Wow!
So many great ideas! Thanks! I think that maximum speed would not be more than say 10 mph? I occured to me on the way home that if you look at the water pressure on the bow you would have to subtract the depth reading for as you go down the pressure on the bow sensor would be reading the water pressure at that depth! Doppler sounds good and wouldn't the return become shorter the closer to the bottom you get? Robert maybe great minds think alike? I put together basically a motor to turn a shaft and on the end of the shaft I can mount the pressure sensor a given distance from center to calculate distance travelled. There is also a timer to tell how long it takes for one revolution. The sub itself is 8" in diameter, 6' long, and 1:100 scale. Mounting a pressure sensor just inside one of the torpedo tube openings should not allow any air to accumulate.

Best, Ed