Finding data with shaft RPMs at the end of watch - Page 2 - SailNet Community

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Old 03-22-2011
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Yes. I'm used to recording hourly turns from the RPM counter. And at steady speeds over longer hauls it's probably pretty accurate for DRing with the Nav making the adjustments. Even though they record the turns I think everyone uses GPS now for distance traveled.
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Old 03-22-2011
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Originally Posted by Waltthesalt View Post
I'm used to timig a run over a measured distance (average both ways) to get make a RPM/speed table. Figure distance from the speed using the table. Calculations assuming a slip value are infreior. Slip can also change at different rpms
That's a great idea that could also be done by us amateurs, using a GPS. That way we wouldn't need to run a course, only try different rpms and let the GPS "catch up" to the rpm changes. It could really tell us if the prop had barnacles or if the bottom was getting dirty (or both).

As an aside, I'd love to make a chart showing fuel consumption versus rpm. Not exactly sure how to go about that one, short of adding a small tank to the top of my diesel inlet, then overfilling the diesel up into that tank.

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Old 03-22-2011
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Does anyone on this forum actually have a recorder that adds up the rpms over a period of time or are you just saying ran @1800 for 4 hours. How accurate is your tach and won,t rpm vary as conditions change(like prop comes out of water)
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Old 03-22-2011
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It would be easy enough to add a rev counter to the propshaft on a boat. Epoxy on one small neodymium magnet and a dummy counterweight, put a digital counter and a reed switch up next to the shaft, $30-60 and there you go, a rev counter if you can't find one off the shelf.

Actually there are plans on the web to use a simple calculator as a counter that way too. Make it $10 that way.

And...I'm still not inclined to do it, sorry Boasun. "Too Much Stuff!"
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Old 03-23-2011
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I don't think that using GPS is a good idea to determine the distance traveled. GPS calculates the distance in 3D which means upward motion of the boat due to waves is also included in the distance traveled.

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