Flat bottom/vertical sides/flat deck/ flat everything

Discussion in 'Sailboats' started by Ron Cook, Apr 3, 2009.

  1. Andy
    Joined: Aug 2003
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    Andy Senior Member

    Ooh any more info? Looks interesting!
  2. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    The canting ballast keel has an interesting history.
    Back in the old BOC race days, moveable water ballast started to appear. The race committee got conerned that they'd end up with a bunch modern sand-baggers on the open ocean. So they tried to come up with a way to regulate (limit) it. There were several posible choices:
    1.) allow the moveable ballast to be a limited percentage of the fixed ballast,
    2.) limit the amount the moveable ballast could heel the boat in stil conditions, or
    3.) a combination of these two.

    What they chose was the first option. They limithed the amount the moveable ballast could heel the boat in still conditions.

    IMHO, they picked the wrong option. This is because this option would encourge wider boats. The wider the boat, the less a given amount of moeable ballast could make it heel.

    At that time, the BOC open 60 class had evolved into a relatively narrow hull, with a deep fixed keel, and a lot of sail area.

    The ten-degree moveable ballast heel rule changed that. Boats got wider and wider. The increased Beam, along with the light hull weight and huge sail plan, made planing possible.

    This new type was much faster, but it had a huge weakness. If it was ever turtled, it had an unusual proclivity for staying that way. One would think the heavy bulb on the very bottom of the airplane wing like keel would certain right it--especially if there was any seaway. And there certainly would have to be, as this type was very difficult to capsize by wind alone. This is pecause the topsides were kept very low. And the spars were usually very light.

    But, if the boat turtled, and even if it lost its rig in so doing, it was remarkably stable upside down. This is even with the ballast bulb and keel still in place. The reasons for this were two-fold:
    1.) the low sides (in relation to the Beam) and the very short but deep keel gave little for breaking waves to hit. The waves would tend to climb the inwardly sloping sided and go right over the inverted bottom. They might hit the keel, but the keel was so short that there was little area for them to hit. And
    2.) the heavy ballast-bulb, high up, provided excellent snap-roll resistance, so even the slope of the wave could not entice the boat to flip back over onto its bottom.

    Another problem with shifting water ballast is that it could end up on the wrong side at the wrong time. Then it would be far easier for wind and wave to turtle the boat. Not only that, but the water ballast being on the wrong side, while the boat was right side up, would then be on the right side to make it even more stable upside down.

    The canting ballast keel changed all that. First, all the ballast was now under the boat, so there was no need anti-heeling ballast, usually above the Waterline. So now the boat could be lighter.
    Not only that, but it is atleast somewhat plausible that the crew could crank the ballast over to the leward side of the turtled boat, and increase its odds of self-righting.

    The problems with the canting ballast keel are;
    1.) it requires expensive engineering for both the boat and the keel, I seriously doubt that building a light monohull, with one of these, would be cheaper than building two hulls, an
    2.) It requires a motor of some kind to move the ballast to the desired side in a timely manner. And this motor will likely require an engine to power it.

    Also, the only advantage of this system is that it keeps the boat more upright in strong wind conditions.

    For a boat expected to plane, this is a pretty good deal. But for one that is likely too narrow to do so, the benifit seems more dubious. One possible benefit is that it would be much more capable of bashing into head-seas. But a deep bulb on a fixed keel may do almost as well, if the boat is designed to sail while heeled.
  3. Paul Scott
    Joined: Sep 2004
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    Location: San Juan Island, Washington

    Paul Scott Senior Member

    The magic of 1:6 beam to lwl? Int 110, 210, 510, Lawley 225, etc….

    edit- not only that, but point of max beam is at 50% length? And pointy ends!:eek:
  4. Paul Scott
    Joined: Sep 2004
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    Location: San Juan Island, Washington

    Paul Scott Senior Member

    intrigued by what I was seeing to similar images below, I built a boat, and it was beguiling how fast the thing would go without a lot of observable waves- it had 2.5’ wings on a 36” beam hull, 16’ long, 100 sq ft single sail. What intrigued me on the computer screen was wave drag going down to almost zero at higher Froud #’s- & as long as I could keep the hull flat, when I was sailing, it seemed real enough, even with low prismatic - is it?

    Attached Files:

  5. Paul Scott
    Joined: Sep 2004
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    Location: San Juan Island, Washington

    Paul Scott Senior Member

    For anyone interested, here is a vid of said square boat sailing- I was using a formula windsurfing centerboard, too small for light winds, so not great upwind, but after the first tack, wait a bit, about 3:10 or so, and on a shy reach in 4 K TW she scoots. I was taking a stab at very little rocker, and low prismatic, so you’ll notice toward the end, even when I’m standing on the stern, the bow does not want to come up. :oops: Fun boat to sail, pretty effortless, needed to be sailed flat to take advantage of wave formation…. Kind of a frankenboat from odds and ends laying around- like a Force 5 mast & Boom, etc.. Rocker has its uses! Think of her as a class B sailing canoe, I guess. When I do another, the seat will move fore and aft, and there’ll be more rocker.

    Last edited: Feb 17, 2023
  6. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . . .

    More rocker for easier steering?

    I enjoyed the video.

  7. Paul Scott
    Joined: Sep 2004
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    Location: San Juan Island, Washington

    Paul Scott Senior Member

    Thanks! Actually more rocker so I could trim bow up a bit going downwind, as the bow seemed to be always threatening to go under DDW (it never did actually), and also not get stuck in irons in light air tacking. Steering was never a problem, had a nice light feel. I think this was the canoe where I found 3 planks of 17’ long~ 1/2” thick Port Orford cedar that were all warped the same way, with a little rise (1/2”?) at the bow, and a nice tiny curve the rest of the way back that I used for a central spine and the sides, so I didn’t need any chine logs, and they were only a couple of pounds more than the 4mm Okume ply I was using for the rest of the boat. Fast easy build. (Also the boat project where I became allergic to cedar.) most of the bulkheads were XPS. The Aussies at the time were messing with little or no rocker, so I thought I’d try it, even though it didn’t look very good as far as wave drag on the computer screen. compared to 3.5” 55% rocker. I used to design and build windsurfers, so I was going with as little freeboard as I could. The bow and stern were blocks of balsa, and just using varnish never soaked any water. Fun to shape- I wish I’d used more balsa.

    That’s me playing the Bach.
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