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Old 29-01-2018, 21:49   #16
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Re: How much horsepower?

175lbs, 16" height width, looks like almost 3' long with the pulley.

So yes, might want to scale back to a couple units at half the size. That would also help with the side-load issue.

Echo-Tech's units put out serious amps at much lower rpm too, maybe even allow for direct coupling.
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Old 29-01-2018, 23:42   #17
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Re: How much horsepower?

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What on earth do you need a 12kw alternator for ?
I don't, I am fine with the usual alternator.
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Old 29-01-2018, 23:49   #18
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Re: How much horsepower?

Keep in mind the lower HP numbers were from way back when it was hard to fit a higher HP engine. Modern engines can get 2-3 times the HP from the same size package and in the big picture, the cost to go from say a 25Hp to a 35Hp engine is often negligible allowing you to run at 80-90% of hull speed but still have some reserve for bad conditions.

I also wouldn't worry too much about engine longevity due to being under-loaded. As long as the engine gets up to full operating temperature periodically, you don't have to run at 80-90% of max output all the time. Running at 30-50% will typically be sufficient. If you are in no wake zones a lot, it wouldn't hurt to occasionally open her up for 15-20minutes when you get the chance. Reality is the vast majority of marine engine failures are lack of maintenance. Very few die from too many hours.

You don't say specifically what boat designs you are looking at. I would check similar models to see what is spec'd by the manufacturer and go with something close to that.
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Old 30-01-2018, 09:50   #19
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How much horsepower?

I donít remember when someone said, I wish I had a smaller motor.
Of course you can get excessive with anything, but I donít think that 5 HP per ton is excessive myself.
Iím at 3.3 HP per ton, and itís plenty, until I get into that 30+ kt headwind, then I wish it was 5 HP per ton.
The actual fuel usage for a 75 HP engine to produce 25 HP and for a 40 HP to produce 25 HP is very small, it amounts to the increased internal friction of the bigger motor, and if it got bigger by a turbo and the smaller motor is actually the same displacement but NA, then the bigger motor may actually burn slightly less fuel for the same power as the small one.
In other words, itís so close that it doesnít matter.
Of course as you get closer to max power output, fuel use per HP increases, this can easily be seen by looking at an engines fuel consumption per RPM curve, itís usually a reverse bell curve.

The Boat the young girl sailed around the world had a little motor as well as the main I think, idea being to extend fuel range by motoring slowly with a tiny motor.
However I donít think it was a sound idea, go the same speed with the main motor and use the space and weight the little motor took up for extra fuel, and I think she would have had even more range.
I bet her Father felt better knowing she had a back up motor is the real reason.
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Old 30-01-2018, 10:03   #20
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Re: How much horsepower?

Pay attention to the HP curves also. Some lightweight engines are rated at very high rpms.... but down at reasonable rpms they are actually low HP engines.
ie: If you need 25 HP and the engine you buy is rated 25 HP at 3600 rpms, you may be 16 HP at 2400 rpms. So you will be working the engine hard all it's life.
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Old 30-01-2018, 10:22   #21
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Re: How much horsepower?

And these HP/ton numbers are of course real displacement figures, in case of a 33' boat I suppose I can add 2 tons (full tanks, rigging, etc). I see e.g. Westerly Fulmars (net 4.5t) on 20HP, 33's (net 6.5t) on 35HP, and being sceptical of their motoring performance...
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Old 30-01-2018, 10:27   #22
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Re: How much horsepower?

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And these HP/ton numbers are of course real displacement figures, in case of a 33' boat I suppose I can add 2 tons (full tanks, rigging, etc). I see e.g. Westerly Fulmars (net 4.5t) on 20HP, 33's (net 6.5t) on 35HP, and being sceptical of their motoring performance...
I had a Westerly Corsair, about 8.5 tonne, with a Vovlo MD2030 (29hp) with a fixed 2 blade prop and never felt I needed more power. Even when I was fighting a stiff head wind and 4.0-4.5kt current (Hell's Gate NYC).
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Old 30-01-2018, 10:37   #23
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Re: How much horsepower?

