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Old 09-12-2009, 06:45   #1
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Props...

Does anyone know the difference between larger props turned with higher torque diesels and smaller props turned at higher speeds on ouboards???

Is it safe to assume that a smaller prop turning at a higher speed can move a similar amount of water through the disc and thus produce a similar amount of thrust???

Thanks in advance.......

S
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Old 09-12-2009, 08:09   #2
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you prop-heads please jump right in there I am looking foreward to this one myself..
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Old 09-12-2009, 11:07   #3
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There are books written on the subject and you might want to check some of the threads posted on proper prop size using the search engine at my signature.
"Skene's Elements of Yacht Design" is a good start.
The short answer to your question is that there is a difference. There is more efficiency in larger props turning more slowly but each prop has to be mated with factors concerning displacement of boat, engine and transmission.
regards,
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Old 09-12-2009, 13:52   #4
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I've found Dave Gerr's Propeller Handbook to be a useful reference.... it's a fairly inexpensive book, if this is something that interests you. Parts of it are also on Google Books.

Here's my short (but slightly longer than John's) answer to your question, w32honu:

The efficiency of a propeller is closely related to a coefficient known as Bp. Bp is the square root of the shaft horsepower, times the shaft RPM, divided by the 2.5 power of the speed at which the propeller is advancing through the wake (a bit less than the speed of the boat).

Smaller Bp makes for a more efficient prop.
Power = force times distance over time. That is, power = thrust * boat speed.
Larger prop diameter means lower RPM to absorb the same power, and vice versa.

For a constant shaft horsepower, you can see that there will be many combinations of RPM (thus diameter) and advance velocity that will give the same Bp (ie, for props of identical pattern, the same efficiency).

Let's picture a large diameter, slow turning prop moving at a low advance velocity that has the same efficiency as a smaller diameter, faster turning prop at a higher advance velocity. The same power is being transmitted to the water in each case.

But the thrust is not the same: the large prop is producing a lot of thrust at low speed, while the small prop is producing relatively low thrust at high speed. Thus, we start to see why a slow tugboat's 500 hp engine is turning a prop the size of a man through a giant reduction gear (high thrust at low boat speed) while a speedboat's 500 hp engine is turning a much smaller prop at much higher RPM (low thrust at high boat speed).

There are, of course, many other factors involved, but this should cover the general idea....
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Old 09-12-2009, 15:42   #5
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Thanks guys,

I appreciate the input. I will have to ponder the above technical data and perhaps crunch a few numbers. I do have a copy of "Skene's" and I it seems that I have run across some of Dave Gerr's work.

My interest is trying to "ballpark" the relationship of an inboard diesel and an equivalent outboard.

Would a smaller h.p. outboard provide a similar amount of thrust (smaller prop at higher rpm) as a larger h.p. Deisel at a slower rpm??

Would a 6h.p. outboard equate to an 18h.p. deisel?? ....etc.

Thanks,

Steve and Jules

s/v Honu
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Old 09-12-2009, 17:00   #6
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Thanks guys
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Old 12-12-2009, 12:54   #7
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Quote:
Originally Posted by w32honu View Post
Thanks guys,
Would a smaller h.p. outboard provide a similar amount of thrust (smaller prop at higher rpm) as a larger h.p. Deisel at a slower rpm??

Would a 6h.p. outboard equate to an 18h.p. deisel?? ....etc.
In general, a larger diameter, slower turning prop will produce high thrust at a low boat speed. Similarly, a small diameter, fast turning prop- if it is absorbing the same power as the big prop- will be best for producing lower thrust at higher boat speed.

If you try to use the small, fast prop to push a large, heavy, slow boat, the advance velocity (the speed at which the prop is moving through the wake) is very low, while RPM is high. This combination produces a high Bp, and thus a very inefficient prop. Most of the power is wasted thrashing the water around in circles, with relatively little power going towards useful thrust.

A large, slow-turning prop on the same large, heavy, slow boat will have the same advance velocity and the same shaft horsepower, but the lower RPM allows a relatively low Bp and thus a more efficient conversion of shaft power to useful thrust.

If you have a light, fast boat, the advance velocity is high, so a low Bp can be achieved with a relatively high RPM. Steep reduction gears and large diameter props no longer have an advantage, as we move to lighter, faster boats.

A propeller, frankly, doesn't know or care what's on the other end of its shaft. It could be petrol, gas, electric, hamster wheels. All that matters, as far as the prop is concerned, is the torque on the shaft and the speed of the shaft (and, therefore, the power transmitted through the shaft). With a suitable gearbox, any 100 hp engine can be made to look like any other 100 hp engine, as far as the prop is concerned.
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