SWATH speed and stability opinions

Discussion in 'Boat Design' started by jon haig, Jul 25, 2019.

  1. jon haig
    Joined: Jun 2019
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    jon haig Junior Member

    swathoption.png please give your opinions on the following design.
    same upper house size 30' x 30' with 50' length of tubes
    option # A...vertical supports are ,front 10' tall x 6' long by 12" wide in middle like pic.
    rear support is same size as front ,middle support is 3' x 6'' wide in middle.
    option #B.... everything the same except not vertical, width between tubes is 40' instead of 30'
    same hp in both boats.
    the vertical Swath boat will go X speed with Z amount of hp. with the tubes 6' bellow the water.
    *****The only question i would like an opinion to is ????
    what will be the difference of speed of the option #B boat.
    the images are to visualize the angle of the vertical supports and the vertical shapes i described.NOT the design..
    THE DESIGN OF Both is Identical Except for the angle of the supports.

    Also ...might the angle create unwanted lift..????
    or more drag??

    FIG-1-768x543.jpg
     
    Last edited: Jul 26, 2019
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Like everything. It all depends on the number of horses you have ...
     
  3. jon haig
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    jon haig Junior Member

    thanks for fast reply.
    The speed with option #A is known ,it is X.
    ***The only question i would like an opinion to is ????
    what will be the difference of speed of the option #B boat.
    will it be faster or slower Because of the angle of supports.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Calculate what, if any, the increase in WSA will be, ....and that's it.
     
  5. jon haig
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    jon haig Junior Member

    what about the angle, is angle irrelevant??
    might the angle create unwanted lift?
    or might it create more drag?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I refer you to my reply above.
     
  7. fastsailing
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    fastsailing Junior Member

    Are you sure you wrote what you intended to, and not something else?
    I quote again what you did indeed wrote:
    Width of circular tube is the same thing as its diameter. length to diameter ratio is so low for both, that resistance will be enormous. The larger diameter even more so. Compared to that the angle of supports is irrelevant. The design can and will not be the same if both tubes are not the same diameter and length and displacement too.

    If you intended to write something else, then the answer would have been very different, but unknown at this time.
    There is an optimum length for a given displacement for submerged bodies. If the versions have different tube dimensions, the closer to the optimum is obviously better having less resistance. Struts should be located symmetrically to the tubes, not tangentially which will increase resistance.
    Angled struts will make additional resistance where penetrating surface (air/water interface). The difference in speed will depend on how much extra resistance will be there. Nobody can estimate or calculate that with information provided. It will depend on displacement, speed, length and diameter for the tubes, as well as the angle and size of struts.
     
  8. jon haig
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    jon haig Junior Member

    thank you, i corrected..it is distance and angle between tubes that is the only difference in the design.
     
  9. BlueBell
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    BlueBell Ahhhhh...

    Option A will be faster than Option B under most circumstances.
    But why do you ask such a limiting question?
     
    Last edited: Jul 26, 2019
  10. jehardiman
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    jehardiman Senior Member

    Option B is a very bad idea because it is a construction nightmare and throws away all of the seakeeping advantages of a SWATH hullform.
     
  11. Yellowjacket
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    Yellowjacket Senior Member

    B is the first letter in Bad... The outward angling of the supports would tend to make the boat roll outboard when turning. I rode in the Lockheed Martin's COBRA SWATH and even with vertical supports it rolled outboard in turns a lot more than I expected... The angling of those supports would increase that tendency. Don't even think of going there...
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That's because it is lightly damped and has a small restoring moment.
     
    Last edited: Jul 27, 2019
  13. W9GFO
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    W9GFO Senior Member

    Option B requires longer struts to put the tubes at the same depth - which will mean more drag. Option B also has the tubes 5ft wider on each side than the house, making docking more hazardous.
     
  14. BlueBell
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    BlueBell Ahhhhh...

    And what is the, unlabeled, unreferenced, pretty picture all about in your original post?
    It looks like a SWATH hot-tub.

    View attachment 148954
     
    Last edited: Jul 28, 2019

  15. Yellowjacket
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    Yellowjacket Senior Member

    Yes, that is true, but you miss the point.. With the "B" design lateral slip will create negative lift outboard and positive lift inboard on turns Since the righting moment is even lower, given the much narrower width 0f the water plane at the surface, you would have real problems even at pretty low speeds. Given that the COBRA had a good bit of roll, when you add the additional rolling moment of that design as well as the reduced righting moment you'd have potentially real issues with roll stability as the speed increases.

    and oh yea, it has higher drag...

    As I said in the previous post. It's a bad idea...
     
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