Hydrofoils/SWATH question

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

  1. Red Dwarf
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    Red Dwarf Senior Member

    Is there a point in a design where hydrofoils and SWATH can overlap and is that bad?

    Let me explain a bit more. If the foils are so large that they displace a significant volume you end up with a "mutt" that has both features. I found this happened when I ran some numbers for a 12 tonne hydrofoil design that lifts off at a low speed, 3 m/sec. The foils themselves displace 4 tonnes of volume and a relatively small hull is all that is needed when stationary.

    I imagine as you raise the takeoff speed the hydrofoils quickly get small enough that they don't have significant volume but I am interested in having the boat fly at a lower speed than most hydrofoils.

    Is there anything inherently wrong with this approach? I imagine all the extra surface area for the large foils creates significant drag but of course there is no conventional hull wave drag. Maybe with a low takeoff speed (large foils) there is a cross over point where the skin friction penalty is so great the boat requires more power than a conventional hull.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

  3. 1J1
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    1J1 Senior Member

  4. Red Dwarf
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    Red Dwarf Senior Member

    Thanks for the links. I have looked at those designs and the one closest to the concept I am interested in is http://www.navatekltd.com/seaflyer.html The lifting body foil is far bigger than I like and I don't agree with the lifting body concept anyway. Lifting bodies are not good in aircraft and I expect no better under water. Have you ever seen a lifting body glider? Oh yea, the one in the 6 Million Dollar Man intro, that went well.

    Ad Hoc - I keep asking because I am having real trouble finding enough info on the web. I guess people do not release any details due to contractual agreements. All the stuff I find on SWATH and hydrofoils is too basic and close to useless for learning and playing with a design.

    I know you have made it clear that the SWATH seakeeping benefits are due to the small waterplane area. But OTOH all the hydrofoil articles claim hydrofoils have excellent seakeeping as well. I expect this is because once the hull is flying the WPA is only the hydrofoils support struts.

    Plus the SWATH I have seen have fairly small foils just for active stability. Why not make the foils large enough to lift the vehicle a couple feet? That way at rest it is sitting on the hulls and above takeoff speed it has low WPA. This would eliminate the ballast issues of a SWATH and also provide a stable platform at rest.

    So my question is can a low speed hydrofoil (5-20 knots) be made to work efficiently? I think that whatever volume the hydrofoil displaces is that much less lift the hydrofoil must produce which helps achieve a low speed takeoff.

    I wish there were more technical papers showing all the failures in hydrofoil/SWATH designs. That would help me learn from others efforts. Most of the info I find are the success stories which are more fluff and hype than detailed analysis.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Well, that is simply it. But I’m not wholly sure you understand this!

    A swath has a small waterplane area, the thin struts. BUT, it is also, a constant draft vessel. What that means is whether the loading is light load, or full load, the draft remains the same. Why?....because the operational profile of a swath is to have the draft roughly mid-way on the vertical struts. On top of that, it is a displacement vessel, i.e. loads of drag. Also it is very important to get the LCG and VCG in the same place under ALL conditions of operating too.

    So, the foil, when it has lifted the hull it is attached to out of the water, is running on just the foils, guess what, yup, those thin struts have almost no waterplane area. The best example of this is the Boeing Jetfoil. And no unsurprisingly it has excellent seakeeping, low motions.

    The fins are either passive or active. A swath has a low GM, which means it is “easy” to manipulate the motions. The force required to restore the vessel back to upright, is much easier with a low GM than with a high GM. A conventional multihull, like a catamaran, can have a GM in the 40-50m range on larger cats, or 10-20m on small cats. The Queen Mary had a GM around 0.3-0.5m. This low GM gave the QM a nice long slow period of roll, she was renowned for being very sea kindly.

    The period of motion, in this case roll, is dictated by the GM and the “total mass inertia” of the vessel. The total mass inertia being the radius of gyration or mass moment of inertia, however you wish to define it. So to have a long period of roll requires a high mass moment of inertia and a low GM; like the QMary.

    But small vessel aren’t like that, especially cats, they have high GMs, which means the displacement (to get the mass inertia) needs to be even larger to result in a long period of roll. Clearly a small vessel which has a large GM is not going to have a very large displacement!

    So, this is where the beauty of swaths comes into play, they do have low GMs, even those on small vessels circa 20-40m have a long period or roll (more sea kindly) because the GM is low, and so the small fins can damped out and act as a restoring force to counter the opposing roll too. Since the force to infleunce is now much smaller owing to the low GM, ergo improved motions beyond the naked hull.

