Catamaran crossbeam repair

Discussion in 'Wooden Boat Building and Restoration' started by Ryan Glister, Mar 9, 2021.

  1. Ryan Glister
    Joined: Mar 2021
    Posts: 10
    Likes: 2, Points: 3
    Location: Sandgate

    Ryan Glister Junior Member

    Hi Upchurchmr

    Thanks for the reply. This is the forward crossbeam not the rear one, but i take your point.

    Cheers

    Ryan
     
  2. upchurchmr
    Joined: Feb 2011
    Posts: 3,132
    Likes: 174, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    My mistake, its obvious when I looked better.

    OK, I take back most of the nasty comment I made.
    But does it support the forestay?
    Personally I wouldn't want the mast to come down.
    Unless I made another bad assumption.
     
    fallguy likes this.
  3. Ryan Glister
    Joined: Mar 2021
    Posts: 10
    Likes: 2, Points: 3
    Location: Sandgate

    Ryan Glister Junior Member

    Hello

    Manged to get out to the boat today to do a few odd jobs.

    The string test showed at most 5mm of difference between the centre of the beam and the outside of the beam.

    Also checked the padeyes/chain plates where the seagull striker wire joins to the hull. There may be some ingress here but the block that the chain plate is secured to has no signs of rot.


    Upchurchmr ive attached a picture showing how the loads are distributed through the rigging. The forestay is secured to the anchor plate which is in turn attached to the seagull strike. The rigging of seagull strike transfers the load to the hulls.

    Ive also attached a photo showing how the beam terminates inside the hulls. It doesnt appear to be glassed in.

    At this stage am waiting on a spot at the boat yard and will follow up with photos of the repairs 20210314_112943.jpg 20210314_112943.jpg 20210314_112956.jpg 20210314_113006.jpg 20210314_113016.jpg Screenshot_20210314-132146_Video Player.jpg
     

    Attached Files:

  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,639
    Likes: 838, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Ha! Cabbage Tree Creek, I wondered whether it was that Sandgate, or the UK one ! What's it like to find a mooring there ?
     
  5. upchurchmr
    Joined: Feb 2011
    Posts: 3,132
    Likes: 174, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Ryan,

    As your picture shows, the forestay loads pull up, causing compression on the crossbeam.
    What happens if the crossbeam crushes from those compression loads?
    The crossbeam fails, allowing the forestay to pull up the center of the crossbeam.
    This pulls the bows inward, loosening the forestay, allowing the mast to fall to leeward until the windward bow takes all the load.
    The loose forestay then allows the mast to thrash about, causing huge loads in the chainplates when waves and boat motion take the mast from leaning windward to leaning leeward.
    I'm assuming fairly large waves, since that is then the front crossbeam would probably break.

    No matter what, you are going to have a serious problem.

    I'm sure you intend to fix the rot, just pointing out what happens if you trust people who say its not critical.

    I don't know what your "string test" shows.
    If you don't know what the measurement was when built, you don't know if it was just built that way or is caused by problems.

    Good luck.
     
  6. Ryan Glister
    Joined: Mar 2021
    Posts: 10
    Likes: 2, Points: 3
    Location: Sandgate

    Ryan Glister Junior Member

    I got in at a time when a fellow was leaving so got lucky but its worth giving the club a call and checking availability because a few people have come on and off since I've been there. Im really happy with the club
     
  7. Ryan Glister
    Joined: Mar 2021
    Posts: 10
    Likes: 2, Points: 3
    Location: Sandgate

    Ryan Glister Junior Member

    Thanks for your help upchurchmr. Like Fallguy said, i think we are going down a rabbit hole. The string test shows that there is no flex in the beam across its length. The forestay is not pulling it up and the seagull striker is not forcing it down. These two things are not in equilibrium as the tension on the bridle running through the seagull striker is tensioned significantly more than the forestay.

    The boat also has 11 stays in total including a forestay, inner forestay and baby stay.

    Anyway i came on here to ask how to fix this problem, and that is what i intend to do. I hope this alays your concerns
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,639
    Likes: 838, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    OK, thanks for the tip.
     
  9. fallguy
    Joined: Dec 2016
    Posts: 4,281
    Likes: 737, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Based on his initial post; it appears the rot is related to a padeye improperly bedded.

    I figured he was fortunate he didn't have that rot in the other beams is all. And he needs to watch for it. I consider that beam to be less important than the others, but not unimportant. And if the forestay is at risk of failure, then as important. When you tell someone they are lucky; sometimes it is done to wake them up. And he cannot accept such extensive rot in the other beams is all.

    You're lucky that rot isn't in a beam further back is the same as what I wrote; perhaps written better now.

    Now, the reason for the string test is simple. It is to understand current state and for future repair work. If the forestay has been overtensioned to compensate for the rot, for example, that would be terrible, but is not the case, it appears. Assumptions? Yes. He also must support the beam for the work and decide if the beam has sagged. Based on the test, all we know is it is possible the beam has sagged 5mm. The only way to really be sure is to now use a straight edge on the hard and see if it rocks on the bottom at the rot, etc. Then you could, at least in theory, remove that rocker by properly supporting the work. A decision the builder must make... Unlikely the beam would have been built with rocker at the rotten spot..but he needs to look for it now.
     

  10. fallguy
    Joined: Dec 2016
    Posts: 4,281
    Likes: 737, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    One of the important things to do got me yesterday. I built this removable panel and bonded some cleats to the back of the joint. I had already painted inside the box and thought I could squeak by without grinding the paint back. I had to reach way in to screw in a thru hull for a heating system intake and in doing so applied some pressure to the bond. Think 220#. The cleats broke off on two corners. The bond was onto the paint. So frustrating to prove out what you know. This little light panel cleat in a cabin probably the only time in its life experiences a small load ito psi and doesn't cut the mustard. One shortcut I took on a non-structural element. The bond area was say 6", so apparently the rating of the paint is about 30 psi only. Ugh. I put 2000 psi glue on a 30 psi rated substrate. And it failed.

    For this repair, he also must be very careful to first get the wood dry and then prewet the seams with epoxy before applying the bonding pastes. All wood bonds require prewetting. It is easy to think you can skip it. Occasionally, you see old wood suck resins and the surface becomes dry to the touch almost. But here he must dry the wood out so he can prewet it and it is a little funny, but important.

    And another important thing is the timbers inside need to be sistered well and prewetted and epoxy bonded. The sistering even needs careful thought and he ought to consider a single larger timber dadoed for the originals remaining on each side, all surfaces prewet and then epoxy putty bonded. Cutting away less timber and more plywood is also important..when you start cutting; try to not cut the timbers until you know their state. It will be a better repair that way as well. A lot of guys will go in recip saw mad. Don't. Use an oscillating tool and remove only the soft ply first. Take some pictures and then post back.

    My only aim is to help.
     
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.