model aircraft tachs are small handheld optical devices, generally for 2 blade props. it'd only be good for "right now" general idea of rpms. "mini tach" for briggs and tecumseh powered lawn equipment could be "fun" to set up, the engine coils fire every stroke like a 2 stroke, it'd need a strong proximity sensor, not sure how strong a magnet and coil it needs. add a briggs flywheel and coil into the mix (center hole bored to size, air fins removed?) digital display could be dash mounted might be nice, you'd know boat's speed by rpm and prop pitch with a small list after calculating. -best idea I can come up with, "mini tach" are 40-45 bucks. maybe pirate a flywheel and coil from a weedeater engine to trigger it? same can be said about reversing the math with a very low range speedometer, ballpark calculating 63360 (mile in inches) dividing prop pitch to arrive at general range rpms. miles per hour, rev's per minute.. if you already have a speedometer you can whip up a general mph = prop rpm chart. same for amps vs speed vs range ballpark calculations. --- On Thu, 2/2/12, Robert Jorgenson <falcolnxp@yahoo.com> wrote:
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