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prop speed

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Old 01-08-2008, 07:32 PM
  #11
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Your question was size & speed?

We ran diesels nothing else, we tried to keep prop speed at 4,000 - 4,500 rpms, which is pretty fast. The big props breaking and then re-entering the water drag the diesel down it can't recover because you've lost some boost.

Size - whatever you could spin at that speed, we liked small dia and more pitch.

At 3,400 rpm even at 1:1 you won't see big speed numbers.

Without any fancy math or fromulas, use the example of a gas engine behind a stern drive. When do start to see real speed? In the last 1,000 to 1,500 rpm of engine speed.

5,500 rpms X 1.5 reduction = 3,666 prop rpm

prop needs to operate above 3,000 rpm

The problem is getting on plane, with small props, they cavitate to no end and you have no useable cruising speed. Big props take forever and you over load the engine trying to break those blades free.

Get yourself a 2 speed and run a slight overdrive you'll have cruise and top end
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Old 01-08-2008, 07:40 PM
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Quote:
Originally Posted by HabanaJoe View Post
Your question was size & speed?

We ran diesels nothing else, we tried to keep prop speed at 4,000 - 4,500 rpms, which is pretty fast. The big props breaking and then re-entering the water drag the diesel down it can't recover because you've lost some boost.

Size - whatever you could spin at that speed, we liked small dia and more pitch.

At 3,400 rpm even at 1:1 you won't see big speed numbers.

Without any fancy math or fromulas, use the example of a gas engine behind a stern drive. When do start to see real speed? In the last 1,000 to 1,500 rpm of engine speed.

5,500 rpms X 1.5 reduction = 3,666 prop rpm

prop needs to operate above 3,000 rpm

The problem is getting on plane, with small props, they cavitate to no end and you have no useable cruising speed. Big props take forever and you over load the engine trying to break those blades free.

Get yourself a 2 speed and run a slight overdrive you'll have cruise and top end

cheers mate thats the answer i was looking for but just when you mention that i have a 2 speed first gear is 1.30-1and the second is direct

thanks again
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Old 01-08-2008, 07:43 PM
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If you can ex-change for OD gearing you'll be happier?

Last edited by HabanaJoe; 01-08-2008 at 07:47 PM.
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Old 01-08-2008, 08:47 PM
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funny enough with my set up the last 200rpm seems to give me about 6mph
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Old 01-09-2008, 10:39 AM
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the "good old days" were better because we were younger, single and didn't have 3 kids!!!

It's not that life today is bad but it is different!

As far as cut & paste - you didn't think I was smart enough to do something on my own - did you?
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