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Old 29-01-2018, 16:29   #1
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How much horsepower?

I see boats having very different HP-rating/displacement ratios - e.g. 5tons boat with 20 and another 5t boat with 40HP engine, is there a rule of thumb for the right engine size?

The goal is to cruise at 80% hull speed, allowing for some 30kts headwind (good reason for motoring...) and not-the-best bottom (=large wet surface from a full keel, some growth) without pushing the engine to its extremes (where the tank gets empty quite quickly). Obviously this seems more than the usual "get in/out of the marina" job...
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Old 29-01-2018, 16:34   #2
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Re: How much horsepower?

The answer is "It depends"

My 25 ton ketch has 80 BHP and cruises at 6 Kts on 1400 RPM.

Type of hull etc affects what the designer thinks will be suited to the yacht.
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Old 29-01-2018, 16:56   #3
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Re: How much horsepower?

The rule of thumb 4hp/ton of displacement, figure fully loaded. Years ago it was 2hp/ton. Many modern boats are in the 5.5hp/ton area which is way too much hp to adequately load the engine at normal cruise speeds

Your answer lies in may factors many of which you stated but windage and propeller size, rotational speed and type play a big role too.

The theoretical hull speed formula is 1.34 x LWL1/2. your requirement of 80% of hull speed is 1.25 x LWL1/2. That is a fairly high load and would require you to be up near the 4hp/ton mark. If however you chose the 1.00 x LWL1/2 as your target you can com down in hp/ton and still meet you goal.

BTW a byproduct of not oversizing your engine is that under normal cruise you will be putting a nice load on it to keep it very happy for a long time.
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Old 29-01-2018, 17:04   #4
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Re: How much horsepower?

I think 1 HP per 500 lbs is used as a general rule. People tend to overpower sailboats IMHO. A 15K pound, 30 ft LWL sailboat will cruise at 6 kts on 16 HP. The same boat will take 25 HP to go 6.75 kts. Hull speed is 7.34 kts and would need 33 HP. I guess it depends on how fast you want to cruise and what range you need.
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Old 29-01-2018, 17:18   #5
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Re: How much horsepower?

Quote:
Originally Posted by GTom View Post
I see boats having very different HP-rating/displacement ratios - e.g. 5tons boat with 20 and another 5t boat with 40HP engine, is there a rule of thumb for the right engine size?

The goal is to cruise at 80% hull speed, allowing for some 30kts headwind (good reason for motoring...) and not-the-best bottom (=large wet surface from a full keel, some growth) without pushing the engine to its extremes (where the tank gets empty quite quickly). Obviously this seems more than the usual "get in/out of the marina" job...
My boat/motor combination might be a useful data point. Panope (full keel) will motor at 75% power against 30 knot wind (protected water) at 5 knots (hull speed 7).

7 ton boat, 40 HP. (18 inch dia., 3 blade - fixed prop)


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Old 29-01-2018, 17:19   #6
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Re: How much horsepower?

If your alternators are a significant source of electricity generation, charging holding plates or making a lot of water, those extra loads should be accounted for as well.

I'm trying to get a handle on HP needed for an alt that produces 12kW, and so far it seems to be a pretty significant load.
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Old 29-01-2018, 17:19   #7
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Re: How much horsepower?

The 4HP/ton or 1HP/500lbs rings a bell, thanks to you both . I expect to motor a lot, that's the main reason I asked non-ideal conditions. However, if the difference is <1kts speed, then wouldn't push the 80% hull speed.
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Old 29-01-2018, 17:41   #8
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Re: How much horsepower?

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Originally Posted by john61ct View Post
If your alternators are a significant source of electricity generation, charging holding plates or making a lot of water, those extra loads should be accounted for as well.

I'm trying to get a handle on HP needed for an alt that produces 12kW, and so far it seems to be a pretty significant load.
12kW is 16 HP. That's one heck of an alternator. Then you have inefficiencies. What HP engines are used on 12kW generators? That will give you an idea. There's a limit to what can be run off belts and 12kW drastically exceeds that. An engine crank shaft bearing can only take so much side loading.
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Old 29-01-2018, 18:00   #9
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Re: How much horsepower?

A quick internet search shows 12kW generators are using about 22 HP diesels.
FYI: 12kW at 12V is 1000amps. 24V is 500 amps.
You need a generator, not an alternator.
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Old 29-01-2018, 18:11   #10
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Re: How much horsepower?

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Originally Posted by kmacdonald View Post
12kW is 16 HP. That's one heck of an alternator. Then you have inefficiencies. What HP engines are used on 12kW generators? That will give you an idea. There's a limit to what can be run off belts and 12kW drastically exceeds that. An engine crank shaft bearing can only take so much side loading.
Fischer Panda's 200A @24V is rated at 6kW and uses Kubota's Z482 which is 11HP.

Looks like 28-30 HP would be a better allowance for 500A?
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Old 29-01-2018, 18:17   #11
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Re: How much horsepower?

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Originally Posted by kmacdonald View Post
A quick internet search shows 12kW generators are using about 22 HP diesels.
FYI: 12kW at 12V is 1000amps. 24V is 500 amps.
You need a generator, not an alternator.
Well actual output is 14.5+V and near 29V.

So I guess a 500A @ "24V" is more like 15kW at least as the vendors do it.

And I guess that alt coupled to the right engine would indeed be called a DC Generator.

I have no interest in AC.
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Old 29-01-2018, 18:33   #12
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Re: How much horsepower?

Watermaker:

CruiseRO's engine-driven Seamaker 45 uses around 2.5 HP, and electric components under 15A
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Old 29-01-2018, 19:01   #13
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Re: How much horsepower?

500 amps requires 750 MCM wire. One will fit in a 1.5" conduit. What are you running with all that power?
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Old 29-01-2018, 19:29   #14
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Re: How much horsepower?

Charging a large LFP bank, minimizing noisemaker runtimes.

Actual continuous output more like 3-400A, just don't want to overload the engine, so sizing for 500A.

#4/0 should be OK, not talking a long run.
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Old 29-01-2018, 19:47   #15
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Re: How much horsepower?

What on earth do you need a 12kw alternator for ?
It's hard to take your question seriously without an explanation of what you are trying to accomplish that would require that much power.

PS. That would weigh and be about the same size as a 30hp diesel engine
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