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Old 06-17-2013, 06:30 PM   #21
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Baker-all you need to do is check out a prop calculator. If I take my boat and do the calculation as is, with continuous duty rated JD's, then change only the rating on the engine (raising HP and raising Max RPM to JD's max rating on our engines), there is no appreciable change in either the diameter (about 0.1") or pitch (about 0.3"). On a full displacement hull, this should not be surprising. It takes a calculated 209.5 HP to move our hull at hull speed, it does not matter whether that HP is produced by a 154 HP engine or a 225 HP engine (the lowest and highest ratings on our JDs). Since all else remained the same in the above, including the reduction ratio, it should take the same size prop at the same prop rpms to move the boat. One interesting note, the 209.5 HP is 72% of our HP available at the shaft. With the higher rated engine, the 209.5 HP is less than 50% of the HP available at the shaft-a lot of wasted power.
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Old 06-17-2013, 09:45 PM   #22
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That is what I suspected!
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Old 06-18-2013, 06:27 AM   #23
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One interesting note, the 209.5 HP is 72% of our HP available at the shaft. With the higher rated engine, the 209.5 HP is less than 50% of the HP available at the shaft-a lot of wasted power.

Its not "wasted power" , the reduction is what is required to allow a longer power on rating.

Big difference between 24/7/365 and a picnic boat rating for a couple of hours a month .
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Old 06-18-2013, 10:39 AM   #24
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209 is less than half of 225?

It doesn't matter if 209 HP is delivered by a 154 HP engine?

What am I missing?

Do you guys know the difference between brake horsepower and a propeller curve?

If you have a boat that takes 209 HP to move it at (for example) 10 knots and then repower it with a 150 HP engine (and keep the same prop) you are not going to see 10 knots again. Thottle your existing 209 HP engine back to the 150 HP mark on the propeller curve and let us know what your boat speed is.
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Old 06-18-2013, 10:53 AM   #25
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I think it depends on the oil. Synthetic or fossil?
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Old 06-18-2013, 12:50 PM   #26
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RickB-I was less than clear-we have twins. The calculations were done on that basis. Thus, 308 total HP (and 450 for the alternative calculation). The prop calculator I used calculated a 5.5% mechanical HP loss from engine to shaft, so SHP of 291. It was interesting to me that the continuous duty engine uses a higher % of available SHP than the higher rated engine.
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