How to mount a Thern/Davit style crane to deck

Discussion in 'Boat Design' started by byob1985, Apr 24, 2013.

  1. byob1985
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    byob1985 Junior Member

    Hi - I am looking to mount a Davit/Thern style crane in the middle of my 21ft Carolina Skiff. I am a novice when it comes to hardware installation, so I'll probably have a professional perform the work. My question is: my Carolina Skiff is the older style skiff that is comprised of a deck, then foam core, then hull, not the newer style which apparently has a cavity between the deck and foam core. Will this make installing the crane base difficult? I believe most load bearing cranes (this will haul loads up to 400 lbs) are bolted down through the deck via a sealed base plate.

    Also, is it advisable to mount the crane in the dead center of the boat? I have flexibility to mount it anywhere, but would think just behind the bow plate, the center, and at the stern would make the most sense from a stability and space maximizing perspective.


  2. gonzo
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    gonzo Senior Member

    You will have to laminate (fiberglass) in a large pad to spread the load. Carolina skiffs have a rather thin deck that uses the foam for rigidity. I don't know if you can keep it from delaminating if the crane base is pulling. You may have to install something that goes across the whole deck to the sides.
  3. byob1985
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    byob1985 Junior Member

    Thanks for the help Gonzo. So laminate a big plate onto the deck and then bolt the crane base to it? I was planning on building some supports, either lateral wood braces or weld a metal tripod to the crane mast, in order to brace it and reduce the chance of delaminating.
  4. messabout
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    messabout Senior Member

    Mounting the crane is problematic but do able. There is the question of boat righting moment that should be addressed.

    If you are to lift 400 pounds and the boom end is four feet off the center line, then the result will amount to 1600 foot pounds. Depending on what it is that you are lifting, the effort of the crane will be less while the object is still submerged but when it breaks the surface the apparent load will increase suddenly.

    Let us imagine a four foot reinforced pad to which the crane is attached. The base of the crane post is in the center of the pad. You will now have a lever of sorts. The part of the pad that is farthest away from the extended boom will try to lift and the edge of the pad nearest the boom will press downward on whatever it is fastened to. The force numbers will be big ones if the load is anywhere near 400 pounds. I suspect that you will need to construct some sort of framework that will transfer a generous amount of the load to the sides of the boat so that the frame to side attachment is in shear mode.
    Last edited: Apr 25, 2013
  5. byob1985
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    byob1985 Junior Member

    Ok thanks. Sounds like there will be considerable side forces at play and a solid framework and laminated base plate are required. I guess the concern over not having a cavity between the foam and deck is not that great though. There would still be load concerns regardless.
  6. daiquiri
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    daiquiri Engineering and Design

    Achtung, achtung!!!

    Before you proceed, make sure that your boat has sufficient transverse stability (plus a margin) to counteract the heeling moment induced by the fully-extended derrick arm with the appended maximum load.

  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Agreed Slavi and I'll add, plus a good size margin, as shock loads can double the cantilever expectations. G-10 would be the logical choice, though not the least expensive. Plywood will be the least expensive, which will need encapsulation and sheathing to protect it, so cost differences become closer.

    Simply put, this needs to be worked out and you haven't provided enough details. What is the height of the davit, the reach (arm length), expected loads you like to lift, conditions you'll use this in, length, beam and weight of your boat, plus the location you want the davit to live. With these and likely some other answers to question yet raised, we can offer a good set of suggestions on backing plate dimensions, attachments, etc.
  8. philSweet
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    philSweet Senior Member

    ... and are you planning on a manual hoist, or an electric or hydraulic hothead?

    For the ultimate in what you could do with that boat, I'd build it in four pieces-

    1. A drop in aluminum frame that spans the entire beam of the boat and up the sides.
    2. A hinged table that rides on the frame out board of the boat that lets you capture the load and pivot it into the boat over the side. Swinging loads suck. Look at a lobster trap boat.
    3. An electric/hydraulic winch that pins to the frame or mounts to the center console, which would also need some modest reinforcing assuming a 2:1 purchase.
    4. Shears that pin to the frame.

    However you do it, you should consciously choose how you want the system to fail in the event of a problem, and build that in.

    Please tell us this isn't for man lift or person/diver recovery.
  9. viking north
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    viking north VINLAND

    Might also be a good idea to describe your proosed use to give the engineering guys here a fair kick at the can and better armed to supply you with the info you need. Being a builder I will of course throw in an idea. :) While it would require a small square section of core removal and a solid section replacing it, I would take a serious look at installing a compression type post rather than glassing in large pieces of ply on deck/ cockpit floor. Just an idea that might or might not apply as while I know the make , I am not that familiar with the layout and construction of your boat. All this of course as mentioned in several posts is subject to the stability question. Possibly not good for lifting cannons off the bottom :D

  10. byob1985
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    byob1985 Junior Member

    Thanks all for the great ideas.

    As far as specifics go, the beam of the boat is 8ft from the bow plate to transom. It's probably around 1,600lbs. The davit needs to be at least five feet high (enough for a 1.5' high object clear the gunnel) and the reach needs to be at least 5.5' (long enough to get an object 3' wide) if mounted in the center (half of beam + half of object width).

    The object in question is a wire coated oyster cage filled with delicious oysters. It can be as light as 70lbs. and as heavy as 400lbs. when full and fouled.

    The conditions will be relatively calm. I plan on using an electric winch, but may consider a small hydraulic unit to power the winch for speed.

    I saw a rig this weekend that was more than sufficient. It was a tall mast made of reinforced aluminum (8' tall) with a boom equipped with a pair of block and tackle running along the boom. The mast was laminated into the deck and there were vertical side supports laminated into the gunnels at the point of the mast. Most critical, there was a forward and pair of side stays/shrouds made of wire cables that affixed to the top of the boom and were bolted down onto the deck. Apparently they are the most important element of stability in handling the side, swinging load.

    Hope this helps from a specifics perspective.
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