Shaft RPM?

schrater · Jun 25, 2017

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?

DavidM · Jun 25, 2017

Sounds like you have a 2:1 transmission ratio. Very common.

David

rochepoint · Jun 25, 2017

On your Velvet Drive there should be a plate with the ratio number on it, ours is "Ratio E-2.5".

schrater · Jun 25, 2017

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.

FF · Jun 26, 2017

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.

DougR · Jul 8, 2017

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)

DougR

Shaft RPM?

schrater

Senior Member

DavidM

Valued Technical Contributor

rochepoint

Guru

schrater

Senior Member

FF

Guru

DougR

Member

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