Hydrofoils/SWATH question

Discussion in 'Boat Design' started by Red Dwarf, May 31, 2013.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    But it is just a slim catamaran with a futuristic sounding name! :eek:

    Oh well, if they are willing to part with cash, good luck to ya :D
     
  2. BMcF
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    BMcF Senior Member

    I won't say that is impossible without a lot more thought and some calculation....but that is a very low "take off" speed and very limited top speed for so much of the displacement on the lifting surfaces. I don't recall the exact numbers for the USN HYSWAS but I believe approximately 40% of its displacement was on the foils and her "take-off" threshold speeds were in the range of what you seek as a top speed.
     
  3. BMcF
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    BMcF Senior Member

    Well, yeah...so? Did you have a point? Or perhaps another example that is not just a slim catamaran with a futuristic sounding name or eye-catching exterior shapes?


    :p
     
  4. tspeer
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    tspeer Senior Member

    I think you're talking about something like Mike Stevensen's Hydroflier design.
    [​IMG]
    He also wanted a low-speed hydrofoil.

    There is definitely a cross-over speed for flying because lift-induced drag is inversely proportional to speed squared (when the span and lift are held constant). So you can fly at any speed you want if the foils are big enough, but the drag is enormous at very low speed because you're having to impart a huge downward velocity to the water to generate the lift.

    If your takeoff speed is too low, a lot of power will be required because of the induced drag. If the takeoff speed is too high, a lot of power will be required because of the parasite drag. In between is the takeoff speed that requires the least amount of power.

    The drag due to the buoyancy provided is going to shift that minimum power speed. It'll depend on how much buoyancy is retained after takeoff (reducing the lift required) and the surface area, etc., associated with that buoyancy.

    Whether it's good or bad depends on the performance requirements.
     
  5. Red Dwarf
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    Red Dwarf Senior Member

    Thanks for that and your explanation. You are correct on the balance of lift to buoyancy. I am looking at options and how the numbers work out.

    That picture is a perfect example of one issue with a small craft. The percent of total weight lifted by the foils has to be greater than the useful load. If it is not the vehicle in a light/empty condition will not settle on to the hull when stopped. Instead the submerged hull will just float to the top as has happened in that picture.
     
  6. tomas
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    tomas Senior Member

    That small craft foiler is a monohull with two amas (a trimaran), so what is the benefit of having submerged buoyant bodies? Why not a Tri with foil surfaces added without the extra resistance?
     
  7. Sundevil
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    Sundevil Junior Member


    Well, the failures or ineffective designs don't get promoted or advertised much, so when someone else comes along has a similar idea, it isn't easy to find out that it doesn't work when other people tried it (unless they ask and come across someone who has tried it before).

    I hope there is some way to design something that would work. Have it be pure hydrofoil at high speeds and calm seas, SWATH when it's anchored or at low speeds in semi rough seas, and a normal catamaran at slow speeds and big waves. My thought process behind wanting buoyant hydrofoils is to keep the boat stable, provide more lift at low speeds even if max speed is reduced, and be less likely to nose plant into a wave.

    I would be interested in the 4-6 ton, 10-15m boat size though, and if this type of design would permit an electric motor to work better.

    I don't have the experience of knowing what doesn't work to limit my thoughts though. :D
     
  8. petereng
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    petereng Senior Member

    Hi Sundevil - I think you have to nail down your operational requirements and objectives a bit better before any of your "wants" can be evaluated. Your answer may even lie in a suspended float type of boat like the nauticraft suspended boat.

    http://www.youtube.com/watch?v=IZsE64WQZcc

    Cheers Peter S
     
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  9. Sundevil
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    Sundevil Junior Member

    That is a cool boat in that video. I have had that type of idea before, done a little differently, but still 4 'pods' with a shock absorber on each one. I am impressed that someone has actually made one. I'm not sure how it would handle really big waves in the deep ocean, but that isn't the primary thing I would worry about designing against. It is the 2-3' waves that don't stop for hours that would be the issue. When you try and figure out why people are living on land instead of living on the water, I bet this would be a big issue.

    I have started keeping a list of all the requirements, both functional and performance that I can think of. And then, I also am keeping track of all of the things that would be nice to have.
     
  10. Sundevil
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    Sundevil Junior Member

    http://features.boats.com/boat-content/2013/03/the-velodyne-martini-1-5-active-suspension-on-a-boat/

    Here is another boat that has an active suspension. Although they need to make a video of it working in the waves...

    Put a bigger deck on that, put some basic house boat/cruising items on it, and I think that would be just about perfect.

    I keep coming back to the tri-maran layout with hydrofoils on the center hull that might be buoyant and amas that will have 'shocks' on them to stabilize the main deck platform. The 'shocks' might help keep the hydrofoil wing under water too when at speed in choppy conditions.
     
  11. BMcF
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    BMcF Senior Member

    I agree. Knowing what I do about the power requirements to simply operate active fins, foils and trim tabs to stabilize ships and craft.....:cool:
     

  12. Ad Hoc
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    Ad Hoc Naval Architect

    Not to mention how those thin tubes as space frames will behave when subjected to torsion and bending.
     
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