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Old 10-01-2005, 16:14   #1
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Question Too many Experts

Everyone I ask has a different opinion. Here's the story. I have a 1977 Islander Freeport 41. The original engine was a nissan 40 or 50 HP Diesel, not sure. The previous owner replaced it with a Nissan SD-33 100HP. Now given this information, what changes should have taken place after this tranformation? I have been told that I am overpowered, if so, is this bad, good or indifferent? My main concern is a. What size prop should I have? b. Do I have to get a larger alternator? Have I damaged the engine in any way? etc.... If anyone out there has sufficient knowledge and experience Please contact me.

Thanks in advance,

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Old 28-04-2016, 00:06   #2
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Re: Too many Experts

if a 100 hp fitted in, ok well its all ways good to have the extra power in reserve,BUT would still only be good for shaft and prop married to a 50 a lot of wasted fuel,some of the nissans only put out 60 to 70 at the working end,its good to have a proper marine alt. as the car ones doesn't use all its grunt all the time.

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Old 28-04-2016, 02:39   #3
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Re: Too many Experts

The original post is now 11 years old. The boat owner would have worked out all the things by now.
However the question is a valid one for those who put bigger (read "with more power) engines in their boat, certainly when the HP is doubled, the following could be considered:
- certainly change of prop
- check with a marine engineer if the gearbox is up to it
- likely upgrade of prop shaft in diameter or stronger material
- likely upgrade of the heat exchanger
- alternator? a larger engine does not 'use' more electrical power, except for the startermotor, this can be sorted later, after the installation
- possibly better ventilation of engine room/space as the engine needs more air when revved
- possibly bigger diameter seawater hoses and with that, a larger skinfitting, as the raw/sea water pump on the engine is bigger
- possibly a larger primary fuel filter, if the old one was the right size or undersized
- unlikely bigger fuel tanks, as the 50HP of the old engine was using a similar amount of fuel as the new engine when running at the same output. And I assume here that 50HP is enough for 90% of the time.
Wishing you all sunny skies above, clear water below, gentle winds behind and a safe port ahead,
and when coming this way check,_Australia
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Old 28-04-2016, 05:43   #4
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Re: Too many Experts

Yeah, I'm thinking that after 11 years the OP has got this figured out.

Nonetheless, there are some significant issues with grossly over-sizing the auxiliary as in the original question--and, yes, 100hp is grossly over-sized for a sailboat that displaces 11 tons.

Hank mentioned some of the biggies; prop shaft, heat exchanger, exhaust all have to be considered. Beyond that, in calm conditions it is only going to take 15-20hp to move the boat at hull speed. Going into wind and waves it might need 45-50hp at most. Meaning that you are either going to be constantly running the engine at just above idle speeds, which is bad for a diesel engine in the long-run, or you are going to be wasting huge amounts of fuel running the engine at higher speeds just to push the bow wave a little harder. Not really a good situation.

Oh yeah, and the engine probably weighs nearly twice as much as an appropriately sized one, so you're carrying a few hundred pounds of extra weight in the stern for no good reason. Extra weight, extra expense, extra fuel; all for no benefit whatsoever.

I really do not understand why so many people out there think that putting a huge engine in a sailboat is a good idea. It is a SAILBOAT, for God's sake! You're not going to go skiing behind it! And, of course, they think these gigantic engines make the boat worth so much more than a smaller one would when, in reality, for those who understand the drawbacks, it makes the boat worth so much less.
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Old 28-04-2016, 07:39   #5
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Re: Too many Experts

No... He's still waiting for the answer with the boat tied to the pier.

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