Pressure sensors for measuring pressure field on the bottom of the planing hull?

Discussion in 'Hydrodynamics and Aerodynamics' started by Hildershavn, Oct 20, 2017.

  1. Hildershavn
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    Hildershavn Junior Member

    I know there is an old thread with the same name. It is too old to reply to.
    anyway,

    I am involved in a project where we need to make pressure measurements on the hull of a planing leisure boat 30-45 knots.

    I am looking for information from anyone with relevant experience. I need to find good sensors to use and how to install them as they should not be affecting local flow on the hull and mounting should be easy and as little intrusive as possible.

    Since it is a planing hull, I have been told I should use temperature compensated sensors. Besides this I do not know anything with regards to correctly selecting sensors.

    The boat will run in seawater and have perhaps 15 pressure sensors.

    Any help will be much appreciated.
     
  2. DCockey
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    DCockey Senior Member

    Do you need sensors which mount to the hull surface, or can you put small holes through the hull surface to connect a pressure measuring system?
     
  3. M&M Ovenden
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    M&M Ovenden Senior Member

    Hi,

    I've been involved in a project with pressure taps on an aircraft wing. A fairing that contained the transducers was slipped over the leading edge and secured to existing hard points. Maybe you could do something similar on your boat to avoid drilling multiple holes.

    Cheers,
    Mark
     
  4. CDK
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    CDK retired engineer

    MS5837-30BA from Digikey is a tiny gel filled sensor that can be installed in a 3 mm hole, sealed with an O-ring. It has an I2C interface, so a large number of sensors can be connected in parallel on a 4 wire system. It is intended for underwater use, but the part that has the seal is only specified as "metal". I could not find any reference to seawater.
     
  5. Hildershavn
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    Hildershavn Junior Member

    Well, ideally the sensors would mount on the outside. Then it would be easier to sell the boat to end user after trials. However, drilling holes is a probable option if the sensors would be better/more accurate.
     
  6. Hildershavn
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    Hildershavn Junior Member

    Thank you for this interesting input. I see only one problem with these sensors, though I would love to use them for their small size and simplicity. The gel-membrane seems to be recessed slightly. I am afraid this would make sensor itself affect the flow and pressure locally. Agree?
     
  7. Joakim
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    Joakim Senior Member

    I don't think it is possible to measure pressure accurately without making a hole to the hull. Even though the sensors can be rather small (fits inside a wristwatch) they need a cable (or battery + wireless) and protection from water. So you will end up with a lump under the hull and you will measure something else than the pressure you would like to measure.

    Also the sensor will need a reference pressure (unless it is an absolute pressure sensor), which is not available under the hull. Absolute pressure sensors have the problem that they have 1 bar to start with and thus their resolution and accuracy is limited at low pressures. But at 30-45 knots you will see rather high pressures anyway, so that's not a real problem.
     
  8. tom28571
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    tom28571 Senior Member

    I have wondered about this problem with measuring water pressure under the hull of a boat. I agree with those who say that just about anything under the hull that is exposed to water flow, especially on fast moving boats, will be impacted by the water flow, making the reading higher or lower depending on the local condition. I don't see drilling holes as the answer either as the hole can introduce errors as well. Bernoulli at work in all cases as well as temperature variation .
     
  9. gonzo
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    gonzo Senior Member

    Strain sensors could be used to calculate pressure. They can be calibrated by applying a force on the hull.
     
  10. DCockey
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    DCockey Senior Member

    Standard practice in wind tunnel and tow tank testing to measure surface pressure is small holes with a sharp edge which is perpendicular to the surface, and the small holes are connected to pressure transducers. The pressure in the hole will be the same as the pressure of the air or water going past the entrance to the hole.

    Assumptions are needed about how much the hull deflects at the strain gauge locations in response to assumed pressure fields to deduce surface pressure from strain gauge measurements.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the purpose of getting this data ?
     
  12. Hildershavn
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    Hildershavn Junior Member

    Basis for quasi-static structural calculations. Ultimately the goal would be to see if one could use lower panel pressures than the standards dictate.
     
  13. Hildershavn
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    Hildershavn Junior Member


    I think this is more suitable for steady-state tests. I am looking for an accurate way to measure pressures when driving at high speeds into waves etc. I have been looking at a pressure scanner, which is essentially a manifold with sensors from which a network of liquid filled tubes extent out to wherever you want to measure. Talking to the supplier, they admit that the length of the tubes and the liquid inside them will provide some dampening of the pressure, thus making it more suitable for steady-state.
     
  14. Joakim
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    Joakim Senior Member

    For this purpose a sensor making a lump under the hull doesn't matter, since the dynamic pressure due to speed is small compared to the highest peaks of slamming to the waves. You need to be more conserned about the frequency respond of the measuring system. Most pressure sensors and measuring systems are targeted for slow pressure changes.

    Since you are ultimately interested in panel sizing, it would seem more sensible to measure the deflection of the hull.
     

  15. Hildershavn
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    Hildershavn Junior Member

    Thank you for that reply. Well said, I need an absolute dynamic pressure sensor with temperature compensation. What you think should the frequenzy response should be? I thought something in the vicinity of 750H (with reference to earlier studies/reports).

    We are going to measure deflection also, for the sake of validating quasi-static FE analyses of the hull. We need pressures for future reference when doing our preliminary design of GRP.
     
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