# Looking for plans or suggestions

Discussion in 'Boat Design' started by Kessica, Oct 5, 2006.

1. Joined: Jan 2004
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### Raggi_ThorNav.arch/Designer/Builder

A rude estimate for needed power in a planing boat...

Look at how many HP you have per ton.
Take the square root and multiply with 3.5 to 4.
That's your ESTIMATED speed in knots, if the hull has a proper shape for that speed...

Example:
5 tons
250HP
-> 50 HP/ton
-> 7x4 = 28
I would estimate 25 knots.

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### SyedMember

May I know the limititions of the application of this formula(valid for size X to Y)?
As it does not seem to be applicable for small sized hulls, weighing only 100 kg or so.
Thanks,

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### Raggi_ThorNav.arch/Designer/Builder

I think it's applicable for small hulls also.
Larger boats seem to be more efficient, they go a bit faster with the same HP/Ton. Just remember that in a small boat the weight of the boat is a small fraction of the displacement,
Boat 100kg
Crew 150kg
Engine + fuel 100kg?
Drinks 50kg?
Displacement 400kg?

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### SyedMember

Thank you, Raggi_Thor, for your kind response. When you say drinks, does it mean the cooling water in the outboard or the drinks for crew?

Syed

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### Raggi_ThorNav.arch/Designer/Builder

I was joking a bit, I meant beer to drink

What I mean is that people always bring some stuff, so the total weight of a boat is often more than we think.

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### GilbertSenior Member

For estimates I have been using the old rule of thumb that one horsepower per 50 pounds should give you 25 miles per hour. This is assuming it is a reasonably efficient style planing hull. That is not much help to find top speed but it will give you confidence that it will plane.

Raggi, if the old Ole and Sven jokes I've heard are correct, one of the passengers can pick up the 50 kg of beer and then it won't count.

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### KessicaJunior Member

Ah now thats useful. The dry weight of the Wooden Shoe is 3700 lbs, so divide that by 50 gives 74. So a 70 -100 HP power unit will actually move the boat!

As Ive mentioned before somewhere Im not into speed, but moving efficiently and on the plane without taxing the power unit too much. The plan is to get deisel engine from a breakers yard, reconn it and marinise it at the same time, so I will learn a lot more about the engine for repairs and maintenance, also knowing what its condition is like inside, e.g. the head is not warped and all gaskets are new, a good oil pump, injectors etc. I might spend just as much as a new engine but the knowledge wont be there, and I would hate to have to call out the rescue service for a tow because of something simple that could have been fixed with a little extra knowledge !

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### Raggi_ThorNav.arch/Designer/Builder

Gilbert,
Talking pounds, I made myself a golden 30-30 rule that fits a lot of boats
One horsepower per 30 pounds should give you 30 miles per hour.
And the power requirement is equal to the speed squared.
If you divide 30 knots with 1.414 you get 21 knots with 60 pounds/HP.
If you want to go at 60 knots you can only have 7 or 8 (7.5) pounds/HP...
30/50 = 0.6, the square root is 0.77, multiply with 30 and you get 23 knots with 50 pounds/HP (very close to your rule).

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### Raggi_ThorNav.arch/Designer/Builder

And if you drink all the beer you have to wait a while...

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