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Thread: How to measure outboard RPMs?

  1. #1
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    How to measure outboard RPMs?

    Is there an easy, cheap way to measure the rpm of my outboard while motoring? I want to check the appropriateness of my prop's pitch for this boat? The outboard is a Tohatsu 8 (2-stroke). Is there an easy way to hook up a tach, or am I better off using some sort of strobe? Cheap would be nice.

  2. #2
    Seglare Sven's Avatar
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    I used to borrow a tuning-fork contraption to measure rpms on an ourtboard as a kid. It was a professional instrument, kind of looked like a comb with different length teeth. The teeth that were in tune with the vibration would really vibrate and it was easy to see the rpms by reading them off the scale next to the teeth.

    I have no idea if that device is still made but it was a beautiful engineering solution to a very common problem.



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    Yep. They are still available. It is called a "Sirometer" and is available from "Tecumseh". I bought one a couple years ago for about $20 to use on my tractor. It is part no 670156.
    jerry in Kingston, WA

  4. #4
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    Stewart Warner makes a contact dial Tachometer that reads to 4000rpm for about $60. It will actually read to at least 6000 rpm as the needle just goes past 4000 and counts up again. It will read any spinning object (Crankshaft) as long as you can put the rubber tip on the center. I have checked mine against my digital tach and its very accurate. Its great to get the electric tach on the diesel calibrated, also cool to check alternator speeds at different engine rpms.

    http://www.stewartwarner.com/Catalog/tachhand.html
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    Ben Warner
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    E38 #61
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  5. #5
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    That's pretty cool looking Ben.

    I thought the VibroTach was so slick that I ordered one. I'll see how it works. I love clever mechanical stuff like that. $14 too!

  6. #6
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    Nate ,
    I thought the tuningfork idea is very cool. I'm curious how the number of cylinders/2 stroke/4stroke affects the vibration and reading? Interesting technology...
    Ben Warner
    Pau Hana
    E38 #61
    San Diego, CA

  7. #7
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    The hand held direct reading tachs like the Stewart Warner are neat, but can be dangerous. My friend was checking an outboard while we were zipping down that bay at 35 mph hit a wake and you guessed it. Got his hand into the flywheel. Lots of blood but he healed up A-OK. My German Sirometer requires you have some idea what the speed is, then you can get a reasonable good reading.

  8. #8
    Contributing Partner Art Mullinax's Avatar
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    Non Contact style

    Shimpo makes a good non contact style tach. Place a piece of reflective tape or white out on the flywheel and point the tach and read the RPM.
    Art M.
    71/E29
    S/V Issaqueena

  9. #9
    Moderator Guy Stevens's Avatar
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    Or Make your own

    This site will sell you a kit, some soldering required for $39.00 to make your own non contact light based Tachometer.

    http://www.dsgnspec.com/Tachjr.html

    or search on e-bay for Tachometer Non Contact, and the one all done for $40.00

    Or go old Tech and get the Sirometer here: for $18

    http://shop.briggsandstratton.com/BS...D-ED87573E9F4E


    Guy
    :-)
    Last edited by Guy Stevens; 02-24-2006 at 12:42 PM.

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