Radiused vs Flat Top Beam With Lashings

Discussion in 'Boat Design' started by abosely, Aug 8, 2015.

  1. abosely
    Joined: Mar 2015
    Posts: 122
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Big Island Hawaii

    abosely Senior Member

    On the Wharram Cats, the laminated crossbeams are lashed to the hulls on each side of hull. The Narai Mk IV the beam approx 5" x 10" in cross section with the lashings going over the 5" flat top. The rope is 6-8 wraps of 3/8" Dyneema with frapping to tighten it. Usually the beams are built from Douglas-Fir.

    After a while some have found that the lashings have crushed the D-F beam tops and water got in and of course started to rot. It doesn't always happen, so not sure of if it's softer wood sometimes, harder use, the radius use on edge of beam or what.

    I have two ideas to for the tops of my beams where the lashings lay. But wanted to see if there might be any issues with them thinking of.

    The first is to laminate a 3/4" thick plate on top of the beams where lashings lay, about 6"-8" long & the width of beam top, using Ipe, which it is very crush, wear & rot resistant.

    The second one is where I'm concerned there might be a problem I'm seeing.

    Which is to radius the top and edges of the plate, so the center of the plate is carrying more load than it would be if it were flat and only the edges were radiused. They usually radius the edges of the beams some.
    Not have a big hump in it, but use a 3/4" thick stock and radius the edges down to about 1/4" thickness, leaving the center of plate 3/4" thick.

    The Dyneema has virtually no stretch so it's almost like draping a cable across a 5" wide wood beam. Most of the load gets transfered to the radiused edges. This would give a crush proof top plate/pad and spread the load across the full width of the beam.

    Will cut the ends of the plate rounded so not have concentration of stress in one cross section of beam, since the Ipe is almost like putting a steel plate on top of beam and the bending moment would be concentrated right at inboard of the plate.
    Or at least that's how it appears to me.

    It would be a fairly quick & simple job to do to gain some extra durability.

    Cheers, Allen
     
  2. waikikin
    Joined: Jan 2006
    Posts: 2,334
    Likes: 113, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    All sounds pretty good to me.
    J.
     
  3. abosely
    Joined: Mar 2015
    Posts: 122
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Big Island Hawaii

    abosely Senior Member

    OK, just thought I'd put my idea here see if anyone saw a flaw or potential issue in it. :)

    Cheers, Allen
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.