4hp per ton

[Superflid]

Been advised to post this here.

I've been told that this is a rough guide to the power needed in a displacement hull.
I've just bought a 25 foot fibreglass sheathed wooden fishing boat and will need to fit an engine and box. The weight, I've been told, is around 3 tons.
Would a 16 hp diesel be big enough or would it be better to go big and stick in a 4 cyl car/van engine?

Advice so far is that 20 - 30 hp would be minimum.

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[Bejasus]

Hi Ken, I take this not for the microplus, so a bit more info such as draft & beam and length at the waterline would be helpful. Type of engine and drive. I.e. inboard and shaft drive etc.
Can give you a more comprehensive answer then.

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[jfm]

Re: 4hp per ton is a bit mean imho

Seems a very rough rule of thumb. My brother has a 38 tonne displ boat with 2x 250hp, and it just gets to its about-to-plane speed. That's 13hp/ton.

Gut feeling would say 30hp for a 25footer, not 16hp. 30hp is about 13hp/ton. So 4hp/ton seems a bit weedy to me. Even if 30hp is more than you need, nice to have say 10hp in reserve

Obviously there are factors other than displacement that dictate hp requirements, so we're doing back-of-fag-packet maths here, but 30hp sort of seems about right

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[Superflid]

Whoops, must update my details!

It's 25 feet long, 9 foot beam. Not positive on waterline length, but is pretty near 25 feet as the stern is vertical. Draught is said to be 2 feet.
Engine will be inboard with single prop.

I'm looking at the moment at a Landrover diesel which would have the right sort of characteristics (slow running slogger!).

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[Mirelle]

Can a rag and stick owner comment?

4hp per ton is the number often used to calculate auxiliary power for sailing yachts.

I reckon this would make a rather low powered motor vessel; if for example you are regularly punching the tide over a bar you would want more.

The only motor boat I possess is a 16ft clinker launch, weight about 3/4 ton, with an 8hp Yanmar, which is ample for her; the sailing yacht is 9 tons with 15hp, which is definitely not enough!

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[Bejasus]

If we take the boat as having a 24ft lwl, 9ft beam and 2 ft draft, then:
using an inboard engine with shaft drive, using a 1" shaft with 2-1/2 ft between the bearings and a 10" x 7" prop with a 1:1 gearbox, then you need an engine of min. 17hp. this will give you 16hp shaft hp and 1 hp for transmission losses with 34 ft/lbs of torque at the prop. This will let you reach the maximum hull speed of 7 knots with a static bollard pull of 357 lbs. This is assuming a maximum engine speed of 2500 rpm. This does not allow for running ancilaries such as an alternator etc. Any thing above this should be suitable to drive the boat, but if going to sea, I would definately want something in reserve.
I have a 25ft displacement hull at 9ft 6in beam and 1.8 ft draft and I have just fitted a Ford 1.8 diesel, 2:1 gearbox to allow me to get some decent rpm. HP at max rpm is 60. Prop is 12" x 10". Should go well but I haven't had a chance to run it yet. Have to say though, that I am only running on rivers.
Hope this helps

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[Superflid]

Certainly does help!

Will be basing the boat either at Connah's Quay on the Dee estuary (which has some "interesting" tidal currents!) or Boston, which is more benign. But...... the sea is still the sea......

The 4 cyl option is looking like the only option. The Landrover motor should be more than up to the job.

Thanks for the facts and figures.

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