keel locking mechanism

Discussion in 'Sailboats' started by bretag, Oct 24, 2013.

  1. bretag
    Joined: Sep 2010
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    bretag Junior Member

    Is there a relatively simple mechanism (design) for locking a ballasted daggerboard in position when lowered-to prevent small side to side clearance movement?
    The other issue is just how important is this if clearance is minimized, but is still sufficient for unimpeeded vertical motion.
     
  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Google mirabella 5 drop keel ron holland.

    Someplace on the internt is a explanation and drawings of her drop keel bearing details. Its a pdf file..ive seen it in the past
     

    Attached Files:

  3. fastwave
    Joined: Jan 2007
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    fastwave Senior Member

    A simplen wedge form of the top part of the fin will fix the side movement when in the lowered position.

    In sailing conditions clearance is not a problem. You get a problem when the net force changes from one sode to the other.
    Rolling in waves at rest or downwind for example. It is a combination of gravity as well as sideforce
     
  4. daiquiri
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    daiquiri Engineering and Design

    Hi Bretag,
    How big the boat? How heavy the ballast? How do you plan to lift and lower it?
    Part of the answer will depend on your answers to the first part of questions.
    From the hydrodynamic point of view it will probably not be so important, but could be from the structural point of view. One of purposes which the clamping system serves for is to ensure that the contact points with the hull are well known and hence the transmitted forces predictable - instead of letting them change randomly the position and intensity as the keel changes it's lateral and angular position.
    Cheers
     
  5. bretag
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    bretag Junior Member

    hi,
    the boat is 32' long, 8.5' beam, displacement max 5000lbs. Construction cedar strip plank with glass skin(s). Draft max will be 6.5', min 2.5', ballast wt 1200lbs. Keel will be glass/epoxy with lead bulb & jackscrew lifting/retracting.
    I plan it to be a high performance version of my present boat which i designed & built of steel with a strip ply cabin. (steel, trailerable, lift keel 4.5/2.5', disp approx 6500 lbs)
     
  6. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I am attaching here a schematic of the locking mechanism which has been used for retractable stabilizer fins on board an existing ship we have been working on. The same principle can be used for a retractable keel.

    It requires 2 wedge-shaped locking pins. As the pins make their way horizontally (pushed by an electromechanical or hydraulic actuator), they also press the keel down (thanks to the wedge profile) against the mechanical stops near the bottom. In this way, the keel is firmly locked in 4 fixed points which can thus be adequately designed and supported, and surfaces prepared and protected for the expected loads.

    Hope the drawing is clear enough.

    Cheers
     

    Attached Files:

  7. bretag
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    bretag Junior Member

    Thanks for the sketch, the mechanism is perfectly clear.
    cheers
     
  8. philSweet
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I gather the days of building the slot 3" wider than the board so you can rod out the oystershells is over. I made the mistake of fitting a tight board once. Had to carry oliveoil on the boat to lube the board. How do you calculate the moment forces if the board is tight? I figure the land is at the top and the bottom. The hold-down should only be a bit stronger than the weight of the board. You usually want it to release in the event of a grounding. Do you know what screamers are? They are used by alpinists to reduce shock loads in a fall and can be put to good use on boats as well.

    http://www.yatesgear.com/climbing/screamer/
     
  9. peter radclyffe
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    peter radclyffe Senior Member

    what is the expansion, contraction fore and aft on a 4 metre chord steel lifting keel in the tropics and the north pole
     
  10. philSweet
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    philSweet Senior Member

    I'd be more worried about bashing into a balk of timber and crushing the leading edge just below the slot. Royally screwed then, you are. I've literally chain-hoisted a boat out of the water by the board trying to get a board unstuck. Had to secure the boat, loosen the chain two feet, and drop the boat to get it to budge. It's amazing what one square foot of surface area can produce in the way of shear resistance. The cause was a bit of rust that had dropped down from the housing top. A simple lanyard hold down that is fused to break at, say, 2 X board weight should do the job. If it needs to be tight, use some sort of track system that gets the surface area down to the smallest practical amount to lesson the risk of binding. You really do not want the whole of the board cheeks as contact area.
     
  11. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I had the following idea. See attch. When board is lowered, it lands on the ends of the clamp and flattens it out, rotating to cams in to grab the board. It looks like it would be fairly bulky, but two or three could be stacked in a carrier to easy manufacturing and reduce size.

    A solution in search of a problem?
     

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  12. michael pierzga
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    michael pierzga Senior Member

    Actually swelling and corrosion of the drop keel assembly is an issue.

    Not long ago a relatively new superyacht, 100 footer, drop keel, professional build, well known architect... was hauled at the yard to find out why the keel would not function. By the time it was over the keel had to be cut off and the remaining stub buried in the keel box blasted to pieces with a super high power water jet.
     
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