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Old 02-04-2013
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Re: Repower confusion (horsepower)

Low RPM, and low piston speed is always an engine lifetime extender, all other things being equal.

Smaller, higher RPM engines just wear faster.

You want to load a diesel, but - and a BIG but - the little diesel's we put in our aux sailboats are not the same as the continuous duty heavy diesels of lore.
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Old 02-04-2013
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Re: Repower confusion (horsepower)

Along with to make a 6 cyl be equal to my twin cyl yanmar, you would have really small cylinders! Frankly, 2 large vs 6 smaller in many cases is better! I have a 750CC 3 cyl turbo in my trackhoe, do not remember the CI or Ccc of the boat motor, both motors are 16HP. It is smaller, hieght wise, but 50% longer in length. Which could cause some issues.

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Old 02-04-2013
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Re: Repower confusion (horsepower)

It seems like 3 cylinder diesels, with balance shafts, are common in Europe. VW and Audi use them in smaller cars. I've also heard that BMW is bringing a 3 cylinder gas engine to the US, which they claim is smooth. Couldn't some of this technology trickle down to boats? When it comes to repowering, if one brand was significantly smoother and quieter than others, I'd be very interested.

Apparently, the main vibration mode with an inline-3 is fore-aft rocking. No wonder the engine mounts in a boat have to be solid. The good news is that just a single balance shaft reduces the rocking a lot.
Bristol 31.1, San Francisco Bay

Last edited by MarkSF; 02-04-2013 at 09:53 PM.
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Old 02-05-2013
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Re: Repower confusion (horsepower)

Yanmar is particular fussy about their mounts. They have different stiffness mounts for fore and aft. Not sure what that means.
My installer says that that the Yanmar mounts seem soft to him which makes him more concerned about alignment that usual.
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Old 02-06-2013
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Re: Repower confusion (horsepower)

Have you considered getting in touch with Ted Brewer?
Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)
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Old 02-07-2013
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Re: Repower confusion (horsepower)

One of the things with those programs is they assume perfect conditions. Figure you are going to use up to 10hp for the alternator, your going to need a few hp to counteract the power of a head wind, and a few for transmission losses. My guess is that their program is telling you what you need in shaft horsepower, not engine horsepower.
Greg Rubin
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Old 02-10-2013
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Re: Repower confusion (horsepower)

Ahhh! A question close to my heart! I've been trying to perfect my prop pitch and run RPM vs hull speed in my Formosa 41 since I bought it. I think, after this next prop pitch adjustment, I'm finally there.

Why all the fussing and obsessing? Because my engine is EXACTLY sized to my boat. Or, another way to read that, is it's underpowered. I would much prefer to have a couple extra HP so that I didn't have to get EVERTYTHING exactly right in order to go hull speed.

My boat:
28,000LBS + several K for cruising weight.
31' LWL but this increases with speed due to hull shape aft (I believe).
Perkins 4.108 with a 2.1:1 reduction.
Prop is an 18LHx9 (soon to be an 18LHx10)

With my 35hp engine (it's only a 45HP when going downhill) and a perfectly clean bottom I can go hull speed at cruising throttle 75-80% of max rated RPM) but once the bottom gets dirty I'm going slower. Also, I have a standard alternator and no extras drawing off the engine.

I've never had a problem motoring into heavy seas, but she's a heavy boat with a gentle entry and high clipper bow so I think I need less reserve HP to plow into steep seas than my old plum stem lifeboat did.

For your boat (which has similarities to mine) I would say that 30-35 would be an absolute minimum, with 40-45 being better. That way you've got a little extra in the bank for pushing hard against seas, or to beat slack tide at Dodd Narrows (remember you can go faster than hulls speed), even with a less than perfectly clean bottom.

BTW, have you looked at the orientation of the engine and how that compares with your access? Are the fuel filters, injector bleeding bits, and oil filter all going to be against that bulkhead or right by your access hatch?

Are any of the engines you're considering self-bleeding? That would be a real plus. What about glow plugs and cold start options in case you go to Alaska?

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Last edited by MedSailor; 02-10-2013 at 12:57 PM.
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Old 02-14-2013
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Re: Repower confusion (horsepower)

The Beta rep and I had the same discussion a few years back on my Beneteau First 456. He recommended the 37 and I installed the 50. One of the factors in choosing Beta was that they could live with the same 2" exhaust system as the 4108, while Yanmar insisted on 3".
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Old 02-15-2013
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Re: Repower confusion (horsepower)

Either the 43 or the 50 will fit fine, I have good enough access that even though some components are in different places it will still work OK. The only technical problem now is that I'll either have to have a custom flange made for the output shaft of the transmission to connect to the U-joint on the end of the shaft to the V-drive, or use an adapter plate and use my old Volvo trans (which I'm not inclined to do as some parts are difficult to get already). I'm leaning toward the 50, mainly because I like to run engines easy, and the transmission on the 50 is an upgrade from the standard one on the 43.

Stu- Giving Ted a call is an excellent idea, don't know why I didn't think of that (duh).

Don- you are absolutely correct that you need to work through all of the changes that will need to happen, exhaust hose sizing, raw water through hull size, etc can turn into big expensive projects if they need to be increased.
SV Laurie Anne

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Old 02-15-2013
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Re: Repower confusion (horsepower)

Interesting thread. I've messed with turbos on cars for a long time including some high HP 'street' cars. The issue of turbos causing problems is IMO outdated. They've been using turbos in big trucks and now passenger trucks for a long time. If you can take a smaller motor and get the same relative displacement out of it as a bigger motor then that's a plus. If you add in direct fuel injection then you have an extra benefit of cooling and better fuel economy. More complicated but more efficient and generally longer lived.
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