I put a 17 hp in a 31 ft boat I built, according to common prediction numbers it was enough. It wasnt. I had a 30 ft full keel boat after that with an old school 25 hp low rpm engine. It was plenty, even bashing back to CA from Cabo against seas and winds.
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Old 30-01-2018, 10:43   #24
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Re: How much horsepower?

There's been a lot of good replies here and there is obviously a range of engine HP that can work in any single boat...but the determining factor I believe is what size and type of prop the boat will have.

Unless you have a continuously variable prop or an Autoprop to a lesser degree, we are all riding single speed bicycles. Determine what limitations you may have fitting a prop (hull clearance/aperture size) and fit the largest prop you can. If you like to sail, a folder or featherer and then back calculate how much HP you need to drive that prop with a max shaft speed of about 1250rpm.

Too much HP on a non-variable pitch prop is such a waste. I agree with Cheechako, match your cruising speed hp needs to the prop curve and that will tell you how big the engine needs to be.
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Old 30-01-2018, 10:55   #25
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Re: How much horsepower?

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Originally Posted by kenbo View Post
I had a Westerly Corsair, about 8.5 tonne, with a Vovlo MD2030 (29hp) with a fixed 2 blade prop and never felt I needed more power. Even when I was fighting a stiff head wind and 4.0-4.5kt current (Hell's Gate NYC).
That's where things start to get interesting, it may be the prop but a Discus (same weight as your corsair, but LWL: 28.4') owner said his 35 horses motor tops ~6kts.

Just another taught, sometimes space available for props is limited, this means, HP available at the prop will be limited by its size:



I have yet to find a working horsepower-calculator for sailboats...
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Old 30-01-2018, 10:58   #26
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Re: How much horsepower?

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That's where things start to get interesting, it may be the prop but a Discus (same weight as your corsair, but LWL: 28.4') owner said his 35 horses motor tops ~6kts.
Definitely a prop issue if the engine was in good nick.


I have yet to find a working horsepower-calculator for sailboats...
Send me a PM and I'll send you one developed by Oxford University
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Old 30-01-2018, 11:04   #27
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Re: How much horsepower?

There is an optimum diameter prop based on engine HP and gear reduction ratio for every boat. If you increase HP or reduction ratio to a point where the optimum diameter prop won't fit the boat, you have wasted your money on too big a motor. It won't push you any faster or better into the wind. It will push you, just less efficiently than a correctly sized engine, gear, and prop. The largest diameter prop that will fit the boat is usually the most efficient and will offer the best control. It will also have the most drag when sailing.
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Old 30-01-2018, 11:06   #28
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Re: How much horsepower?

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Originally Posted by kenbo View Post
There's been a lot of good replies here and there is obviously a range of engine HP that can work in any single boat...but the determining factor I believe is what size and type of prop the boat will have.

Unless you have a continuously variable prop or an Autoprop to a lesser degree, we are all riding single speed bicycles. Determine what limitations you may have fitting a prop (hull clearance/aperture size) and fit the largest prop you can. If you like to sail, a folder or featherer and then back calculate how much HP you need to drive that prop with a max shaft speed of about 1250rpm.

Too much HP on a non-variable pitch prop is such a waste. I agree with Cheechako, match your cruising speed hp needs to the prop curve and that will tell you how big the engine needs to be.
That's good advice! It should be obvious that the propeller MUST be matched to the engine and gearbox, but some people miss it.


Concerning the idea that the engine should not be too big, so that you are not harmfully underloading it at normal power settings --

In my opinion, there is a lot of mythology concerning this. It is indeed true that running a diesel engine at such a low power setting that the combustion chambers are cooled down too much by all the air going through, but this effect does not appear as any particular % of maximum power.

This effect will be more related to power used per cc of displacement per given RPM, rather than power used as % of maximum, because it occurs because the volume of air going through a diesel engine is not reduced by a throttle plate at low power settings like with a petrol engine -- the same volume of air continues to flow through it, cooling it down inside if only a tiny bit of fuel is being burned. So following from this, it is really harmful to run a diesel engine at elevated RPM with little load -- like a constant speed generator with no load, the classic case of polished bores. But if the load is reasonable for the RPM, this problem won't occur.

