Speed Vs Hp Math

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Dave Ullrich

OK, I'm VERY bad at advanced math. I'd like my Cat 30 to cruise around 6.5 knots under power with power leftover to pound into a headwind. The boat weighs about 11,000 pounds. It has a waterline of 25 ft. This should provide a hull speed of about 6.7 knots, so my idea is possible with enough power. So how much power would I need? Right now I have the little 11 hp Universal 5411 and a 2 blade prop and it provides a cruise speed of about 5.5-5.7 knots and a top end around 6.0-6.3 knots. But from what I've seen here, others with a LOT more power don't go any faster than I. Is something wrong with their setup, or do you just need a huge power increase to create a small increase in speed?
 
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Justin Wolfe

My experience

Dave, I have a similar boat. 32' with a 26' waterline and 11,800 lbs. We have a Yanmar 27hp, 3GM30 and get 6.4 knots with a 2 blade prop in flat water at 2800 rpms (our cruising rpm). So I would suggest that you would get virtually the same boat speed if you had a 3GM30 too. What does a brand new 310 & 320 have for an engine? That is probably a good choice too.
 
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Dick McKee

Don't forget your final drive ratio

It's kinda hard to compare if you dont know the final drive ratio of the transmission. I was suprised to find out that the Yanmar 4JH2E in our 430 had the possibility of 3 final drive ratios from the factory. Those others you refer to with more power might have a lower final drive ration or a lower pitch on their prop...Hope this helps Dick McKee S/V Constellation
 
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