Resistance of submarines at different depths.

Discussion in 'Hydrodynamics and Aerodynamics' started by gonzo, Apr 26, 2013.

  1. gonzo
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    gonzo Senior Member

    I know about propeller efficiency, but at higher pressures it should be able to turn faster before cavitating.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Would the prop make more noise at those higher rotational speeds?
     
  3. DCockey
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    DCockey Senior Member

    Separation of a foil moving in water will not be affected by ambient pressure at any speed a submarine will travel.
     
  4. johneck
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    johneck Senior Member

    Submarine propellers do not cavitate, at least not enough to affect efficiency. They know at depth vs speed when cavitation is likely to occur and stay within the non-cav envelope.
    The resistance will change with density and viscosity but the propulsion effiiciency will also change and thus the max ship speed vs depth should not change (once they are away from the free surface).
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    On searching the web for data I was surprised to discover that the viscosity of water reduces at high pressures. This is very unusual and presumably indicates something trange about the molecular bonds of water but I'm not a chemist. It varies quite rapidly with temperature however.

    Water's density does increase, but at the depths that a regular submarine can survive it would be a fraction of a percent I suspect. I used to work with high pressure water systems up to around 300 MPA and at that pressure - equivalent to depths of 35,000 m - density increased by around 50% if I recall correctly: but it was a long time ago.
     
  6. Hampus
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    Hampus Junior Member

    Yes, it can spin faster without cavitating. The higher the pressure the faster it can go before cavitation kicks in.

    Yes, and most other hings as well. There would be increased noise levels from the engine, bearings and water streaming around the hull.

    /Hampus
     
  7. DavidJ
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    DavidJ Senior Member

    There is added drag when operating close to the surface. Along with the submerged viscous resistance there is also a component of wave making resistance. Significantly more power can be required when operating at periscope depth compared to when running deep. There was an Australian paper about this using an 80m SSK shape a couple of years ago.
    "Hydrodynamic Design Implications for a Submarine Operating near the Surface"

    From that paper:
    Speed (Knots)_______________________________5____7___10__12____15
    Effective Power When Shallow (KW) ____________40__100__290_530__2250
    Effective Power When Deep (KW)______________30___70__190_320___890
    Percentage Increase in Required Effective Power_30%_40%_50%_65%_150%
     
  8. tomas
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    tomas Senior Member

    Is there a rough rule-of-thumb to define "Deep"?
    Can it be 5, 10, or 20 hull diameters, for example?
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    jehardiman noted earlier that beyond 5-7 diameters, the sub is not making
    appreciable waves. Of course, it depends on the ambient waves too: 5-7
    diameters is not much in a huge storm.
     
  10. tomas
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    tomas Senior Member

    Thanks Leo. You just reminded me of an older thread where someone had posted about being in a submarine 300 feet below a particularly strong storm system and that it was making the sub roll quite heavily. It surprised me to read that.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    In calm-water it's quite easy.
    You can do a little study with Michlet to find how quickly wave resistance drops
    with submergence depth for different L/d ratios. There's probably a nice non-
    dimensional parameter that will collapse the results into a useful form. ;)
     
  12. powerabout
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    powerabout Senior Member

    I was tied up alongside in Bermuda once with a US sub, over many beers the guys told me once they came close to the surface to deploy the antenna and didnt know there was a hurricane in the Caribbean, he said the sub was picked up at depth flung into the air then crashed back into the water, carnage on board many broken arms and legs.
    That must of been a large wave....
     
  13. jehardiman
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    jehardiman Senior Member

    There are a lot of things that influence both of those effects. BG, appendage size and shape, relative wave direction, ships trim, water depth, wave period....big decision tree matrix. Rule of thumb is that in deep (+3000 ft/1000m) water, wave effects will penetrate to their period wavelengths and, yes, you need to consider the whole spectra with depth acting as a pass filter (i.e. spectra at depth). So beyond ~100ft, only wave periods of ~7 seconds or more need be considered. That also means however, that large storm swell cannot be avoided in most operational situations.
     

  14. tomas
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    tomas Senior Member

    Thanks jehardiman. That is clear and helpful.
    I was not conceptualizing it correctly.
     
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