SA/D

Discussion in 'Sailboats' started by TiPegleg, Sep 5, 2021.

  1. TiPegleg
    Joined: Sep 2021
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    Location: USA

    TiPegleg Junior Member

    Not looking to race from dock to dock at all, I’m more concerned that passage making will require extra supplies and the engine becomes a necessity in light air rather than an option. This was never going to be a “fast” boat, but only being able to sail in a gale would get old fast.
     
  2. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Not necessarily.

    I once had a 17 foot sailboat that originally sailed like a pig.

    At one point, it refused to move any way but sideways. Then it refused to change tacks.

    I solved the problem of it only going sideways. It had a very high Aspect Ratio swing keel. What I was doing was causing it to stall, instead of create upwind lift. It took me a while to get used to the idea that I had to fly it through the water like an airplane wing.

    Getting it to change tacks (come about) proved to be a bigger challenge. I solved it only after reading about 19th century sailing work boats. To get them to change tacks, with their long keels, the forward most sail had to be left aback. That is left on the side it was on during the old tack. As the bow came into the wind, that sail would back wind and push the bow over onto the other tack.

    I used this technique with the jib. Soon. I was able to get this boat to change tacks smartly. In fact, with the help of a crewmate, I was able to tack up a 40 ft wide channel, dead to windward.

    Now guess what the S/D on this boat was. It was 21. It had 145 sf
    of sail for 1,000 lbs of boat and crew.

    The boat you are considering has a great deal of heft to it. It would take a considerable amount of time for it to lose momentum when coming about. My 17 footer lost such momentum almost instantly.

    What this means is that a heavier boat often can get by with a lower S/D than a lighter one.

    Yes. The boat with the higher S/D is going to be faster over all. But maybe not as much so as one would think. Especially when things start getting rough.

    I have designed a boat with a very low S/D. This was because I wanted it to have an especially large carrying capacity combined with a high range of stability and shallow sailing draft.

    To get the higher carrying capacity, I had to do with less ballast. To get the higher range of stability, I had to go with a shorter rig. So I ended up with an S/D of about 11, with the boat fully loaded (empty, it had an S/D of almost 15).
     
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  3. TiPegleg
    Joined: Sep 2021
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    TiPegleg Junior Member

    So how did this sail?
     
  4. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    It was never built.

    I live on a very low income and cannot afford to keep a boat of that size.

    The design was mainly an intellectual exercise.

    It was to design a low-cost sailboat that is capable of crossing an ocean.

    The whole idea is obsolete right now, as there are plenty of ocean capable sailboats available that can be purchased and repaired for a fraction of the cost of building this boat.

    I only brought it up as a way of explaining how and why a sailboat with such a low S/D would be designed and built.
     
  5. Tedd McHenry
    Joined: Feb 2020
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    Location: Surrey, BC, Canada

    Tedd McHenry Junior Member

    By "fly it through the water" do you mean keep the lateral forces light so the keel's angle of attack to the water is low?

    I had (recently sold) a boat with a swing keel that had an aspect ratio of about 6. That boat liked to be reefed quite early, which I assumed was because of being fairly light and also a bit tender. But reading what you said caused me to wonder if reefing early was also helping the keel work better.
     

  6. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    What I mean by 'fly it through the water' is to treat it like an airplane wing. And that is to keep the angle of attack low.

    And this has more to do with how the sails are sheetfed in than with the course the boat is sailed.

    Just about every book on how to sail, I came across, was crap. Long after I discovered the secret myself, I read a single sentence in a magazine article.

    It went something like this:

    "Let the sail out until it flaps, then pull it in until the flapping stops."

    It's not quite that simple, but that's about 90% of the truth. The other 10% is that you pull it in just a little bit further until you feel the boat start to scoot forward. It's at that point that you have the proper sail trim.

    Pulling it in any further, without changing course relative to the wind, is going to have the opposite effect to what is intended. The boat will start to slow down.

    Keeping this balance is "flying the keel through the water."

    During my first season, I got about 3.5 kts at best.

    During my second season, I was up to 4.5 kts.

    On my third, 5.5 kts was more typical.

    And all three seasons were with the exact same boat and sails.

    Only the tiller nut was changed.

    In answer to your other question, I say yes.

    This is because your boat may be limited to 'hull speed', which is about 1.34 times the square root of the Water Line, measured in feet, and indicated in knots.

    Once this speed is reached, adding more power only increases the speed very slightly. This means the keel is producing only slightly more lift. But the leeward pressure of the sails is continuing to increase. This effectively increases the angle of attack of the keel.
     
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