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#### wallster

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##### Member
RL24 (Rob Legg) Kookaburra
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101 Posts
Discussion Starter · ·
Is there a formula for how much HP you need per foot of boat? Just got a 24 foot and have a 4hp outboard Mercury. I assume that will be fine.

#### Faster

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##### Senior Member
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19,468 Posts
It's more about the weight and the intended use. A 4 hp outboard is going to be fine for a 24 foot sailboat, esp if the primary use is simply getting out of the marina/ramp area to where you can set the sails, and the occasional return trip if the wind dies.

For more prolonged trips against wind and tide you may wish for a bit more power. Time will tell. In the meantime you'll appreciate the lightweight 4 hp.

#### chuck53

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##### Registered
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1,656 Posts
Faster is right on this. In most cases, 4 is enough. If you were boating in strong currents, 6 would be better. Anything over 8 would really be overkill.

#### newhaul

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##### islander bahama 24
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1,842 Posts
The formula usually works out to be two hores per half ton on a full keel boat a trailersailer about half that will work just fine

#### eherlihy

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##### Learning the HARD way...
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I have this note ferreted away. I found it somewhere in my online reading.

A Diesel Engine produces about 20lb of thrust per horsepower

Wind resistance for a 35' boat (my boat) 225lbs of thrust at 15 knots, and 1200lb at 30 knots. A smaller boat will require less thrust per given wind speed. (there is a chart somewhere)

Detail; the very approximate calculation goes like this:
Take the engine power in Watts. If it is given in HP, then multiply by 746 to obtain the power in Watts. Call this power BSP (brake shaft power).

Multiply BSP by transmission and propeller efficiencies. You can assume around 0.5 as a maximum total efficiency of such a small prop. Call this power PE (effective power).

Convert the speed to meters per second (m/s). To do that, multiply the speed in knots by 0.514 . Then multiply again by 0.95, to approximately take into account the eventual effect of the hull wake. Call this speed Vp.

There might be no wake effects if this is a planing hull with props clear of obstructions ahead. But still I'd keep that 0.95 factor above, because it will give a safety margin to the calculation of the thrust.

To obtain thrust given by the prop, divide PE by Vp: T = PE / Vp .

This will be the available thrust in Newtons, for that speed and engine power. I'm using all SI units, to avoid errors related to inconsistent units. To convert this thrust to lbs, multiply by 0.225 .

That's all.

If you think you'll need to do more such calculations in future, and/or have them more precise, I suggest you to buy the excellent Dave Gerr's book "The Propeller Handbook".

#### dabnis

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##### Banned
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Is there a formula for how much HP you need per foot of boat? Just got a 24 foot and have a 4hp outboard Mercury. I assume that will be fine.
Looks like you may be on a lake? Depends on how much headwind & chop you may encounter? The 4HP should work in all but heavy wind & chop conditions?

Just put it on & see what happens.

Paul T

#### Barquito

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##### Barquito
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3,900 Posts
Your Rob Legg 24 is a really light boat. I think the manufacturer said 4-10hp. I would think anything in that range that you come across for a good price would be fine. Will help you finding a good deal with your boat, in that. you don't need a long shaft OB.

#### wallster

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##### Member
RL24 (Rob Legg) Kookaburra
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101 Posts
Discussion Starter · ·
Thank you everyone for your responses. I figured the current engine would do, was just curious if I should be keeping my eye out for a bigger one. Ill save a few \$\$ then and put it towards other upgrades or basic materials.

#### newhaul

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##### islander bahama 24
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1,842 Posts
At 1500 lbs the 4 that came with it would be plenty spend the money on sails or some other upgrades
Here is the factory site for the boat http://www.rlyachts.net/index24.asp
An excerpt from the page Designed to sleep 4, the interior is open and clear of unnecessary bulkheads to provide comfortable living. The midship toilet, galley and icebox areas are convenient and the cabin can be subdivided if required. Auxiliary power is provided by a 4 to 10 hp standard shaft outboard sited in a special well designed to prevent cavitation.

#### Puddin'_Tain

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##### Don't call me a "senior"!
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968 Posts
My old boat was about 1500 lbs, and I used a British Seagull (about 2 hp, or was it 1 gull-power?) to push it at hull speed in flat water.

#### svHyLyte

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##### Old as Dirt!
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3,491 Posts
For a good discussion of the subject and computations based upon David Gerr's engine HP calculations, see (click on) Boat Speed Calculator

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