Hydrofoil questions

Discussion in 'Boat Design' started by Panos_na, Jun 4, 2007.

  1. Panos_na
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    Panos_na Junior Member


    I have some questions about Hydrofoil boats.

    1) What about Froude number? In conventional boats Fn = U/(gL)^(1/2)
    L = lenght of waterline

    What happens with hydrofoils, in foilborne condition? How we can compute the Fn??

    2) What are the types of resistance on a hydrofoil boat in foilborne condition?
    Of course the total drag of the strut-foil system : Drag = induced(vortex) + paracitic(form,friction) . Should we have in mind about other types of resistance for example, from spray on the strut?

    I think that an important parameter is the air resistance on the boat.

    3) What about ventilation on surface piercing hydrofoils? when is that phenomenon take place?In which speed?

    Does cavitation exist on surface piercing hydrofoils, or these hydrofoils cannot reach cavitation speeds, due to ventilation?

  2. tspeer
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    tspeer Senior Member

    Froude number applies to foils, too. Lots of Froude numbers - based on chord, depth, span, etc.

    Most hydrofoils operate at high enough chord Froude numbers that the surface can be considered essentially flat, and a linear surface condition gives good results (the infinite Froude number approximation). This is often known as biplane theory because the surface behaves much like the mid plane between two biplane wings, and an image of the foil above the water is used to calculate the surface effects.

    This is the same image used to calculate the effect of a solid boundary, but the strengths of the singularities in the image are reversed in sign compared to those used for the solid boundary. That's because the boundary condition for the solid surface is no flow through the surface, requiring the image system to cancel out the vertical components; but the boundary condition for the free surface is constant pressure, requiring the image cancel out the horizontal components.

    See Wadlin, Kenneth L; Christopher, Kenneth W, " A method for calculation of hydrodynamic lift for submerged and planing rectangular lifting surfaces," NACA-TN-4168, January 1958.

    It's possible to locate the rear foil in the part of the wave from the forward foil that is moving upward, recovering some of the wave drag. Of course, this depends on the Froude number based on the distance between the two foils.

    Spray drag is there, but it's a surprisingly small contribution. There are interference drags of various sorts, such as the junctions betwee struts and lifting surfaces.

    Induced drag is not at all simple for a hydrofoil. The presence of struts, other foils and the free surface can significantly alter the spanwise lift distribution and increase the induced drag. At the surface, the induced drag is doubled compared to running deeply submerged, due to the downwash from the image foil.

    I agree. Although the water is much more dense, the topsides have a lot of frontal area, and typically very poor coefficients of drag.
    A necessary condition for ventilation is that there be flow separation on the hydrofoil. Other necessary conditions include local pressure less than atmospheric, a pathway from the surface to the separated zone, and the formation of a stable cavity.
    Cavitation can happen with surface piercing hydrofoils just like it does with fully submerged hydrofoils, and for the same reasons. Cavitation can lead to ventilation if it causes flow separation. Cavitation in the trailing vortex wake can also form the required pathway for air to get to the foil and cause ventilation.

    You need to order the AMV CD's from the Internatinal Hydrofoil Society. You'll find a wealth of information there on hydrofoils, including design handbooks that cover all the topics you've listed.
    Hunter25 likes this.
  3. RatliffFranklin

    RatliffFranklin Previous Member

    Hydrofoil Speeds

    Tom - Have you come across any good objective data on upper speed limits for hydrofoils? I've read claims that there are essentially no upper speed limits for supercavitating foils but these were made by people with commercial interests
  4. tspeer
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    tspeer Senior Member

    I don't know of any inherent speed limits for supercavitating foils. There may be practical limits, however. For example, the leading edge can be very highly loaded, so material strength becomes an issue. And, of course, you've got to have the power! Stability and ride quality also become issues.

    FRESH 1 was going 80 kt shortly before it flipped, and at the time the Navy had the goal of doing 100 kt.

    Especially for powered craft, I don't see hydrofoils as a way to make a boat go faster. I think hydrofoils are better viewed as a way to give a fast boat tolerable ride quality and better seaworthiness in a seaway.

