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Old 21-08-2009, 01:37   #16
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Thank you all so much , for your time taken to make these comments.
Valuable information indeed .

Best Regards Jim

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Old 23-08-2009, 11:39   #17
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my old 2GM (15 hp max, 13hp continuous) pushes my 9000lb 30'er at 6.5kts in near calm, and only loses a knot or so, in about 15-20kt headwind (about 3-4ft chop).

I am no expert, but I read alot pf people recommending about 25-30 hp etc, for these size boats. I think that you do run a risk in being overpowered, in that you will often be under-revving your engine. Of course, you can rev it higher every hour or so, etc, to prevent engine damage, but diesels are meant to be owkred hard, not cruised at 1500 rpm, etc.
If you are cruising with engine driven pumps, huge alternators, etc, then you may be able to justify the larger engine!

That being said, if I were to ever repower, I would like something a bit larger, like 18hp, or so, but often the choise goes from 14 to 20/22 with nothing in between!

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Old 24-08-2009, 03:16   #18
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Thanks, Northeaster , all good info ... seems to be pointing towards the same conclusion , ie ...30 foot 4 ton boat be safe use minimum say 15 hp. Thanks for your time writing NE,er.
Great Regards Jim.
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Old 24-08-2009, 13:44   #19
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Originally Posted by David M View Post
Current is irrelevant. Current is the water speed with respect to the bottom. You're boat only "knows" of its speed through the water. Torque is a useless figure without speed...and that determines power. You can have a ten thousand foot-pounds of torque on a propeller that turns one RPM...and you will still have very little horsepower.

You wont notice any difference between a one or two cylinder, 12 horsepower engine because the power is directly and linearly converted to forward force..and that is what makes it move. If the prop is sized properly your engine will be able to reach the RPM at which 12 horsepower is generated. Power is power.
Current would only be irrelevant, if one didn’t care about one’s speed over the ground (in real space).
Although, in some senses, “the boat” may only know it’s speed through the water, I’m only concerned with it’s speed through space. In fact, a counter-current, of a high enough magnitude, can stop a boat dead in it’s tracks.

I’ve stupidly departed Staniel Cay’s Big Rock Cut, against a flood tide. That 2 mile passage took me TWO HOURS, and I knew the counter currents. The counter-current turned my ("normally") 6 knot boat*, into a 1 knot barge!

* 2GMF15 on a 6,800# 28.5 footer
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 28-08-2009, 21:18   #20
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My previous boat was a 28' S2, with a 2QM15 Yanmar. This was enough power, but certainly wasn't excessive, and I wouldn't have wanted to have any less. Have a friend with a Hunter 33, with that same engine, and it is underpowered in a headwind or sea.

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