Deck stepped mast beams?

Discussion in 'Sailboats' started by Pylasteki, Nov 24, 2011.

  1. Pylasteki
    Joined: Apr 2008
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    Location: North Carolina

    Pylasteki Junior Member

    Hi Guys,

    I'm curious what some of the racing guys are doing, when it comes to mast beams on deck stepped boats.

    My boat originally had an oak beam, bolted to a 3/4 plywood bulkhead. The beam split right in the center, where the mast step was bolted to the beam.

    I am curious, what it would take to calculate the load on this beam, and what sort of foam composite beams are being used these days to spread the load.

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Are you trying to replace the broken beam? The original design lasted a long time and probably broke through neglect.
  3. PAR
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    PAR Yacht Designer/Builder

    Gonzo has a good point Zach. The important thing is to first determine why it failed. Typical reasons are egged out fastener holes, often from lack or worn out bedding. This isn't the fault of the design or beam, but a maintenance issue. Another area commonly over looked is the through deck wire connection for mast lights. Again, bedding, boot or other water seal, is neglected and moisture gets at the structure beneath. The cause it usually obvious with inspection.
  4. Perm Stress
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    Perm Stress Senior Member

    If breakage is not for neglect, calculation should be done for point load on the order ~1.5 times displacement, for dinghies and small keelboats. Quite a load to handle without a bulkhead or pillar.
  5. John Riddle
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    John Riddle Junior Member


    What kind and size of boat are you discussing?

    What is the depth and thickness of the oak beam and the thickness of the ply bulkhead?

    Is the bulkhead solid all the way to the bottom of the boat or is there an opening beneath the mast? Your description of the structure could be interpreted in a number of ways.
  6. Pylasteki
    Joined: Apr 2008
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    Location: North Carolina

    Pylasteki Junior Member

    It's an old Pearson Triton I'm rebuilding. The original beam was oak, 2 5/8's tall, by 2 thick.

    I actually already built a 3 inch x 3 inch laminated ash and epoxy beam, but have a curiosity about foam cored laminates and making a box beam to do the same job, and how to calculate what the beam would be able to take.

    I also fiberglassed the deck solid, where it was cored about a foot to each side of the mast, spanning across the opening between the bulkheads and overkilled the tabbing with a few layers of 1708 the whole width of the underside of the deck above the beam.

    Since I already have the beam made as a template, I was thinking to make a flange across the top of it, and use it as a relatively small easy to do vacuum bagging and infusion project with my scrap divinycell and nidacore and other bits of string to short to keep...


  7. alan white
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    alan white Senior Member

    The Triton is a pretty solidly built boat and any repairs should be of the same character as the original construction.
    If you use white oak to build a new beam of the same proportions as the original one, (the wood species originally used was likely red oak), the heavier white oak is a bit stronger. That's how I'd do the repair. Choose a curving grain to match the cabin roof----- even stronger than straight grain and far less work than laminating to a curve.
    It would be silly to build a complicated beam from odd materials. The original design was fine, strong enough to last a long time on most of these bulletproof 50 year old boats

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

    I'll agree that the Triton was a very well built design and one of the few of that era that actually was engineered (overly so) for it's build material.

    I'm not sure if red oak was used on the beam, but white oak is nearly twice as strong and only slightly heavier, while live oak is slightly heavier still, but over twice as strong as red.

    Finding stock like this can be costly, so a laminate may be a good option, but instead of a composite beam, it might be better to just lay in thin strips of hardwood, say 1/2" thick, by 2" wide. This would be a 5 layer sandwich and likely stronger then what was originally there. The epoxy glue lines would help mitigate moisture ingress if you had a minor leak too. Your ash beam sounds like it'll work fine, though I don't like ash on boats except for trim, in a laminate it's much less prone to rot, then a solid timber would be (assuming encapsulation).

    Go with what you have (the ash laminate) and save the composite work for your next project.
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