    The thing is, even if you could go any speed in a hydrofoil, why would you want to? After a certain point, it makes much more sense to fly in air than to fly in water. I believe that's why you haven't seen anyone develop a really high-speed hydrofoil. It wouldn't be competitive with an aircraft for pretty much any mission.
  5. SailDesign
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    SailDesign Old Phart! Stay upwind..

    I've always seen a figure of 70 knots as a practical limit for hydrofoils. Don't know where it came from though.
  6. RatliffFranklin

    RatliffFranklin Previous Member

    Thanks, Tom.

    It seems sporting/racing applications might be the most fruitful area for innovation with hydrofoils.
  7. RatliffFranklin

    RatliffFranklin Previous Member


    Before supercavitating foils were developed, it was around 70 knots where hydrofoils would cavitate and lose lift.
  8. terryohio
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    terryohio New Member

    i just istalled a sting ray xr111 hydrofoil stabilizer on my 1988 20 ft cuddy with 165 hp. the boat planes out quicker, rides much smoother at all speeds and turns are much more responsive. i also have noticed a dramatic decrease in fuel consumption
  9. BarendGrobler
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    BarendGrobler Junior Member

    A very good source for basic hydrofoil theory, covaring the most vital resistance elements is a book by Peter Du Cane, High-Speed Small Craft. There is one chapter dedicated to hydrofoil design. This theory is the basis on which most of our designs are based (with a couple of basic alterations and additions taken from other sourses and tests)
  10. strvng2
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    strvng2 New Member

    If the time is taken to browse aviation theory (lift..loss of...) remember the linguist of FOIL.
    If the reference is obscure...allow a secound chance...Check the history of aircraftwings and you will find a fini at its height that can only be overcome by speed.
  11. RatliffFranklin

    RatliffFranklin Previous Member

  12. RatliffFranklin

    RatliffFranklin Previous Member

    Peter Du Cane


    The highly innovative design of John Cobb's 1952 jet boat was largely by Peter Du Cane.
  13. Panos_na
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    Panos_na Junior Member

    So, someone cannot use the equation Fn = U/(gL)^(1/2), to compute the Froude on hydrofoils and compare with other types of ships?

  14. tspeer
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    tspeer Senior Member

    The waterline length of a hydrofoil is the foil chord. At the speed most hydrofoils travel, the Froude number based on chord is so high that the limiting value of wave drag at infinite Froude number is a good approximation. This leads to the linearized boundary condition in which the water surface is a flat plane with constant (atmospheric) pressure. Biplane theory can then be used to compute the hydrodynamics in this operating range because there's negligible influence of Froude number.

    Froude number based on length does have a place in scaling hydrofoils, however. Suppose that the weight of a hydrofoil craft scales as L^3. Areas scale as L^2. At the same speed, a large hydrofoil must either have comparatively larger hydrofoils or operate at a higher foil loading (lift per unit area). There is a limit to how high the foil loading can be at a given speed (CLmax), so this results in impractically large foils for larger craft.

    The solution is to use Froude scaling for the hydrofoils. Instead of maintaining the same speed for the large and small hydrofoils, the two craft are allowed to operate at the same Froude number. This results in a higher takeoff speed and higher cruise speed for the larger craft. Since the lift of the hydrofoil at constant lift coefficient is proportional to U^2*L^2, this allows the hydrofoils to be sized in proportion to the rest of the craft (chord~L, span~L). In effect, the higher speed fills in the "missing dimension" required to match the L^3 weight growth with L^2 growth in the foil area.

    Cavitation, however, does not scale with Froude number. For subcavitating hydrofoils, this places an upper limit on speed regardless of size. As a result, if Froude scaling is used to size the hydrofoils, large craft have a much narrower range of operation between takeoff speed and cavitation-limited top speed. This tends to make hydrofoils a small-craft game. They can take advantage of the large difference between their low takeoff speed and cavitation-limited speed. Very large craft can find themselves cavitation limited at takeoff, and hydrofoils are of no use to them at all.

    Of course, there are other ways to scale hydrofoils for specific objectives. Reynolds number scaling would result in speeds inversely proportional to chord, so that a small model would have to operate faster than the full-scale article. However, cavitation can be severe constraint in this case. If the lift coefficient is allowed to vary between large and small models, then the cavitation behavior will be different. One has to decide what is the most relevant phenomenon to match, and then distort other aspects of the design as required.
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