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Old 06-25-2017, 09:14 PM   #1
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Shaft RPM?

I used a simple handheld digital laser tachometer to verify my engine RPM gauge. However, the handheld (pointed at the shaft) showed an RPM reading (691) that appeared to be exactly half of the engine RPM (1400). I can't imagine that my engine gauge is off by that much, but is there something else going on here? Does my Velvet Drive transmission lower the RPMs to the shaft? Or do I just have a problem with my handheld?
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Old 06-25-2017, 09:28 PM   #2
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Sounds like you have a 2:1 transmission ratio. Very common.

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Old 06-25-2017, 09:32 PM   #3
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On your Velvet Drive there should be a plate with the ratio number on it, ours is "Ratio E-2.5".
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Old 06-25-2017, 09:45 PM   #4
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Shaft RPM?

I have a single Lehman 120 with a Borg Warner Velvet Drive transmission. Further research shows my model has a 2.1:1 ratio, so gauge was pretty close. I hadn't realized the prop was rotating so much slower than the engine.
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Old 06-26-2017, 05:23 AM   #5
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Folks that like rules of thumb ,

A shaft spinning at 1000RPM can use the prop pitch in inches to get the boat speed in Knots , with a prop with no slip.

No slip is not possible , even on hydroplanes, but with the rule you can easily work out how much slip the boat has.

1000 shaft RPM 22 inch pitch 22K theoretical would see 11K at 50% slip.

50% slip is not uncommon.

Great fun for those that enjoy marine mental masterbation.
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Old 07-08-2017, 03:45 PM   #6
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Engine RPM divided by transmission reduction ratio = shaft RPM

Shaft RPM X propeller pitch divided by 1056 = theoretical speed in MPH (no slip)

Shaft RPM X propeller pitch divided by 1215 = theoretical speed in KTS (no slip)

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