5m Ocean cruiser twin bulb keel

Discussion in 'Sailboats' started by HelgeS, Mar 20, 2018.

  1. HelgeS
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    HelgeS Junior Member

  2. Angélique
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    Angélique aka Angel (only by name)

    Looks to have a lot of lead, maybe try to put the boards a bit more forward . . ?

    [​IMG]
    image from goodoldboat.com

    The sketch comes from post #1 in the thread: Centre of Lateral Resistance, Centre of Effort and lead
     
    Last edited: Mar 20, 2018
  3. Angélique
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    Angélique aka Angel (only by name)

    Just a thought; make the boards hinged on the top side, so you can hoist the windward bulb hiking a full board length outwards just above the water, the increased distance from the centerline would make the bulb's weight a lot more effective, plus above the water it weighs more, also it would reduce more of the resistance in the water than it gives in the air.

    Good luck !
     
    Last edited: Mar 20, 2018
  4. PAR
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    PAR Yacht Designer/Builder

    There's lots of potential issues associated with this arrangement. Maybe canting the boards a touch, which wouldn't steal too much interior volume. Pigeon toe them a degree or two as well. Of course additional drag from the windward board/bulb will slow her and decrease some windward ability too. Windage in the rig doesn't help things much either.
     
  5. HelgeS
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    HelgeS Junior Member

    It is very easy to build this model. I started with the transom 165mm and the sides 60x510mm. Bending them out with a 200mm frame 200mm fra the transom.
    The bottom piece is 60mm wide and put 25mm deep in the middle and 200mm forward,
    60mm wide and 15mm deep at the transom . The ply is 3mm. The rest follows that. The boat starts building on a flat bead and the cabin top is added later. This makes the frames to be put in a very accurate position.

    I plan to put the keels on inserts, bolted on the outside, to avoid conflict with the cabin roof, which is high. Keel (full size) 40x145cm, 5x9mm plywood, attached 2x half bulbs a 35kg, with a 10mm aluminium 30x40cm. Epoxy glued and screewed in the middle of the plywood keel.

    The plan is to make a boat which is able to cross oceans with keels not moving around. Chafe is a threat, and at capsizing, it have be in place and not come loose. At harbour outpointing keels will be in conflict with the keyside. I have already a KRILL2, but the keel is moving and making noise in high waves, scary.
    Orust Orust beste best http://paradis-2.blogspot.no/2017/08/orust-orust-beste-best.html
    Krill2 http://paradis-2.blogspot.no/p/krill2.html
    Nesten Oseberg, det gikk for seint. http://paradis-2.blogspot.no/2013/06/nesten-oseberg-det-gikk-for-seint.html
     
  6. HelgeS
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    HelgeS Junior Member

  7. sharpii2
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    sharpii2 Senior Member

    Try moving the mast aft, so the the center of the combined sail area lines up with the trailing edge of the keels. This will cause a slight weather helm, which will make the boat want to point into the wind. this is good because then the rudder, being used to counter act this, will cause windward lift.
     
  8. HelgeS
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    HelgeS Junior Member

    Just rigth, you are the first one to understand this without going traditional. I just did it and got the windsurfer effect. I moved the keels more back, align with the true sailing course, moved the mast aft, and it was sailing. Now its time for testing assy spi and downwind sailing. 5m seil-kjølbak1.jpg
     
  9. gonzo
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    gonzo Senior Member

    What is the reason to have twin keels if they don't cant or can be raised? They have no advantage over a single keel and a lot more cost and complication.
     
  10. PAR
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    PAR Yacht Designer/Builder

    . . . and drag . . .
     
  11. Angélique
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    Angélique aka Angel (only by name)

    I can think of one advantage, if you lose one keel, you will only list, and still be able to sail one way, instead of immediate turtling, so we're talking about a safety feature here, I would guess . . :rolleyes:

    P.S.

