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Old 18-12-2015, 02:42   #1
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Engine size for boat size

I am in market for a 35' - 40' mono. Mostly coastal cruising and island hoping but over long distance - many months per year. I am looking at used production boats (5 -15 years). I have been "cruising" the forums for months and do not want to the " production boat" discussions, but am keen for advise on this topic. What size engine for what size boat. The online sales sites suggest commonly 40hp Yanmar for about 35' but when you go to 40' it is up to near 55 hp. That seems a big jump. I do not want to be going backwards when the winds are down in an opposing tide but not carry something so overpowered (and big) I will be running it just over idle. I know space will be a limiting issue but the 40 hp seems a little small for size. Keen on you experienced advice.
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Old 18-12-2015, 04:16   #2
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Re: Engine size for boat size

Greetings and welcome aboard the CF, Astro.

A common Rule of Thumb is 1 hp per 500lb total weight (1kW per 304kg).
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Old 18-12-2015, 05:01   #3
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Re: Engine size for boat size

My ten-ton, 31' cutter sports an 8hp outboard. We'll do 5kts in flat water, but I have to be careful with tides.
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Old 18-12-2015, 05:33   #4
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Re: Engine size for boat size

My IP 38 (24,000 lbs unladen) will easily run hull speed with a 4-JHE (40 HP) and propped like it is, I don't get the 40 HP, she will make 7 kts at 2,000 RPM and 7.5 at 2200. I have a 33' waterline, so hull speed is 7.69 kts?
I don't know what it makes at 2,000, but suspect it's down there about 20 HP or maybe less. I seem to remember 1 hp per 1,000 lbs was considered the minimum?

Said another way, I've never seen an under powered production boat, if anything I think an argument can be made that they are over engined because that is what sells, people will almost always assume the boat with the bigger engine is better.
Now I believe my IP has similar wetted area and weight to say maybe a 45' average production boat?

Bigger engines burn more fuel, are heavier, require bigger exhaust, more expensive props that have higher drag etc. Now if you want to motor at hull speed all the time, then you need a big motor, but knock off a half a knot, you don't need that big motor and fuel burn is way down and range way up.
Of course this is just my opinion

On edit, my fuel burn at 2000 RPM and 7 kts is 1 gallon per hour, to get to hull speed, another .7 kt, I think doubles my fuel burn, be the same with a bigger motor too, so motoring at hull speed is wasteful, and maybe more importantly my range goes from 400 NM to 200 NM
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Old 18-12-2015, 07:09   #5
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Re: Engine size for boat size

According to Ted Brewer:
3-4 bhp per ton (1HP/500 - 600#) is adequate
5 bhp per ton (1HP/400#) is ample
6 bhp per ton (1HP/300#) is often too much.
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Old 18-12-2015, 07:50   #6
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Re: Engine size for boat size

I think loaded I fall to the less than adequate point, but can easily make hull speed etc.
So there of course seems to be some it depends on that recommendation.
Either way, have you ever seen a Hunter / Catalina etc. made in the last ten yrs or so that was under engined? There may be several, I just haven't noticed if there are.
Dockmate has an M25 on a about 35' steel ketch, he could use a bigger motor, but seems to get along just fine, and I think an M25 is actually a little less than 25 HP?

55 HP in a 40' production boat as they seem to tend towards the light side, might be just a little more than you need?
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Old 18-12-2015, 08:09   #7
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Re: Engine size for boat size

I have a 5hp outboard on my 6500 lb Bristol 27.

I will have my main up if I think I might run into problem like trying to get out into the bay against the wind and tide etc.
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Old 18-12-2015, 08:33   #8
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Re: Engine size for boat size

OP is right, there is a tendency though for newer boats to have bigger motors.
I don't know if this is a marketing thing or maybe people are buying sailboats as Trawlers now?
Myself I'd rather have a larger fuel tank, than a larger engine.
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Old 18-12-2015, 08:58   #9
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Re: Engine size for boat size

My Tayana 42 also has a 4JH E rated at 40 horses. I weigh 28,000# nominally but more in live aboard cruising mode. My waterline is 36 ft. I have a MaxProp. This engine does just fine and uses about .75 us gallons of fuel per hour at 2200 rpm pushing us at 6.5 knots. 2500 rpm gets us up to 7 knots in most conditions but at a higher fuel burn.


