Folding Twin Keels Using Angle Strut?

Discussion in 'Boat Design' started by mcm, Apr 15, 2010.

  1. mcm
    Joined: Sep 2006
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    Location: Port Townsend, Wa., USA

    mcm Senior Member

    Analytic 10 Steel, is a 10m or 31ft, steel, twin keel, certified class A cruiser.

    My questions are:

    How does a twin keel fold flat using angle struts?

    Does the folding force act through the hinges or the strut?

    Is the folding force hydraulic or electric motor?

    Does that strut and those hinges look strong enough to withstand the forces expected of certified class A cruiser?

    I've only seen hinges like those on the wing ailerons of aircraft, and where does one buy a strut like that?

    View attachment A-10.pdf
  2. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    I'm going to caveat my comments by saying that I design aircraft not boats.

    Looking at the design, which is essentially similar to a strut supported wing, I will admit to being a bit puzzled. You want the struts to be in tension when loaded normally, because they don't really look up to taking the buckling stress if they were primarily loaded in compression.

    My first thoughts were that you would want the windward ballast keel down when sailing, to gain the modest benefit from some additional righting moment. Doing this would both limit the effectiveness of the asymmetric ballast end plate effect on the low aspect keel and, more significantly, load the strut in compression.

    Looking closer, and assuming that the intention is to sail with the leeward keel lowered, leeboard style, then I realised that the lifted windward keel would shift ballast outboard, just like the crew sitting out, so would be quite useful in terms of reducing heeling. This is fine structurally, as the leeward strut will be in tension, and in terms of gaining some benefit from reduced keel tip vortex losses from end plate effect (the keel effective aspect ratio will be greater than its physical aspect ratio).

    It's a clever idea, but the mechanical complexity would concern me. Having the keels jam up would present a problem, plus there's some added drag from the struts, particularly as they a surface piercing.

  3. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    The strut is pulled up through a slot in the topsides fitting. The exact mechanism for pulling it up is not shown.

    The folding force would be tension in the strut.

    It could be almost anything, including a line led from the top of the strut to a winch on deck.

    That would depend on the details of the construction. However, being a hinge, it takes no lateral bending moments, only shear force. So the loads are considerably lower compared to the keel attachment of a cantilevered keel.

    I would expect the strut to be fabricated by the builder. It looks like it's just a piece of steel. There would have to be stops welded to the upper end so the side loads on the leeward keel do not pull the strut through the topsides fitting.

    It's not clear from the drawing whether the strut locks firmly into position in the topsides fitting when the keel is down. One thing that concerns me if the strut is left free to move vertically, in a seaway dynamic loads could cause one or both of the keels to move about as the boat rolls. That could cause hammering of the topsides fitting by the strut as the keel moves up (or the boat rolls down while the keel is more stationary) and then slams into the stop when the motion is reversed.

  4. mcm
    Joined: Sep 2006
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    Location: Port Townsend, Wa., USA

    mcm Senior Member

    I think you're right, the strut is pulled up through a slot, and that makes the locking stop mechanism the vital mystery.

    But there's no way to contact them.

    Their contact link doesn't work, and their discussion-forum is not up.

    There must be a standard locking stop mechanism that's designed for those tension and compression loads.

    But how do i find them?

    In other words the same type of hinges used for cantilevered keels should be more than stiff enough to hold these 900kg each twin keels tightly in place?

    Where do i find those hinges?
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