And this is a unique and profound advantage of turbocharged engines in cruising boats -- all the power you get under boost just increases the range of healthy power output, without taking away from what you can use at the low end without harming the engine.

Take the 4JH Yanmars, for example -- the same block is used for engines from 50 hp to 135 hp and more. Whatever is healthy for the 50 hp engine is equally healthy for the 100 or 135 hp engine because the displacement is the same and thus volume of air going through at a given RPM is the same (except when you are on boost, but you don't any problem in that case, because you only get boost when you have hot expanding exhaust gases).

So I agree with those who say you never hear anyone complaining you have too big an engine. It takes a lot of power to bash through steep headseas, and there may be cases where your life might depend on being able to do so. My next boat will have a lot of power, more per ton than my present boat (which has 100 hp for 20 metric tonnes light ship). And needless to say, with a prop to suit (a Hundested manually variable pitch one). It will be a turbo engine so that I can run it at a fraction of its rated maximum without harm (and for other reasons as well).
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Old 30-01-2018, 11:15   #29
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Re: How much horsepower?

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That's good advice! It should be obvious that the propeller MUST be matched to the engine and gearbox, but some people miss it.


Concerning the idea that the engine should not be too big, so that you are not harmfully underloading it at normal power settings --

In my opinion, there is a lot of mythology concerning this. It is indeed true that running a diesel engine at such a low power setting that the combustion chambers are cooled down too much by all the air going through, but this effect does not appear as any particular % of maximum power.

This effect will be more related to power used per cc of displacement per given RPM, rather than power used as % of maximum, because it occurs because the volume of air going through a diesel engine is not reduced by a throttle plate at low power settings like with a petrol engine -- the same volume of air continues to flow through it, cooling it down inside if only a tiny bit of fuel is being burned. So following from this, it is really harmful to run a diesel engine at elevated RPM with little load -- like a constant speed generator with no load, the classic case of polished bores. But if the load is reasonable for the RPM, this problem won't occur.

And this is a unique and profound advantage of turbocharged engines in cruising boats -- all the power you get under boost just increases the range of healthy power output, without taking away from what you can use at the low end without harming the engine.

Take the 4JH Yanmars, for example -- the same block is used for engines from 50 hp to 135 hp and more. Whatever is healthy for the 50 hp engine is equally healthy for the 100 or 135 hp engine because the displacement is the same and thus volume of air going through at a given RPM is the same (except when you are on boost, but you don't any problem in that case, because you only get boost when you have hot expanding exhaust gases).

So I agree with those who say you never hear anyone complaining you have too big an engine. It takes a lot of power to bash through steep headseas, and there may be cases where your life might depend on being able to do so. My next boat will have a lot of power, more per ton than my present boat (which has 100 hp for 20 metric tonnes light ship). And needless to say, with a prop to suit (a Hundested manually variable pitch one). It will be a turbo engine so that I can run it at a fraction of its rated maximum without harm (and for other reasons as well).
On your next boat with a Hundested, make sure you also fit an exhaust gas pyrometer. That way you can tweak the prop pitch and the revs to the sweet spot for your particular engine.

Also you current boat seems to be around 4hp/tonne fully ladened, correct?
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Old 30-01-2018, 11:20   #30
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Re: How much horsepower?

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There is an optimum diameter prop based on engine HP and gear reduction ratio for every boat. If you increase HP or reduction ratio to a point where the optimum diameter prop won't fit the boat, you have wasted your money on too big a motor. It won't push you any faster or better into the wind. It will push you, just less efficiently than a correctly sized engine, gear, and prop. The largest diameter prop that will fit the boat is usually the most efficient and will offer the best control. It will also have the most drag when sailing.
I don't disagree. What I was suggesting was to reverse engineer the engine to max diameter prop that can properly fit the boat, that's a subtle difference.

If the engine is sitting in the boat then yes, you match the prop to the gearbox and hp.

Here's a question for the gearheads, do you size the prop to the max or continuous rating of the engine?
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