Displacement Hull Speed of a Tuna

Discussion in 'Boat Design' started by naturewaterboy, Oct 26, 2006.

  1. Excalibur
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    Excalibur Junior Member

    In regards to SWATH hulls... If you design the underwater body to get the CG as low as possible and maybe add a ballasted keel, could you get enough stability to go to a monohull? Its true that such a design would have very limited deck area, but if one was williing to put up with that in return for a high displacement speed, would it work?
     
  2. Excalibur
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    Excalibur Junior Member

    Holy Gadzooks! I only hit the send button once, promise!
     
  3. PI Design
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    PI Design Senior Member

    Marshmat has it spot on. We tend to design for rigid materials, which fish aren't. As has been said before, surface vessels operate in a different medium to fish, in as much as they are in the air/water interface. That significantly changes the problem as there are now waves to contend with. Submarines are sort of similar to dolphins/whales etc but the practicalities of design mean that you can't copy a dolphin design carte blanche. Most modern subs have moved away from the tear drop shape, due to the difficulties of fitting all the maachinery in. The US Navy (amongst others) has researched flipper propulsion, and it does have some advantages over rotary propulsion, but it would be nih on impossible to make a full size sub with ths sort of system. I dispute the comment that submarines are less efficient due to their higher wetted surface area. An attack sub (albeit nuclear powered) can definitely outsprint a frigate.
     
  4. yipster
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    yipster designer

    mean something like this?
    i did some model testing with swath and -very delicatly- it does work
    [​IMG]
     
  5. Excalibur
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    Excalibur Junior Member

    i did some model testing with swath and -very delicatly- it does work

    Looks something like the General Dynamics proposed LCS. By "very delicately" do you mean that it only works with small sea states, or that you are not willing to get further into the design? Just curious. I was thinking of an actual monohull with no outriggers. Kind of like a WWII submarine when surfaced deck awash, with a hull width much greater below the waterline than above. I note that they had a length/beam ratio in the 10:1 area but had sufficient roll stability.

    Oh, and as far as an attack sub out sprinting a frigate, very apples to oranges there. The frigate's prop will cavitate MUCH sooner than the subs due to the compressability of sea water at atmospheric pressure (lotsa air bubbles near the surface). Not a fair contest at all.
     
  6. yipster
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    yipster designer

    excalibur, a 2 ft model of this drawing i tested. speed ( reduced wave resistance ) and stability in wild water worked fine, trick was calculating volumes for the pods and keels together with displacement and cg and than load must be centered on deck still, works and is exiting but not as easy as it looks. have those legs 4 ft high made but need the framing etc to have a real live go. together with our other boat its the short money that slows it up. i looked up lcs but dont think it is a swath while general dynamics / lockheed martin is into those also. surfaced subs would be great freighters but forget now what was against them.
     
  7. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Robert, I think what you can save in drag is overestimated.
    Look at the many boats with rotating keels or keels with flaps (rudders) to increase lift so the boat travels with no leeway. These boats are not necessarily faster than others in the same class.
     

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  8. jehardiman
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    jehardiman Senior Member

    OH boy, there is a lot of fluff flying in this thread. A target rich environment.

    1)
    MikeJohns, Marshmat and PI Design are correct, the technique used by fish to swim is not directly applicable to rigid man-made structures. A fish is not more hydrodynamic, in fact it need higher drag to swim, they use it to "push-off" against. It is in the manipulation of the flow field that it excells. Google up MIT's ROBO TUNA project and several other "swimming AUV" projects I can't recall now. I think some were posted here.

    2)
    All bodies have wavemaking drag, it is just the physical manifestation of the pressure disturbance of the body moving in the fluid. For submarines it is called newtonian wake and can be visable on the surface to depths of several hull diameters depending on body shape.

    3)
    I've seen the design of that..."craft". It is frighting...crush depth is way too shallow, it draws combustion air from the passanger compartment, it relies on a poppet valve in the induction valve....IMHO an accident waiting to happen. Just an extreme toy, and not that of safe one.

    4)
    Actually, submerged bodies are more efficient than surface bodies, what they lack is power density. A modern nuke is about 7,000 tons with ~35,000 shp to make ~30 knots (http://www.fas.org/man/dod-101/sys/ship/ssn-688.htm); that's about .16 shp/(t-kt). A modern DDG is about 8,000 tons, makes the same speed (31) but uses 100,000 shp (http://www.fas.org/man/dod-101/sys/ship/ddg-51.htm) for a shp per ton-knot of 0.4, i.e. twice as much. Subs speed is limited by power density, they spend an inordinate amount of displacement on structure and life support. Over come the power density issue and you go faster, which is why torpedos are used to catch ships.

    5)
    This one is near and dear to my heart as I did my senior design thesis on SWATHs. In the proper design regiem, a SWATH is faster than and similar power and displacement mono or displacement cat. The SWATH hull shape accentuates the humps and hollows in the power curve, so there are specific areas that you want them to operate in. And thier peaky response RAOs allows them to carry this higher speed into higher sea states than either a mono or a cat. However, there are real physical limits based upon powering options and structural fabrication. This is why SWATH come in basic "sizes", these are the design points where thier characteristics excell.
     
    Last edited: Nov 7, 2006
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