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Old 18-12-2015, 10:00   #10
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Re: Engine size for boat size

Quote:
Originally Posted by Astro2 View Post
I am in market for a 35' - 40' mono. Mostly coastal cruising and island hoping but over long distance - many months per year. I am looking at used production boats (5 -15 years). I have been "cruising" the forums for months and do not want to the " production boat" discussions, but am keen for advise on this topic. What size engine for what size boat. The online sales sites suggest commonly 40hp Yanmar for about 35' but when you go to 40' it is up to near 55 hp. That seems a big jump. I do not want to be going backwards when the winds are down in an opposing tide but not carry something so overpowered (and big) I will be running it just over idle. I know space will be a limiting issue but the 40 hp seems a little small for size. Keen on you experienced advice.
it's all about weight and a fair bottom. my lancer 36, 10,500 lbs, does 7 kts running my yanmar 18 hp at 2000 rpm while sipping a mere half gallon an hour. my 30 gallon tank gives me 60 hours of motoring. why motor at just over idle? diesels love to be run hard.
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Old 18-12-2015, 10:01   #11
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Re: Engine size for boat size

It depends on a lot of factors like hull shape, prop and weight, but a common old rule of thumb was 1 hp per foot of waterline length. We have 30 HP on LWL of 27 feet and it is more than enough to push the boat a 6 knots. We have 25 hp previously and even that was enough in all but the strongest currents and headwinds, but it never really was a problem.

40 HP on 35 feet sounds pretty jacked up like one of those new "sailing trawlers".

We worry less about the power and more about the reliability. Its a sailboat, so we sail as much as possible. :-)
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Old 18-12-2015, 10:06   #12
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Re: Engine size for boat size

The 1 to 500 is a useful estimate.Bigger issue is to make sure you have easy access to the engine, both front and back. Some boats' access is all but impossible(hinckley pilot 35) and you may not find a mechanic willing to work on it. Others, like Hans Christian's, allow you to completely uncover the engine. You will need to replace water pump impellers,transmission roll pins,and the like more often than you want to.
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Old 18-12-2015, 10:34   #13
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Re: Engine size for boat size

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Originally Posted by reed1v View Post
The 1 to 500 is a useful estimate.Bigger issue is to make sure you have easy access to the engine, both front and back. Some boats' access is all but impossible(hinckley pilot 35) and you may not find a mechanic willing to work on it. Others, like Hans Christian's, allow you to completely uncover the engine. You will need to replace water pump impellers,transmission roll pins,and the like more often than you want to.
===

That's excellent advice in my experience. An engine with difficult access is very often poorly maintained.
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Old 18-12-2015, 10:52   #14
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Re: Engine size for boat size

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Originally Posted by reed1v View Post
Bigger issue is to make sure you have easy access to the engine, both front and back.
Excellent point.

I have one of those boats where even changing the oil is a challenge cos you simply can't reach anything. Engine oil I can manage, gearbox (hope that's the right English term for it) oil I can do only if I don't mind seriously scratching up my arm ... Which I do, actually, but not much choice in the matter.
Can't really see what you're doing either, which is always fun with oil.

You do NOT want to have to take your engine out simply because there's no way to reach anything. That's what I ended up doing (on edit: not for changing the oil tho ).
Not a major issue in itself, but seriously ... shouldn't be necessary.

I'll be making an extra access panel this summer so I can at least access the engine from the side too.
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Old 18-12-2015, 10:59   #15
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Re: Engine size for boat size

Bigger is better, run at 75-80% of top rpm.
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