Large Hydrofoil/heaveplate

Discussion in 'Hydrodynamics and Aerodynamics' started by Sherkin, Feb 13, 2014.

  1. BMcF
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    BMcF Senior Member

    I would hazard a guess that your design would be very "tender" but otherwise "stable" when sitting still. You probably misunderstood my earlier comments; I was referring only to lack of inherent stability in foil-supported low-waterplane vessels when underway. They just "sit there and float" when not underway....the hulls are in the water at that point.

    I couldn't begin to offer an opinion on the underway stability of your concept.
     
  2. tspeer
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    tspeer Senior Member

    Computer control at zero speed is not possible with hydrofoils. If you want a small waterplane area at zero speed, it will have to be done 100% with buoyancy. You may be able to use hydrofoils to control ride height and roll when underway.

    Trailing edge flaps on your horizontal wings would be a good way to go for this. At the very least, you will need to have a port and starboard flap on the aft foils, which would give you the ability to control pitch and roll. A flap on the forward foil would make it possible to control heave directly as well (instead of indirectly through pitch). It's not a good idea to try to control roll using the forward surfaces. These may require large actuators, which would be expensive.

    The actuators and control system would also need to have high availability due to safety considerations, as an actuator failure could drive the craft to capsize or pitchpole, and that means they'd need to be redundant and even more expensive. It may be more economical to segment the flaps into a large number of small flaps, each with its own actuator. The smaller actuators may be production items, and each actuator in itself could be single redundant because it wouldn't need to be exceptionally reliable. There would be enough control power in the remaining surfaces to compensate for an actuator failed hard-over.

    Ride height can be controlled by foils when moving, but you may want to include a ballast capability to vary the floating height and trim. This could be very useful for matching the height of the dock for crew transfer. Of course, the ballast system needs to be highly reliable, too, or you'll end up like the Ocean Ranger.
     
  3. Sherkin
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    Sherkin Junior Member

    Thanks BMcF and tspeer you are both really helping a lot.

    tspear, what do you think of the long cord length 4-6m at a max speed of 20 knots? Any issues with this? or if I made the cord length longer?

    And any cross-section shapes come to mind? I'm thinking a standard teardrop will be fine as I want to reduce drag.

    I understand the need for active control but as you say they are at risk of failure so there has to be enough redundancy. Is there anything I can do to the design to passively prevent most of this?
     
  4. tspeer
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    tspeer Senior Member

    I've no idea how much area you need to meet your heave damping requirements at zero speed.

    If you assume the boundary layer is fully turbulent, which I think is a pretty good bet for this application, there's little difference in the drag of different shapes. The main thing you can do to reduce the profile drag is to make the foils thinner.

    In general, your desire to have a small waterplane area is at odds with passive stabilization. Stability and disturbance sensitivity are opposite sides of the same coin. You get stability by having forces that change when the craft alters its shape at the waterplane. But that also means that when the water surface itself is moving, it will affect the craft.

    You might use buoyant surface piercing foils instead of vertical struts. They could have the same waterplane area when at rest, but develop lift when underway. That way the change in lifting area with height would stabilize it when moving.
     
  5. Sherkin
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    Sherkin Junior Member

    That is an intriguing idea, thanks.
     
  6. Jim Caldwell
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    Jim Caldwell Senior Member

    Why not a cat?
     
  7. Sherkin
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    Sherkin Junior Member

    Mainly because cats are the industry standard right now for an offshore wind farm crew transfer vessel. Swaths are able to operate in higher sea states than them and our design remit is to come up with something that can operate in higher sea states than the Swaths.
     
  8. Sherkin
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    Sherkin Junior Member

    If I used a symmetrical supercavitating hydrofoil, would it cope at low speed?
     
  9. Sherkin
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    Sherkin Junior Member

    At the moment the design has three heave plates. The reasoning being that one large heave plate could cause plow in and other anomalies. Is this reasoning correct or could I have one large heave plate that is the entire length and breath of the vessel?
     
  10. BMcF
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    BMcF Senior Member

    What method of propulsion are you considering..and how would it be arranged? Your very conceptual sketch would imply that the propulsion engines are "up in the main hull somewhere" and that the power is transmitted down/through the struts to ..propellors?

    Either of the configurations you suggest for the submerged planform area could have underway trim stability issues. The only craft I've worked with that come even close in concept to yours, had only two transverse foils, not three. Or four underwater hulls, vice two (SLICE v. conventional SWATH). I'm afraid I can't offer much in the way of definitive comment since your concept is well outside of my experience box.
     
  11. tspeer
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    tspeer Senior Member

    There's no reason to use a symmetrical supercavitating section for this application. The supercavitating section will have excessive drag due to its blunt base. It will also lack structural stiffness due to the thin leading edge.

    A conventional subcavitating section will have more cross sectional area for the same physical thickness and will maintain attached flow to the trailing edge for low drag. The best section may well be something like a flat plate, with long parallel sides, an approximately elliptical leading edge, and a straight wedge for the trailing edge with rounded transitions between the wedge and parallel sides. This will maximize the cross sectional moment of inertia to give maximum structural stiffness for a given physical thickness. It will also be the cheapest structure to build because perhaps 80% of the area can be constructed from flat plates.
     

  12. johnhazel
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    johnhazel Senior Member

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