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Re: Leeboards

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  • JeffreyM
    Remember that the foils we re talking about are practically flat anyway. So it s not about resistance while standing still, but lift when moving. A flat
    Message 1 of 81 , Feb 18, 2013
      Remember that the foils we're talking about are practically flat anyway. So it's not about resistance while standing still, but "lift" when moving. A flat board and a foil shape both "fly" when moving, but a flat board stalls more easily. Recall what your boat does when it sails from a standing start (e.g. after barely making it through a tack): at first the boat falls off as it goes partly sideways, but responds and sails closer to the wind once it gets some speed. A foil-shaped board will (theoretically) respond at lower speeds, therefore more promptly. Again, this makes more difference in a rudder than cb or leeboard. (The effect is more pronounced in a fatter foil, but there comes to be a trade-off in increased drag.) Anyway this really isn't rocket science: you just have to shape your board a bit from thickest about 1/3 back, to thinnest at trailing edge.

      --- In Michalak@yahoogroups.com, "prairiedog2332" <nelsarv@...> wrote:
      > That is the opposite to what Bolger writes. Broad flat surfaces prevent
      > more sideways resistance than NACA foils. But you cannot directly apply
      > air foils to water foils according to both Jim and Bolger because the
      > speeds are different as is the density of the media they are moving
      > through. And of course planes are under a lot more power than our
      > sailboats. There is obviously a speed through the water where the NACA
      > foil wins even in boats as attested by ocean racers. But the designs we
      > are interested are nowhere in that ball park?
      > Jim suggests that sailing about 6o degrees off the wind gives the most
      > fun experience in his designs. If you have to reach a destination
      > directly upwind best fire up the OB. These boats are not racers but
      > more about having fun when out on the water. In my view they give the
      > about best bang for the buck in that regard.
      > Of course some really good sailors add some tweaks and get better
      > results than Jim attest to.
      > Nels
      > <br>--- In Michalak@yahoogroups.com, "JeffreyM" <JMichalsbr@>
      > wrote:<br>><br>> I recall that the idea of foils vs. flat is that foils
      > will work at wider angle of attack than flat. In other words, a flat
      > board works fine going exactly edge-on through the water, but stalls
      > (loses effectiveness) pretty quickly at any kind of angle. This would
      > happen, for instance, in a puff while traveling at low speed: the sudden
      > wind pressure would push the boat sideways more than forward, and the
      > boat would sideslip until she had enough way on to lessien the angle of
      > attack and come out of the stall. This is even more important for
      > rudders, which have to work at high angles of attack in sudden turns.
      > Otherwise "hard rudder" would just check speed but not turn the boat
      > effectively.<br>> <br>> <br>> --- In Michalak@yahoogroups.com,
      > "prairiedog2332" nelsarv@ wrote:<br>> ><br>> > If interested in
      > leeboards reading Phil Bolger's "Boats With An Open<br>> > Mind" is
      > probably the best source of information when it comes to<br>> >
      > homebuilt boats. However the info is spread over several different<br>>
      > > chapters with different hulls and leeboard shapes so is not easy
      > to<br>> > summarize. The other useful source is of course Jim's build
      > book.<br>> > <br>> > I think what caught Jim's eye was Bolger's Dovkie
      > prototype design which<br>> > had only a single leeboard but when it was
      > put into production Edey and<br>> > Duff decided to go with 2 shorter
      > boards as they felt the public wasn't<br>> > "ready" to accept a single
      > one. Bolger tells the story of several<br>> > production models out
      > sailing together when the owner of the original<br>> > appeared from
      > shore, sailed across their sterns, circled around and<br>> > across
      > ahead of the pack and took off back to shore. E&D who used to<br>> >
      > write a newsletter about their exploits never mentioned this
      > incident:-)<br>> > <br>> > <br>> > Bolger and Michalak both agree that
      > the board doesn't have to be<br>> > foil-shaped either. Flat sided
      > boards work best to prevent leeway at the<br>> > relatively slower
      > speeds of these designs, especially in light winds.<br>> > They cannot
      > be compared to "air foil" shapes. The board should be as<br>> > stiff as
      > possible without getting too thick and recently it seems making<br>> >
      > them a bit wider than on the plans improves up-wind performance.<br>> >
      > <br>> > Bolger also wrote that asymmetrical leeboards offered little
      > advantage<br>> > in reality as opposed to theory nor did toeing them in
      > slightly. The<br>> > time you lose raising one and lowering the other
      > when tacking cancels<br>> > any gains and the added drag cancels leaving
      > them both down.<br>> > <br>> > To me, a boat with a centreboard or
      > bilgeboards or daggerboards seems an<br>> > unnecessary complication and
      > hassle both in building and use and take up<br>> > valuable space inside
      > a small boat.<br>> > <br>> > Nels<br>> > <br>> > <br>> > [Non-text
      > portions of this message have been removed]<br>> ><br>><br>
      > [Non-text portions of this message have been removed]
    • Samantha Roberts
      Yes, you are using just the end of the ellipse to shape the leading edge. ________________________________ From: Anders Bjorklund
      Message 81 of 81 , Jun 21, 2013
        Yes, you are using just the end of the ellipse to shape the leading edge.

        From: Anders Bjorklund <andersbjorklund5@...>
        To: Michalak@yahoogroups.com
        Sent: Friday, June 21, 2013 8:08 PM
        Subject: Re: [Michalak] Leeboards

        I think I got it. Thanks. It looks like the tangent line back to the 20%
        chord would meet the ellipse VERY near its forward tip then. Interesting.


        On Fri, Jun 21, 2013 at 2:10 PM, Samantha Roberts <
        samanthaeroberts@...> wrote:

        > A 30 deg ellipse is a circle viewed at 30 deg from the plane in which it
        > lies (if viewed at 90 deg, it appears as a full circle). This makes a 30
        > deg ellipse twice as long as it is wide. So if your plate thickness is 1",
        > the ellipse you need to draw is 1" long and 1/2" wide (50% of plate
        > thickness).
        > Now draw a smooth curve that lies tangent to the ellipse and reaches full
        > thickness at 20% chord. Since John does not specify either this curve or
        > the one from 60% chord to the T.E., I assume he means that those shapes are
        > not critical. On the general principle of not trying to make water flow
        > around sharp corners, I would make those two curves come smoothly into the
        > flat section from 20% to 60% chord, but I am not sure how important that
        > might be.

        [Non-text portions of this message have been removed]

        [Non-text portions of this message have been removed]
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