4 hp per ton?

[Superflid]

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?

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[duncan]

I am about 2.8 for a fiberglass planning hulled 25ft craft so I suspect you are going to be closer to 4 tons wet and loaded.
If you compare this to a sailing craft then 20 - 30 hp should be fine; a sound diesel giving 40 odd providing that margin for wind/tide/sea would be my choice.

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[snowleopard]

you might do better to post this on the mobo forum.

for raggies, 3hp per ton is considered ok for an auxiliary. for a mobo you need more than that if you are to go to windward in heavy weather. my guess is that a 3-cyl engine (25-30hp) would be nearer the mark. an inboard diesel will give more shove to a displacement boat than a similar-powered outboard.

before deciding i would look at what power similar-sized boats have and what their performance is.

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[Evadne]

Re. displacement sailing boats I did a survey of our class a while back, long-keel approx. 4 ton displacement sailing boat: Least was 6.5hp (Yanmar 1GM7, 1.6hp/ton) typical was 8-12hp (2-3 hp/ton) and one chap has a 1500cc 4-cylinder BMC job shoehorned in, somewhere in the 20's or 30's of hp I believe. 10hp achieves displacement hull speed in most conditions, less than that requires flat calm. The 1500cc engined boat can make her way up the Humber agin the tide, I am informed, which is no mean feat. All are inboard diesels. Propellor choice is also critical, probably more so than out-and-out hp.

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[snowleopard]

if x h.p. will get you up to hull speed in flat water, you'll need 2x in rough conditions with headwinds to achieve the same. beyond that any more power is a waste of money. the problem in this case is of course determining what x is before buying the engine.

that applies to displacement hulls. if it planes you can virtually keep on adding power and getting more speed, likewise with most multihulls: i have 10 h.p. per ton and get 11 knots (36ft waterline).

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[oldsaltoz]

G'day Ken,

When considering engine size, don't forget about other equipment you may want to add later; a compressor for refrigeration, an extra large alternator for rapid recharge or to run an inverter, deck hose pump, perhaps some hydraulics for an anchor winch, the list goes on and it all consumes power...

Avagoodweekend.



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[JamesS]

As a quick reply, I was recommended 4hp per ton and it worked for me giving 6kts through the water under calm conditions although this will no doubt depend on hull shape.

Cheers

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