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  #1  
Old 03-18-2008
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Horsepower to Displacement?

We're about to repower our 31 foot Pacific Seacraft Mariah. Most likely with a Yanmar, I'm just not sure about which model.

How many HP would you suggest for a 16,000 pound boat? The current engine is only 18hp. Do you think 21HP would suffice, or would you jump to 27?

The boat is new to us, I didn't get the chance to evaluate the 18.

Thanks!
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Old 03-18-2008
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If you use your engine to get in and out of tight spots like marinas, docks, crowded mooring fields, etc. or use it to get home after the wind dies than 21hp is plenty. If you rely on it in bad weather when you no longer wish to sail than you might want more horsepower.
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Old 03-18-2008
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I have a 10,500 lb 38 footer, and have a 3GM30, which is advertized as a 27 hp, but for all practical purposes is a 24 hp Yanmar. Its fine in most conditions but in a short chop and strong current or headwind is about as little as I would want. The Mariah has more drag (so needs more in strong currents) but less windage (so is a little better in a strong headwind) than my boat would be.

Jeff
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Old 03-18-2008
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I just read a few days ago that you should have 2HP for every 1000 lbs. of displacement....so according to that, you need a 32 HP....
I'm a relative rookie, so It's not that I KNOW, but just passing on info I read that sounded good to me....Willy
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Old 03-18-2008
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I guess I'm over powered with 14,000 and 50HP then. I sure needed ALL of it getting off a Lee shore in 50+ once though so I guess I'll keep it.
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Old 03-18-2008
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Quote:
Originally Posted by Jeff_H View Post
I have a 10,500 lb 38 footer, and have a 3GM30, which is advertized as a 27 hp, but for all practical purposes is a 24 hp Yanmar. Its fine in most conditions but in a short chop and strong current or headwind is about as little as I would want. The Mariah has more drag (so needs more in strong currents) but less windage (so is a little better in a strong headwind) than my boat would be.

Jeff
Current shouldn't have any effect on engine horse power. If the water is moving (current) then you simply have to have a boat that goes faster than the current. The limitation will be hull speed unless your engine won't drive the boat at hull speed.

Headwinds have a huge effect.

I have a 14,000 lb boat with a 2QM20 (20HP) and it is fine. I need to have a small bit of main up and motorsail when the wind is over 40 knots or I barely make any way to windward (unless I just sail). I heard that the traditional rule of thumb is about 3 HP per ton but most modern boats have more because they motor more often in bad weather than in the past.
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CharlieCobra, While shopping for boats I ALWAYS look for more HP than is normally available. I'm sure there are occasions where 200 HP would come in handy.....nothing worse than seeing a dock coming at you faster than you can backpaddle!
Willy
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Plumper, now that I think about it you are precisely right in the case of the Mariah 31 with her full displacement hull.

In the case of my boat with its semi-displacement hull, the higher hp results in speeds over theorical hull speed.

Jeff
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Quote:
Originally Posted by buckeyesailor View Post
CharlieCobra, While shopping for boats I ALWAYS look for more HP than is normally available. I'm sure there are occasions where 200 HP would come in handy.....nothing worse than seeing a dock coming at you faster than you can backpaddle!
Willy
In this case it was a twenty foot tall breakwater. It was so bad, the ferry couldn't get more than 200' off the dock and went back. We were looking for the hole and had gone around the point downwind. It took a bit but we motored back out until we found it and surfed through into the marina.
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Rockter will become famous soon enough
A 35 hp Volvo moves my 11 ton long keeler reasonably well.
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