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Old 21-02-2012, 09:38   #1
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What Size Engine ?

We are repowering with a Yanmar 54 HP [4JH5E] but the logstics seems more involved than expected! Some suggestions are pushing us to the smaller 40HP 3JH5 saying it is ample for our 1977 Moody 39. ---11 000 kgs.

We have also been advised to go for the slowest/lowest gearbox ratio for effeciency....
Thank you to those who have commented on one of these points, is there anybody with more info PLEASE!!

Many thanks again - Greg
PS; My present engine is a Ford 2401E and I cannot establish the gearbox ratio anywhere, not even on the net!?????
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Old 21-02-2012, 10:06   #2
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Re: What Size Engine?

contact these guys with the engine number,it will be one of their conversions and will probably be able to tell you what you need to know,since they built the original

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Old 21-02-2012, 10:08   #3
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Re: What Size Engine?

their site is
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Old 21-02-2012, 10:33   #4
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Re: What Size Engine?

The general formula is 1.5 to 3 hp min. per ton, depending on wetted surface. The more wetted surface the higher the ratio.

My 8 ton IOR boat is pushed along very well at hull speed, at 75% RPM's, with a 40 HP Yanmar.

With a lower RPM's you can use a larger prop, but unless it's a folding/feathering prop it will create more drag while sailing.
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Old 21-02-2012, 10:37   #5
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Re: What Size Engine?

I'd expect the 40 HP to be on the lower end, and the 54 HP to be in the middle of the range, for your 24,200 Lb ("light load"?) boat, which "could" use up to 60 HP or even slightly more.
See also ➥ Engine Size
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Old 21-02-2012, 10:46   #6
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Re: What Size Engine?

I stand corrected! I went back and checked my notes and it was 1.5 to 3 hp per 1000#, not ton.
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Old 21-02-2012, 11:06   #7
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Re: What Size Engine?

No more horsepower than you need to reach hull speed for a full displacement boat. That is one of the parameters.
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Old 21-02-2012, 16:36   #8
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Re: What Size Engine?

Quote:
Originally Posted by David M View Post
No more horsepower than you need to reach hull speed for a full displacement boat. That is one of the parameters.
Except if you encounter (perhaps very) unfavourable conditions; whereupon a little more can be very useful.
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Old 21-02-2012, 17:44   #9
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Re: What Size Engine?

Quote:
Originally Posted by Greg Debi Josh View Post
We are repowering with a Yanmar 54 HP [4JH5E] but the logstics seems more involved than expected! Some suggestions are pushing us to the smaller 40HP 3JH5 saying it is ample for our 1977 Moody 39. ---11 000 kgs.

We have also been advised to go for the slowest/lowest gearbox ratio for effeciency....
Thank you to those who have commented on one of these points, is there anybody with more info PLEASE!!

Many thanks again - Greg
PS; My present engine is a Ford 2401E and I cannot establish the gearbox ratio anywhere, not even on the net!?????
Yikes. I had a 51 hp Yanmar on a boat that only displaced 9,000 kg, and it seemed just right. I don't think you'd be doing yourself a favor going to such a small engine.

Also, I think you're getting bum advice to go for the slowest/lowest gearbox ratio. Yanmars tend to be high-rev engines, and your engine will not be happy in such a reduction, ESPECIALLY IF IT'S UNDERPOWERED.

Check with the folks at Yanmar about the best idea for a tranny, and about how much boat they think that little 40 hp will push.
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Old 21-02-2012, 18:18   #10
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Re: What Size Engine?

Here are some resources that might help you figure it out, or at least check that someone else's work is not way off:

A good online calculator -- which has a better explanation and details than the spreadsheet
The famous spreadsheet -- which is a bit more complete (and yields more information) than the above calculator

It's a bit confusing to say "slowest/lowest gearbox ratio", since a lower gear ratio makes for a faster prop. From the spec sheet of the 4 cylinder Yanmar you are thinking of, a 1.47 ratio yields a 2041 RPM prop, and a 2.63 ratio yields a 1141 RPM prop... It may be best to just plug numbers into the calculators above and get a feel for how it comes together.

As others have said, the engine, reduction ratio, prop, and hull all have to be matched with each other. I think your engine dealer should certainly help you with this, but I'd still check that what they say looks right.

I think hp/displacement is a good rule of thumb. I have cruised on both ends of that range and was happy. There is of course a lot more to an engine than HP, so I would pay more attention to the details of the engine than just the HP if it is within that range.
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Old 21-02-2012, 21:24   #11
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Re: What Size Engine?

If it will fit in the room you have, then theres never enough horsepower ! LOL Im a firm believer you must take into account heavy weather useage, and fast flowing tides ect, we have places in the PNW that if your boat can't move against a 10 knot current your gunna wait for a tide change !! a little extra horse power really comes in handy ! Just my 2 cents Bob and Connie
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Old 21-02-2012, 21:48   #12
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Re: What Size Engine?

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Originally Posted by bobconnie View Post
If it will fit in the room you have, then theres never enough horsepower ! LOL Im a firm believer you must take into account heavy weather useage, and fast flowing tides ect, we have places in the PNW that if your boat can't move against a 10 knot current your gunna wait for a tide change !! a little extra horse power really comes in handy ! Just my 2 cents Bob and Connie
Seriously 10 knots?

That means you would need a water line length of about 45 foot to get that much speed out of it.

That's a pretty decent sized boat!
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Old 21-02-2012, 22:29   #13
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Re: What Size Engine?

It is up to you what size to use. For me I have 1/3rd hp electric on a 4ton sloop. The calculation of 1.5hp per ton I think is not really valid or useful. For example, you could have a 20hp engine which pushes the boat at the same speed as a 40hp engine because it has a gearbox and much larger more efficient propeller, but has increased drag under sail and slower acceleration.

You could have a 10hp electric motor which can be overdriven to 20hp but in this case a water cooling pump needs to be running using even more power.

If I had a 40 ton boat I would be happy with 10hp, because I only motor in calms. I do not motor at greater than half hull speed, and at this speed, it doesn't take anywhere near as much power.
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Old 21-02-2012, 23:39   #14
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Re: What Size Engine?

Quote:
Originally Posted by ViribusUnitis View Post
Seriously 10 knots?

That means you would need a water line length of about 45 foot to get that much speed out of it.

That's a pretty decent sized boat!
Here are some examples of Puget Sound (Salish Sea) currents. And this is why I powered my boat to the max limit.







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Old 22-02-2012, 00:02   #15
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Re: What Size Engine?

And sometimes even the pro's loose control.......

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