Wave Resistance on Submarine vs Surface

Discussion in 'Hydrodynamics and Aerodynamics' started by zstine, Jun 17, 2022.

  1. zstine
    Joined: Sep 2013
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    zstine Senior Member

    After a bit of study, I thought that a submarine or torpedo would not exhibit a drag hump at the 'hull' speed, compared to a surface ship, due to lack of wave resistance. So I developed a torpedo with a very thin strut in FreeShip (freeship can't run drag calcs if vessel isn't surface piercing, hence the need for the strut). I also made a surface ship of the same length (16ft) and displacement (700lbs). I expected to see a more smooth parabolic curve for the torpedo compared to the surface ship which I expected a pronounced hump at the ~5.5knt speed. FreeShip uses Holtrop 1988 method, which I'm unfamiliar with.
    Here's the results... not really what I expected. Obviously the <1 inch wide strut is coming up with a much larger wave drag at 3knts than expected, but the surface ship hump at 5.4kn is relatively small. I was expecting to see a larger hump in the surface ship, possibly where the torpedo would have less drag due to lack of wave resistance. But the submerged vessel with the added surface area is showing higher drag at all speeds.. Food for thought.
    upload_2022-6-17_17-24-24.png
     

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  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The Holtrop-Mennen method does not work for this type of hull. Neither for the underwater hull nor for the "normal" hull. No wonder you get weird stuff.
     
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  3. mc_rash
    Joined: Aug 2020
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    mc_rash Junior Member

    Also, a submarine needs to be submerged to a certain depth to seperate from wave making.
     
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  4. Doug Halsey
    Joined: Feb 2007
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    Doug Halsey Senior Member

    Exactly! Here's some data on that from Hoerner:
    WaveDrag_SubmergedBodies.jpg
     
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  5. AJB
    Joined: Jul 2021
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    AJB Junior Member

    Good you all...
    Doug, for the unwashed, what would be benchmark submergence for a (say) 2m x 10% chord bulb?
     
  6. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Typically, 3X it's diameter...
    So 60cm (0.6m).
     
  7. zstine
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    zstine Senior Member

    unfortunately, it is the only tool I have. And while there are several parameters that are out of range for the submerged design, only Bwl/T is out for the 'normal' hull. So when you say 'it doesn't work', what i believe that really means is 'your result will have error'. that is of course true with all analysis. I wonder what the error is compared to reality, 10%, 20%? sometimes you have to cut wood with a butter knife. Work or Not, I'm convinced that given a length and displacement in this realm, the surface-running hull is going to have less resistance at almost any speed than a slightly submerged torpedo body. Would you say that statement is in error?
    "Holtrop and Mennen's method is arguably the most popular method to estimate resistance and powering of displacement type ships. It is based on the regression analysis of a vast range of model tests and trial data which give it a wide applicability"

    Depth issue noted. If motivated, I will increase the draft and rerun the analysis
     
  8. mc_rash
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    mc_rash Junior Member

    I would not say that the resistance of a submarine running near surface is bigger than the resistance running at surface is an error, it's true. The submerged submarine will have wave making resistance but also it has much more wetted surface area than the surface running submarine which results in a larger frictional resistance.
     
    Last edited: Jun 19, 2022

  9. jehardiman
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    jehardiman Senior Member

    I think a better way to say it is:
    While running on the surface, the submarine is subjected to wetted surface drag, form resistance, and all the effects of its surface wake (knowing that the surface wake causes a speed dependent variation in the form resistance). As the vessel submerges, wetted surface and form resistance increases while wake effects continue. Maximum resistance occurs just as the main hull is fully submerged as wetted surface and form resistance reach maximum and wake effects continue. As the vessel continues to dive, the surface reflection continues to apply a wake effect (i.e. Newtonian Wake) that reduces with increasing depth until the vessel is "deeply submerged". Once the submarines is "deeply submerged" the drag of the vessel is only dependent upon speed (and a very, very, small temperature and density variation) as the wetted surface and form drag are constant up until the point of cavitation.
    Also note for submarines there is a bottom reflection also. Resistance increases the closer the vessel is to the bottom. This is why there are blue water submarines and costal submarines, you can have both reflections at the same time.

    Edit: make sure that everyone understands that there is only a very, very small change in resistance due to seawater effect at depth.
     
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