Hull extension

Discussion in 'Boatbuilding' started by PDQAltair, May 29, 2013.

  1. PDQAltair
    Joined: Apr 2010
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    PDQAltair Junior Member

    Specifically, I'm planning a short (24") transom extention to improve boarding on a PDQ 32 catamaran. Rather than burn up forum space, my reasons and some photos are posted here:

    In order to make a mold I have been considering pulling a mold from the aft 30" of the hull, slitting and trimming and sliding that aft in order to form a fair extention mold. The mold would be reinforced with external frames. I would then take the work home where I could do the laminating and internal construction in the shop, returning to the field to gel coat and bond (glass inside and out). All solid glass, no core, since the existing boat is solid below the waterline anyway.

    One problem involves laying up against the existing hull upside-down; obviously I cannot invert the hull. While I know I can lay-up on a ceiling (not much fun), I'm not so sure about a surface with mold release on it.

    And of course, any other practical suggestions and concerns would be of interest.
  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    An option might involve building a "plug" first. Strip building the plug will let you get the transom/extension into a smooth transition. Wood is a lot easier to shape than FRP and not nearly as itchy. You could mock up the whole thing in this manner. You won't have to spend so much time lying on your back, looking up into a gooey mess of glass and resin.

    You'd then need to make the mold which does extend the whole labor process. But it might be more practical in the long run.
  3. keysdisease
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    keysdisease Senior Member

    Maybe take some mica and strap it to the bottom with 30 or so inches sticking out aft? Should extend the hull lines nicely. Lay up enough for it to hold its shape and then take it home, or even do the whole job onsite? I'm guessing you're on the hard?

  4. PDQAltair
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    PDQAltair Junior Member

    Plug. Sure, but not so easy to get fair since there is some compound curvature. Certainly could be plan C. Shaping glass doesn't scare me much; not my first project, just the first one I've approached by pulling a mold directly from the hull.

    Mica. In fact that general method is often used on cats, but depending on the model there can be some very sharp curves, and mine is one of those. Perhaps I could lay-up some very thin glass, strap it on, and then beef it up in place. A good solid plan B.

    Yes, of course the work would be done on the hard, but since it will have to be done in the peak sailing season (the only time it's warm, obviously) I would be trying to do this in just a few weeks total, while it is out for bottom painting. I also live ~ 1 hour from the boat so over 3 hours are wasted each trip, between driving and packing. With a mold I can take much of the work home where I have a full shop, only bonding and matching gel coat on-site. Since there are 2 hulls, a mold makes sense.
  5. nimblemotors
    Joined: Jun 2009
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    nimblemotors Senior Member

    Have you thought about just building a fold-down step? Fold them up underway,
    and fold down at dock or anchor. Much simpler thing to make I'd think, and doesn't increase your LOA.
  6. PDQAltair
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    PDQAltair Junior Member

    a. I don't really care about LOA as a cost. My slip is per month only and other thing don't amount to much.

    b. There are other advantages for a cruising cat. Many are truthfully over weight for the hulls and drag the sterns in certain conditions. This should solve that and as a result, add 0.1-0.3 knots of speed. Small, but material.

    c. Pitching reduction. The Chesapeake is bad for that and this is known to help very similar cats.

    d. Following Keys mica suggestion, I'm thinking of a metal mold, as there are some curves mica would have trouble with. I have the material and the fabrication is dead simple.

    The proposed extension is minimal--2 feet--since this is enough to solve the boarding issue and dragging issue, and should minimize possible handling impacts. I'm keeping it low-profile to minimize quartering wave impacts.
  7. tunnels

    tunnels Previous Member

    ok so you only want 2 feet extension for each hull ! but how far up the back you looking at going ?? if its that small its the bottom part that's most important and faring it into the existing hull shape so with a rolled over flange say 30mm wide all round the upper edges a top could be made and simply epoxied together and no working upside down , with a good thick flange against the hull also they become bolt-on's and just a adhesive type sealer would be easy to remove in the case of getting damaged at anytime .

    All this work could be done at home then take and bolt onto the boat each side if the hulls are identical in shape is just a mater of making one mould to make both of them and the top the same make a wooden painted mould !! take a quick mould off a patch of nonslip some where in the deck and that could be used as the non slip pattern for the mould as its inverted already !!
    If you make mould you could offer them to other people with the same boat !!:D
  8. PDQAltair
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    PDQAltair Junior Member

    The non-skid on PDQs is made by rolling thickened gel coat, not molded. It wears amazingly well, as it is still sharp on this 15-year old boat. However, I don't think a mold could be taken as the pattern has little relief.

    Yup, I've considered a bolt-on variant. Since the transom is already a sugar scoop of a complex shape that would be tricky. But I'm still thinking on it.

    She won't be due for bottom paint for a few months, so I've got time to ponder.
  9. Chuck Losness
    Joined: Apr 2008
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    Chuck Losness Senior Member

    I added a 30" extension to my gulfstar 37 last October. I used 1/8" doorskin to make a mold off the existing hull and then glassed it using 4 layers of 1708 biaxal cloth. I have a lot of photo's. Making the mold and the initial glassing was easy. The final finishing took forever. Send me a private message and I'll try to answer all your questions and give you more photo's

    Attached Files:

  10. PDQAltair
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    PDQAltair Junior Member

  11. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Thanks for the update. Your second blog entry is one of the best reports on a boat modification I've seen in quite a while.
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