Engine HP vs boat size

[beancounter]

I'm sure I've seen a "rule of thumb" for matching size of engine to boat size. I think it was approx 2hp per metre of boat length. Can anyone comment on this? I'd have thought displacement should figure in the calculation as well.
It's crossing my mind, as similar size boats seem to have substantially different engine sizes.

<hr width=100% size=1>Fabricati Diem, punc

 

[snowleopard]

any engine size rule would be related to displacement, square root of waterline or some such. if a 30 footer has a 30hp engine, you wouldn't get very far with 100hp in a 100 footer.

in the past the customary measure was HP per Ton. back in the 50s & 60s we thought of 1 hp/ton as an auxiliary, and 3 hp/ton as a motor-sailer. today the figures are more like 3 and 6+. my motor sailer has 10 hp/ton.

a few minutes scanning test reports etc. in the mags will give you an idea.

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[Samphire]

Yes,It rather depends on the boat,years ago 2hp/ton was usual,nowadays 5hp or more per ton is the norm.If you want to maintain good forward motion into a head wind with choppy sea you do need power.I think many of todays production yachts are underpowered for cruising,but ok for weekending/club racing.

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[ChrisE]

As specific example, I'm just having a new engine stuck into Rival Spirit which is 10 tons, LOA 37.5, LWL 29. Beta did some sums for me and came up with 39hp, as an optimum size. With an autoprop and appropriate gearbox 37.5 hp is what is being put in.

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[snowleopard]

\'optimum\'

if they can calculate the 'optimum' size for an engine, what assumptions are they making about performance? i can see a case for aiming for 90% hull speed for economy or hull speed + 20% margin for bad conditions. did they tell you what to expect from 39hp?

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[ChrisE]

Re: \'optimum\'

I asked for an engine to specced for a compromise between hull speed in reasonable weather, good fuel economy and ability to drive an autoprop which I'd asked to be sized to give good fuel economyin reasonable conditions and reserve of power for punching when needed. Not particularly scientific I know but havnig spoken to several companies about this they all came back with engines around the 40hp mark.

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[beancounter]

Interesting. What prompted the enquiry was 3 boats we looked at a few weeks ago.

Moody 31 31ft 4.5t 28hp i.e. about 6hp/ton
Benny 311 31ft 3.5t 18hp i.e. about 5hp/ton
Hanse 301 30ft 2.9t 10hp i.e about 3hp/ton

In my ignorance, I'd probably veer towards most grunt - but I assumed that I was being a bit simplistic

<hr width=100% size=1>Fabricati Diem, punc

 

[fireball]

Got an 18Hp in a sun oddy 30 - weight 3.5t so between 5 and 6hp/t.
She's got a very slippery hull - fin keel. She motors fine in the harbour against the tide, not had occaision to try in a chop, but I guess she'll be fine. Goes much better under sail anyway.
An Itchen Ferry (27'?) I knew has 38hp for similar weight, but then she doesn't sail to windward....
It really is down to fine balance between the displacement, shape, prop and hp...

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[EdEssery]

Peter Cumberlidge gives data on this in his book "How to install a new diesel" In summary he says work out the boats hull speed (1.4 times the square root of the waterline length) and also your desired cruising speed.

The power needed to drive the boat at these is calculated as follows:

Speed divided by the square root of the waterline length is equal to 10.665 divided by the cube root of the Displacement over the required engine horsepower. When considering displacement don't forget to add the boats payload (stuff on board) to the manufacturers displacement figure. I hope that makes sense. You should then allow for a 3% loss though the gearbox and a further 3% loss through the sterngear bearings.

When you know the power of the engine you need (enough to drive a loaded boat at hull speed) look at the power curve of your proposed engine and figure out whereabouts your cruising speed appears on it. You do not want your crusing speed to be less than 75% of maximum engine revs - if it is you won't be working the engine hard enough which will lead to problems in the long run.

I ran these numbers for my MG C27 and the 9HP engine I have is spot on - just as well as I'm replacing it witan identical but younger engine.

Ed

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[JoeV]

Don't forget the stuff above the waterline matters too, i.e. so not just the LWL and weight but the overall air draft as well. This is has an exponential effect on required HP. So my guess is that sloops will need more HP. I know it probably doesn't matter too much, but I think it it is worth keeping in mind.

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[johmal]

Another very rough rule for small displacment boats:- get the water line length in metres, and that number of HP will give that number of knots. Then double the power for every knot required after that.

All that extra power just goes into making waves!

John M

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[oldharry]

Sorry John - dont get it: The water line length in metres = 7 so that number of HP = 7 to give 7 knots and 14 to give 8 knots?

If so, how come my 20hp on 7metres only gives 6 kts?

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[duncan]

prop? gearbox? w/line less than 7?you aren't getting 20hp?

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[Keith]

Bear in mind that modern engines, the max HP is just that, the max sustained RPM is two hundred lower or there abouts, so in actual fact for a yanmar 2GM (18HP) its max sustained is approx 16HP...........keith

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[oldharry]

Yes I am. Its a big heavy long keeler with a big heavy owner with a big round ...er... tum (what did you think I was going to say on a family board...?).

Yes she will do 7kts just, but to suggest that I can get that speed from just 7hp is totally unrealistic. A fast lightweight racer might just about do it, but not a heavy cruiser.

Even with the original 40hp petrol engine, the best speed she could make was 7.4 knots. At that point she was making a wash like a destroyer, I could see 4 of everything, and HMCG were calling up to enquire whether my exhaust smoke was meant to be a distress signal!

But to suggest that 7hp WILL produce 7knots, 14 hp 8knots, and 28 = 9knots as the formula suggested, is unreal.

Even my old trailer sailer with a w/l length of 6.5 metres was pushing to do 6+ knots with an 8hp engine on the back.














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