folks;
im trying to measure the rpms on my marine deisel engine by some means by measuring it on the flywheel. i had something of a brainwave yesterday and i was wondering if you folks could help. im thinking about buying one of the non contact tachs for rc planes and using it on the flywheel if i used a sharpie marker and made a thick line across the center (to act as the "prop"). the flywheel is about 3.5 inches across, and the absolute max it will be spinning is 3000 rpms. will this work or will i need to paint the whole flywheel for better contrast, or will it even work then?

I think you would have to cut two slots for it to get a reading. Have you thought about the deal that checks rotor speed on a heli? I dont know what it's called. It has a rotating disk inside of it with two slots. You look through it at the rotor blades. There is a wheel on it you turn that speeds up and slows down the disk inside. You simply adjust the speed of the disk until it apears the rotor head has stopped. There is a digital readout on it that tells you the speed the wheel is turning. If they wheel and the rotor turn at the same speed it looks like it's stopped. Pretty neat deal. My buddy says his cost about $90. Kind of high but a cool toy. Later David

If you can borrow one try it out. Make the mark wide and make each "side" of the mark come from the center of the flywheel out. Meaning you'll actually wind up with a mark that's kind pie shaped instead of one line.

If you can shine a really bright light on the flywheel to help with the contrast (which is what the photocell sees on the tach). Remember you'll have to do some math using a 2bladed prop tach or put 2 marks opposite each other on the flywheel.

Can't hurt to try if you borrow one instead of buying it.

paint the front of your flywheel white and then make a big black stripe across it. The photocell in a tach sees the light changes/impulses. That is how some ducted fan guys chech rpm.

I have done it with two pieces of white tape. You may need to use a bright lite to get the reading stable.