    Well, if it's the windward keel you lose, you might first been knocked down or turtled, but only temporarily I'll think, since the other keel will partly right you, leaving a counter list for sailing in the opposite direction, then keep that ‘‘new’’ windward keel in the water, otherwise you would lose your leeway resistance, I believe . . o_O
     
    Last edited: Mar 26, 2018
  12. HelgeS
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    HelgeS Junior Member

    Yes true . I can make a single keel if I want to, but then I need a crane every time I want to go for a trip. The twin keels will have less weight each and will be more easy to handle. I will be able to remove them when the boat is on the water . Out there I will be able to inspect and repair, remove debris etc. The boat I can launch and retrieve from a custom braked trailer. What I loose in speed, I gain in practikal use. I will also be able to cast the bulb at home and make the whole keel by myself without welding. No rust. Only led, aluminum, plywood, epoxy and fiberglass cloth . Good enough reasons? Besides the construction is very easy to make.
     
  13. gonzo
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    gonzo Senior Member

    You can cast the bulb for a single keel at home without welding. Each keel will have less weight than a single, but together will weigh more than a single. Lifting keels and centerboards have been in use for a couple of centuries in all kinds and sizes of vessels without any serious problems. Try a decision matrix or other system where you weigh the pros and cons of a design.
     
  14. Angélique
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    Angélique aka Angel (only by name)

    Maybe I'm thinking far too traditional, but except for construction, sliding or 180° hinged (post #3) up the keels looks far more practical to me for trailering than demounting the keels.

    Also hauling the keels up might become handy when ever in skinny waters and/or when drying out there (you can come later and go earlier then), or when seeking shelter or anchorage in a shallow cove, I'll think.
     
    Last edited: Mar 26, 2018

  15. sharpii2
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    sharpii2 Senior Member

    While writing my previous post, I failed to realize that your boat has an immersed transom. This may have been the real reason it failed to point. The immersed transom creates considerable drag at the very aft end of the boat, giving it a strong tendency to point down wind. If you look at most sailboat hulls, you will see that the bottom of the transom is always above the surface of the water. By being arranged this way, the moving water can take a curved path under the boat then back up to the surface again, without making any sharp turns. An immersed transom caused the passing water to try to make a sharp 90 degree turn, which it can not do. When it fails, it creates a great deal of turbulence, which in turn creates the drag.

    Immersed transoms are used on planing and semi-planing powerboats, because the go so fast that the passing water does not even try to make the turn, but instead moves straight aft past it. Your boat, with even the original rig shown, will not move anywhere near that fast.

    Even a rather tight radius curve back up to the surface is better than an immersed transom. But a tight curve there will still limit the performance of your boat, especially as it nears displacement hull speed. At slower speeds, it will be less of a handicap.

    Your boat can be made to point up wind, even with the immersed transom, if the Center of Area (CA) of the sail plan is moved aft, while the keels are left in there original position. But the CA of the sail plan will have to be moved further aft than I originally said it would. It may have to be moved some distance aft the trailing edge of the keels. By moving the keels aft also, you have significantly reduced the sail area. You have also reduced the length of the steering arm by placing the keels much closer the the rudder(s). In blowing conditions, this could make the boat dangerously difficult to steer.

    As for the twin keels, I see no controversy here.

    This is because the stated purpose of this boat is to make a long distance cruise across open water.

    The twin keels provide the following advantages for this purpose:

    1.) they will tend to make the boat track better, as they will resist quick course changes,
    2.) they will lengthen the roll period of the boat, as they represent a significant portion of the boat's total displacement and they are far out from the center line. This will will reduce the shock loads induced by the boat rolling. The shorter the roll period, the greater the shock load.
    3.) they remove the headache of having an attachment point, which must be heavily re-enforced, in the middle of the hull. By placing the outboard, the support structure can be somewhat simplified. Also, structural problems, due to corrosion and/or fatigue will be easier to spot, without a haul out.

    This being said, they do have a disadvantage when it comes to top performance. A single keel of the same type will always out perform twins. But this may be by seconds on the mile, which will not be noticed when cruising, but would be blaring obvious in a race.
     
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