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Old 03-07-2010, 12:14   #1
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Propeller Power Curve ?

Another thread on here got me looking at the power curves of my Yanmar 4JH3-HTE, which I always thought of as producing 100 horsepower @ 3800RPM, or somewhat less continuous at 3700RPM.

To my surprise I noticed that the crankshaft power output curve, and the similar shaft output curve, told a different story, with peak power of more than 100 hp and at lower RPM. The rated power of the engine is propellor power curve, which goes straight up to 100hp at redline.

WTF?

The curves diverge greatly. At 2200 RPM, the propellor power is less than 20 horsepower, while 50 are being generated at the shaft. Shaft and propellor power converge at redline.

Anybody have any clue what all this means?
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Old 03-07-2010, 12:20   #2
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Your not confusing the torque curve with the horsepower curve? Sometimes they are printed on the same graph.
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Old 03-07-2010, 12:32   #3
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Don MacPherson does a good job of explaining the power curves

Post #4 ➥
Prop power curve - Boat Design Forums

See also ➥ Propeller Loading

Or Google < propeller power curve >
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Old 03-07-2010, 12:51   #4
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Quote:
Originally Posted by GordMay View Post
Don MacPherson does a good job of explaining the power curves

Post #4 ➥ Prop power curve - Boat Design Forums

See also ➥ Propeller Loading

Or Google < propeller power curve >
Thanks; very useful.

To summarize what I think I understood from the referenced materials:

"Propeller power curve" is the entirely hypothetical amount of power put to the water by a hypothetically perfect prop, based on certain laws of physics applied to the power available at the shaft.

The author of the first note denigrates the usefulness of the propeller power curve, but I think it is very useful as a guide to how the real power develops versus RPM. The fact that real propellors are not hypothetical perfect propellers hardly matters.

The one mystery which remains in my mind is why Yanmar would rate the power of their engines based on this hypothetical propeller power, where the absolute number doesn't mean much even if the curve is useful.
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Old 03-07-2010, 12:58   #5
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I found this book helpful when re-engining a boat. It also explains everything forward of the propeller from engine curves, displacement, hull forms to lots of other material related to a boats entire propulsion system.

Amazon.com: The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers (9780071381765): Dave Gerr: Books
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Old 03-07-2010, 13:06   #6
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Originally Posted by David M View Post
I found this book helpful when re-engining a boat. It also explains everything forward of the propeller from engine curves, displacement, hull forms to lots of other material related to a boats entire propulsion system.

Amazon.com: The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers (9780071381765): Dave Gerr: Books
That looks great! I just ordered it.
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