Underwater speed?

[kellyseye]

How about doppler? Modern speed instruments (for deep sea vessels where you can't get an echo back from the bottom) use doppler (iirc at around 2MHz) - you reflect off the bottom. It might work using ultra sonics (think echos sounder pointing 45 degrees forwards).

[Ramius]

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

[AvionicsMaster1]

Holy macaroni! That's bigger than a few compact cars I've rented.

I'm more familiar with aircraft and they do reference airspeed to altitude(pressure density) but mostly to find Mach. They reference airspeed to temperature to find true airspeed. Temp would be easier than a differential pressure to sense I think. Would water density or (there has to be a term for the column of water above the item) induce much of an error? I guess I assumed a local pond or small lake not the ocean.

What depth is absolute maximum? Please don't tell me the bottom of the lake. (It's an old submariners joke.)

[Ioannis]

I wonder, how do you control the sub when it is under water line?

Ioannis

[languer]

The sub itself is 8" in diameter, 6' long, and 1:100 scale.

If you don't mind, I would love to see pictures of this. And of course the method you choose to perform the speed measurement under water.

[Ramius]

I think that the max depth is about 50'? There is a pressure sensor to prevent the sub from going too deep. R/C subs operate at 54 MHZ which has little difficulty penetrating water. The video camera system for the periscope operates at 1.2 GHZ. FYI 2.4 GHZ is the frequency for microwave ovens so to use this frequency would result in you only warming the water around your antenna! I work at Fox Television and have a ham radio license so a friend of mine and I designed an antenna that sends the video in an up direction only so the periscope will work underwater which has been tested. Every 33' the water pressure doubles from surface pressure so if you only used one pressure sensor you would be seeing the depth as speed when stationary. I think differential pressure is the way to go, water pressure on the bow as you move minus the pressure at depth. What I do not know how to calculate is velocity. Pictures are similar to mine.

Thanks to all and best wishes, Ed

[languer]

That is quite awesome. Could you have some sort of towed array to serve a GPS antenna? That way a GPS module can give you the speed, position, etc.

Another method I saw was to use two ultrasonic transducers; one on front and and one on back. The time for a pulse to reach the rear transducer through the water will be a fuction of the speed of the boat (think SONAR).

[Ioannis]

Very impressive! Like it a lot!

Ioannis

[Demon]

What about 2 pressure sensors front and back? The difference tells you velocity.

Robert