Why Are SWATH Tubes So Long?

Discussion in 'Boat Design' started by zstine, Sep 16, 2021.

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  1. BMcF
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    BMcF Senior Member

    With all the advances in composite hull production, I wouldn't rule out a significant reduction in structural weigh being achievable. But then what would you have that is considered "viable payload"? An extremely expensive 27-foot boat with a tremendous off-foil draft that could carry a couple thousand pounds of something . ;-) Our last hydrofoil test craft, at 43 feet, had a payload capacity of over 2 tons. We carried that as water ballast.
     
  2. zstine
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    zstine Junior Member

    In the hybrid design's I have been able to review, the fuel tanks have bladders that allow water to displace the fuel as it is burned as well as dedicated water ballast as needed. Fresh, grey, and black water storage tanks can be made similarly. All these tanks are low, in the tubes. The result is that the 'unloaded' ship weighs nearly equal (within allowable constraints) to the fully loaded ship to maintain stability. That is one way the designers of the past solved the issue. Another way, especially applicable for a design where the strut is a small fin like a 'T' hydrofoil, is to just retract the tube/foils when not in flight like you would a dagger board on a sailboat.
     
  3. TANSL
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    TANSL Senior Member

    There are other types of ships that resort to carrying ballast water when they have no payload in their holds or tanks. That is the solution adopted for dozens of years, nothing specific to the SWATH type. But stability must always be studied when they are not ballasted or, for example, in the intermediate stages of filling ballast tanks.
     
  4. zstine
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    zstine Junior Member

    A 2 ton payload is plenty for a 43ft island hopper/coastal cruiser! That's more than some of the performance cruising catamarans like the Schionning 1201. Very roughly, my concept is something like 'Quest', but with a larger topside ~40ft, larger area/large span foils for lower speed and with cruise speeds in low teens to 10knts, much lower power consumption compared to the 825hp @ 35knts of Quest. I would pursue a small strut foil, like a that of a "T" hydrofoil, which would retract when not flying like a sailboat dagger-board. That solves the draft issue and static stability issues. With a short under-body relative to LOA, a ski or similar at the bow may be required for pitch control. I believe at lower speeds the advantages of a blended wing body (since you need large lifting foil area) vice tube and wing becomes more advantageous, but I have more to explore there. I haven't been able to find any lifting body/blended wing body hydrofoil information out there. Seems hydrofoil designers all want to go fast, but I want to fly at low power, so slow. Are you aware of any blended wing hydrofoil design papers or papers specific to slow/low power flight?
    This concept is not technically 'HYSWAS' as the waterplane area is as small as structurally allowable and is not intended to provide any pitch/roll/heave stability. I'd call it a Buoyancy Assisted Hydrofoil, BAH, haha. yeah, All government contracts are extremely expensive! That doesn't mean it can't be done much cheaper, albeit at higher risk perhaps. Though spending money in design is a risk as well.
     
  5. BMcF
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    BMcF Senior Member

    No..I'm not aware of any serious very-low-speed hydrofoils or hybrids. This blended-wing-body one we built and tested was operated/flying in the 20-30 knot range. To achieve the same foil lift and operate in the 10-15 knot range would require that the foils planform area be increased by a factor of 4.

    You assume too much. Our 43' hybrid foiler was funded entirely by a private party...a naval architect that had developed the design and paid us to build and test it at 1/4-scale. The craft cost approximately $250,000 to build (in 2001..would be significantly more now) , including the necessary flight control systems.

    The conversion from conventional main foil to the blended-wing-body was another $100K..roughly.
     

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  6. zstine
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    zstine Junior Member

    I was referring to Quest regarding large budgets, which with design and build, I'd imagine was much more than this 43' hybrid foiler. For someone else to build that 43ft boat in their facility for only ~$300k seems pretty economical. I'd be very happy to build my similar size concept in my facility, myself for that price! doable...hmm?
    The X-48B (pictured) is the approx size and shape I'm imagining (20ft span, 11ft max chord mid-section), with an added strut to make it a "T" foil. With it's 100 ft^2 area, a 10knt take-off, & Cl of 0.8, that is 23,120lbs of lift, plus it displaces an estimated 4,000 lbs, so 27,120lbs full load... maybe too big. Performance sailing cats in this size range displace 10,000 to 11,000 light ship and 14,000-15,000 full load. While I think this will be somewhat heavier, I don't think it will be twice as heavy. I just did a rough hand calc and determined about 1,411lbs of drag to move X-48B wing at ~10knts through water (~42Hp no losses). But the wing could be a bit smaller assuming I only need ~18,000 of lift (plus 4000 buoyancy = 22k lbs)... just thinking out loud a bit.. maybe assuming too much again. But if 40Hp is ball park for 10knts cruise, then I can work with that to start. Of course I need more power to get over the drag hump while the main hull is still waterborne..... Anyway, I feel like 99% of the feedback I get is "not possible" or at least "not feasible". I'm just not seeing that, not yet anyway. Keeping in mind that the main goal is to de-couple ocean waves from boat movement, so comparison to waterborne hulls is of little use even if they are more practical.
    [​IMG]
     
  7. BMcF
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    BMcF Senior Member

    That Quest project budget was actually very small...that MAPC managed to pull it off as they did was testament to a small group of very dedicated and motivated employees.
     

  8. BMcF
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    BMcF Senior Member

    The seakeeping of our 43' foilborne craft was amazing. I wish we could have afforded to keep it around as a pleasure craft after the trials program ended.
     
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