prop tachometer and sub speed sensor

[tk-7642]

I am interested in knowing how well different prop systems work on my sub, and was wondering if there are prop (or shaft) tachometers out there for sale and speed sensors for sub speed. anyone ever seen these?

[rpm]

you might be able to

you might be able to use a RC airplane tach for RPM's, they sense 2 and 3 bladed propeller blades, so if you have a 3,4,6,8,9 blade prop you could find the rpm by division. Although the small dia of boat props might give weird readings if you dont hold it in the perfect spot.