Foil dynamics questions.

Discussion in 'Boat Design' started by kenwstr, Mar 8, 2005.

  1. kenwstr
    Joined: Mar 2004
    Posts: 27
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    Location: Australia

    kenwstr Junior Member


    Australian Sailing mag did a piece on foiled International Moth class.
    So got into discussions on foils.

    I did some figuring to see whether a 2 handed dinghy could power a foil
    but as this was based on aerodynamic knowledge, I just wanted to veryfy a few things.

    The following is ball park estimate, not intended to be an exhaustive analasys.

    First, If we use a V foil so the tips rise above the surface, there should be no induced drag so only need to consider profile drag with a skin friction componant. I expect there will be some interference or wave drag at the surface but I don't know how to figure these last 2.

    Also, because the foil is a V, it will rise until the lift from the submerged protion is = the total weight of boat and cew, say 250 kg, 2450N. That means we only have the required area submerged at any speed that is sufficient to raise the hull. Therefore the form drag = weight * Cdf / Cl at all speeds in question. Lets also taper the foil (small chord in deep centre) so aspect ratio and Rn are reasonably constant at all lifted speeds. Remember we are after a rough ball park figure.

    So assuming Cl = 1 I get:

    Speed Foil Area
    Kts sqm
    5 0.8
    10 0.2
    20 0.05
    40 0.0125

    I need Reynolds number to look up some polars.
    I found viscosity for water of 1cp on the net and I think this works out around
    944 x 10^-6 kg/m/s
    If the mean chord a is .44 m at 5kts and .055m at 40Kts (tapered foil)
    Rn approx = 1 000 000
    Airfoil polars show a Cdf of around 0.015 at Cl = 1 at this Rn

    Now can I assume this figure for water given that I accounted for density and viscosity in the Rn calculation ????
    Is fluid compresability an issue???

    That means dragg = 2450N * 0.015 / 1 = 36.75N or 3.75Kg force.

    That seems a low figure that suggests almost any sailing dinghy could be foiled. Are my calcs and assumptions correct?
    Are there other major drag componants to consider and how important are they to the final drag value?

  2. Doug Lord

    Doug Lord Guest

    sailing foilers

    In the Moth class surface piercing foils were on the first boat to win a race in that class on foils. They were soon supplanted by the fully submerged foils designed and built by John Ilett of fastacraft( ).
    A Moth mainfoil may have an area around 1.08 sq. ft. and the rear foil about 60% of that. The fully submerged foil does require an altitude control system(wand).
    Go to the International Moth class Australia forum for an in depth discussion of foils.
    Go to for pictures of several dinghy foilers.
    The Moth Class outlawed surface piercing foils mounted off each wing as violating the anti-multihull rule so only foils mounted on the cl are acceptable.
    Foil loading of around 177 pounds per sq.ft. (based on 80% load on mainfoil)works good on a two foil system but loadings have been much higher on some dinghy foilers such as the I14 on Also, the Rave multifoiler operates at much higher loadings but also develops all the righting moment for the boat whereas the Moth foilers RM is from the crew(and crew technique). It's all about when you want to takeoff and whether low speed and mid range foiling is more important than max top end speed. The Moth guys appear to be going for early takeoff....
    See the "foiler design" topic under "sailboats" here.
    Before the two foil Moth(with the possible exception of some windsurfer designs) all sailing hydrofoils were at least three foils. The Moth was a breakthru in foiler design for dinghies and flies with only two surface penetrations; you want to keep struts/surface penetrations to a minimum.....
    Moth forum(click on message board):
    Last edited by a moderator: Mar 9, 2005
  3. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Wrong. Even if the water's surface acted like a solid boundary there would still be induced drag. Contrary to the popular explanation of "flow around the ends", induced drag comes from getting lift by deflecting a mass of water over a finite span. You can tinker with the ends, but you can't eliminate it entirely.

    At very low speeds, the free water surface acts like a solid boundary, but at high speeds it has just the opposite effect. Induced drag is actually doubled at the free surface, and increased by a lesser factor for the portions of the foil that are more deeply immersed.

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