    So, if you now wish to “lift” the hull, why?...for what purpose. The whole MO of a swath is to run roughly mid-span on the struts. If you raise it some, then the lower buoyant tube gets closer to the water surface, which means it is now being influence by the very waves which it is try to decouple from to start with. And then that’s the second reason why. The buoyant tube below the surface, which has the displacement, needs to be far enough away from the free surface, but still being practical for build/weight/structure purposes, so it is less and less influenced by the passing wave and it is orbital motions. If you raise the hull, the behaviour becomes more and more like a conventional hull, a cat, because the buoyant tube is now being influence by the wave more and more and its motions gets worse.

    Low speed….so where does the lift come from? The key aspect of a foil is, it needs forward speed over the surface of the foil to provide appreciable lift. So, if it is going slow, circa 5-10knots, the “hull” part must be very very light to be lifted by such low forces.
     
  6. 1J1
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    1J1 Senior Member

    Regarding the last part, the foil can be thick enough to have a displacement, like on that SeaFlyer, so it would provide the lifting force already at low speed.

    Other solution for a low speed hydrofoil could be, instead of foils, to apply a some horizontal rotating cylinder which by means of Magnus effect would lift the boat at lower speeds, I reckon. Actually it might be a some foil with rotating cylinder as a leading edge.
    Something like this:
     

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  7. tomas
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    tomas Senior Member

    After re-reading your posts, I still don't understand the motivation. If your ultimate interest is in the seakeeping and performance advantages of hydrofoils, why would you want to combine that with the water resistance issue of submerged hulls?

    Wouldn't a better (but still odd) hybrid be to combine hydrofoils with the stability and low resistance of a powered trimaran? Wouldn't that at least be more fuel efficient?
     
  8. DCockey
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    DCockey Senior Member

    How does the draft stay the same as the load changes? Are there ballast tanks which are emptied or filled as needed to offset differences in load and keep the displacement constant?

    Or does the draft actually change with load but the vertical struts are constant section mid-way and thus the waterplane area doesn't change with load?
     
  9. tomas
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    tomas Senior Member

    Here's a couple of SWATH ships (one is a wind-farm tender) with constant strut area:
     

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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Correct. But it is not just the displacement/draft that is being kept constant by the ballast tanks. The LCG and VCG is too.
     
  11. DCockey
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    DCockey Senior Member

    Thanks for the reply.
     
  12. Red Dwarf
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    Red Dwarf Senior Member

    Ad Hoc - Thank you for that explanation. I appreciate the info on GM with example values. It really makes it feel more "real" as opposed to just theory.

    The are two reasons I want to lift the hull with the foils. I'm not saying these are valid reasons, I'm learning here and looking for input.:confused:

    First is directly related to the low reserve buoyancy of a SWATH especially a small one. With a small SWATH even a couple people just walking around will result in leaning. OTOH If the boat comes to rest on the main hulls when stopped or at very low speed it then is very stable to off balance loads while walking around etc. It essentially is a catamaran when stopped.

    Second is the elimination of a ballast system. A conventional SWATH relies on ballast to control draft and stay at the ideal midway up the strut ride height. With foils the height is controlled by the foils and when in motion will always be at the ideal midway up the strut position. Of course I may be trading one complex system for another with the use of foils instead of ballast. The logic is since it will have the complexity of movable foils for ride control anyway why not make them a bit bigger to carry some load.

    So in summary I want the seakeeping of a SWATH but the stability of a catamaran at dock and a simple(?) control of ride height with foils rather than ballast. But I think the stability at dock will be an issue with a small SWATH.

    Here are a couple links to the Silver Cloud a SWATH yacht.
    http://www.yachtsilvercloud.com/
    http://www.youtube.com/watch?v=wt6ANb3EL8w
     
  13. Red Dwarf
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    Red Dwarf Senior Member

    Nice pictures. It is interesting how short those struts are. The hulls can only be a fraction of their diameter below the surface.
     
  14. 1J1
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    1J1 Senior Member

    Obviously a catamaran hydrofoil would be the right choice.

    ShipSpotting.com
    [​IMG]
    © CkIVAN

    Replace foils with lifting bodies and you will have a low speed hydrofoil.
     

  15. dskira

    dskira Previous Member

    A picture will tell you that? You must be a genius. Some people spend their life designing, and you, looking at a picture you just put a definitive assessment.
    Please explain, but not like in your rotor confusion. and without links.
    I can believe it.:confused:
     
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