Underwater speed?

[Demon]

Formula for velocity:

Follow sub in a boat at max speed, look at knots on gps, note reading from flow meter.

Half velocity = 50% knots, etc.

Or is that too simple?

Robert

[NickMu]

Hi Ed,

I knew it is not an easy task to get such system going especially when in my application the transmitter will be above and bellow water at times. I will try some other avenues first and if no success I might bother you with some more questions.

I will be away from my office for a few more days so I do not have access to my notes.
For our test we just calculated the speed of the jet and the volume of the water passing through in a time period. With the water density as a constant we were able to calculate the thrust.
In your case I’m quite sure you will need at least your differential pressure, the surface of the sensor (maybe) and the water viscosity. I will look into getting you a formula combining all the factors from which you can extract the speed as the only unknown.

What will be an acceptable size for the paddle wheel? The one I used was a little over 1” diameter and it is used on Sea-Doo PWC for speed indicator. It gives 4 pulses per revolution and it is a completely sealed unit wit three wires interface (power in and signal out). If you are interested I can send you one as soon as I get access to it. Just PM me your address and you will have a possible back up solution.

For the differential pressure method how hard will be for you to take a couple of readings at half and full speed to see if your numbers will make some sense. From that it will only be a matter of coding.

Regards,

Nick

[NickMu]

What you are dealing with is the case of dynamic pressure in fluids. One of your sensors will read the static pressure and the other one will read the total pressure. The difference will be your dynamic pressure. Using the formula: P = q*V² you can extract your speed.
P= dynamic pressure
q= density of water
V= speed
So you have your readings for dynamic pressure and you have your water density (I’m not sure if you have to compensate for temperature again since your pressure readings are already compensated) and you can calculate your speed.
I had the chance to do some Google search and I found this:

http://www.engineeringtoolbox.com/dy...re-d_1037.html

HTH.

Regards,

Nick

[NickMu]

One mistake on my formula:
The left side should be multiplied by 2 or the right side should be divided by 2.

Regards,

Nick

[AvionicsMaster1]

Just sayin', that's what I said. Even used the same link. Compensation for density in same post. Number 18 if you'd care to peruse.

[NickMu]

Avionics,

I can not explain the duplicates of your suggestions in your earlier posts other than I started my input to this topic late and I did not carefully read the posts before my question to Ed about the RC system.
I felt that I should share the little information I had but the last thing in my mind was to take credit for something that you already posted. Ed asked for a possible formula and, without reading the earlier posts, I gave him the best I could find just to repay his generous information exchange about RC system.
Please accept my apologies for that.
If one positive thing will surface from this is that it could be Ed’s best solution since two separate people thought the same solution which is an “engineering nonsense”. You will always have n+ 1 solutions from n designers. Each one has a different solution and the +1 is the one that should be used.

Again, it was just my superficial attention to this topic and I’m sorry for it.

Best regards,

Nick

[Ramius]

Thanks Nick!
If it is 1" is that the body of the paddle wheel or the pipe inlet size? The reason I am asking and trying to keep everything as small as possible is that the bow are is starting to become very crowded as this is where the torpedoes and missles are plus a mechanism to deploy and retract the bow planes!

Ed

[NickMu]

Hi Ed,

The sensor I used does not have your traditional cylindrical shape.
Just to have an idea of the way it looks go here:
http://www.jets24.com/bombardier-brp...m_store=german
Also if you search Sea-Doo part 278001998 you might find more pictures to get a better idea. A view from the oposite side will give you more information.
The paddle wheel is about 1 ¼” diameter. The mounting plate is about 2”x5” but you can cut most of it off. With a little creative engineering you can hide most of the body and expose only the tips of the paddles.
It is used but in working order. I will send you pictures in a few days when I will get back to work.
My offer still stands but it will take few more days until I will be able o mail it to you.

Regards,

Nick