A marine pitot is about $6.00 and the head unit is about $30. Try and buy the hardware just to mount an aircraft pitot and head unit for that money. There also isn't a lot of trash in the sky.
What you are looking for is the dynamic pressure which is:
q=0.5*rho*v*v
q=dynamic pressure
rho=density of the fluid, in this case water. This is 1.94032 slug/ft^3
v=velocity in ft/sec (55 mph=80.7 ft/sec)
so, the dynamic pressure is 0.5*1.94032*80.7*80.7=6318 lb/ft^2
6318/144=44 psi
The density given is for pure water and is a bit different depending on how much stuff is in it.
My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .Pivot type speedometers are notoriously inaccurate no matter that you do.
My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .
Also, all need to remember that GPS only gives you speed over the bottom, not speed over the water . . .
Marine pitot tubes are also straight pitot tubes. They measure total pressure. And then the gauge I believe is calibrated to show a speed. This can be off if the static pressure is different from what the gauge was calibrated to. Most aircraft use a combined probe called a pitot-static tube or Prandtl probe to measure airspeed. This is more accurate because it takes into account (and measures) both static pressure and dynamic pressure.
So in your case the 55psi you measured is total pressure. To find the dynamic pressure you would need to subtract what ever the static pressure of the water at your location. You can then use 109jb's dynamic pressure equation to work backwards to check the accuracy of your gauge.
My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .
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... On my boat, the pitot is only about a foot below the surface when not on plane. With fresh water at about 8 lb/gallon and 231 cu inches to a gallon that is a static pressure of about 0.2 psi. It can pretty much be ignored. Therefore, simply applying a gauge pressure equal to the calculated dynamic pressure should yield the calculated speed on the instrument at least at higher speeds.
BTW. I see you live in Lancaster. Do you by chance work for Lockheed? I used to work at LMTAS in Ft. Worth.