa.info/cal__hull_speed.htm