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post #27 of Old 02-10-2013
Closet Powerboater
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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?


I have a sauna on my boat, therefore I win.
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Last edited by MedSailor; 02-10-2013 at 12:57 PM.
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