Catamaran crossbeam repair

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

  1. Ryan Glister
    Joined: Mar 2021
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    Location: Sandgate

    Ryan Glister Junior Member

    Hello

    I have a 35ft Shawn Arber glass over ply sailing catamaran. The forward crossbeam is boxed ply and it has considerable rot in a section measuring approximately 30cm by 15cm.

    The previous owner attempted to repair this by laying glass matt and epoxy over the area but this appears to have only made the situation worse by trapping in the moisture. The rot is so bad that a screwdriver can easily be pushed through it. It is essentially mush.

    I have attached a photo and the approximate area is in the part in the red rectangle (it also extends on the back side of the beam where the net attaches to the beam). It has hard to demonstrate with a photo though as the paint is covering the rotten wood. I have tapped over the beam with a hammer and everywhere else feels solid

    Are there any suggestions on how I might go about repairing this? One idea I had was to cut the rot completely out in a square section, glue additional stringers in and then place fresh ply back over the top and epoxy it in place.

    The beam operates primarily in compression, resisting the hulls from pushing in toward one another.

    Any ideas would be greatly appreciated.

    Thanks for your thoughts.

    Cheers

    Ryan
    Resized_IMG-20201230-WA0058.jpeg Resized_IMG-20201230-WA0058 (1).jpeg
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Well, fortunately, that is not a major structural and critical beam.

    I'd be concerned that padeyes were not properly installed and the problem source.

    To properly install the padeyes, you must overbore and fill with thickened resins and rebore to proper dimensions, then use 4200.

    All the padeyes need checking for rot.

    For the repair, you'll need to remove the rotten section and no rot can remain.

    Basically, you will cut out the rotten bits and then you can use 1/2" aquaplas or plywood and cleat the section edges. You make a thickened epoxy mix and screw the cleats from the front overnite.

    Then remove the screws and cut in the replacement pieces. They may need to be tabbed or edge bonded if more than one face is involved.

    There will be a hump, but it won't matter.

    Once you know the ply thickness, you can replace it and sheath it, but overlaps are required and I'd use epoxy.

    This is an overview. You may have more questions and you may need to reframe internal timbers, etc.

    It is a relatively easy repair.
     
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  3. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    You have to cut out and replace the rotten ply, there is no other way. If there are wooden cleats inside, those have to be replaced too. The replacement panel has to be scarfed in place, minimum 8:1 scarf, or you use buttblocks, but scarfing is preferred.

    The work goes a follows: grind away the paint and fiberglass until you see sound dry wood, then continue at least 6". Cut away all bad ply plus 4" of sound wood (that's to remove any rot spores). Now you have an ideea of what's in the corners, wooden cleats or epoxy fillets. If wooden cleats, cut away and replace by scarfing, fillets can be reused. Scarf in new ply of the same thickness using thickened epoxy. Draw the scarf tight with screws, then remove and fill the holes with more thickened epoxy. If the beam is also glassed, reglass the repair area with similar weight glass. Fair and paint.
    Once you cut into the beam you may discover that not only the top panel is affected, but also the sides. Repeat the procedure on the side, but only after you replaced the top.

    Do you know how to cut scarfs in place? If not, just say and I'll explain it. Please mention what tools you have and are comfortable with, there are several methods, from freehand angle grinder to hand plane.
     
  4. Ryan Glister
    Joined: Mar 2021
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    Location: Sandgate

    Ryan Glister Junior Member

    Thanks for the detailed reply Fallguy, this is very helpful. I will definitely take your advice and check the pad eyes. Glad to hear that its a relatively straight forward repair and that the beam isn't critical to the integrity of the boat as well.

    I think the water has gotten in where people stand on the beam to watch the anchor. Their heals have created a depression and a crack in the paint which has drawn in moisture over time.
     
  5. Ryan Glister
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    Ryan Glister Junior Member

    Hi Rumars, thanks for the very helpful reply. I'm not sure how to cut in scarfs but my father is a cabinet maker and says he knows how to. He also has a fairly comprehensive set of tools including an angle grinder and hand-plane.
     
  6. Rumars
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    Rumars Senior Member

    Well, scarfing will be no problem for him, just tell him he has to fit a big dutchmen, he knows what to do. Make sure to prewet all surfaces with unthickened epoxy and wait a few minutes for it to soak in before applying the thickened mix (use microfibers). Epoxy does not like to be clamped to death, just handtight is fine.
    For abrasion resistance the plywood can be glassed, you can add some light glass in the area where people stand even if the beam is not glassed entirely, just feather the edges by sanding. Also add some non-skid there, it makes it safer.
     
  7. fallguy
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    fallguy Senior Member

    You might need to look closely at the beam sort of more globally.

    The emc of the inside of the beam needs to be under about 14%. I recommend when you get the box open, you consider warming the box inside or running a fan in it for a few days.

    Also, if the beam broke under foot, it may need some beefing up.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    God only knows what is going on under the surface there, and surely there is more required of that member than "keeping the hulls apart".
     
  9. fallguy
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    fallguy Senior Member

    My only reason for commenting on the netting beam not being super important is because if his beam were a middle beam; he needs to be grounded for as serious a rot problem at 30cm..

    There is some likelihood that the anchor roller or fairlead mounts also contributed to the ingress. If someone added those without structural sufficiency; the failure could be there.
     
  10. Ryan Glister
    Joined: Mar 2021
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    Ryan Glister Junior Member

    Thanks again Fallguy, I will take your advice about warming it up when I open the beam up. I also plan on removing the anchor roller, fairlead mounts and all through bolts so that I can check for rot as well as rebore and seal the through holes like you said to do.

    Do you mind elaborating on this comment as i'm not entirely sure what you mean "My only reason for commenting on the netting beam not being super important is because if his beam were a middle beam; he needs to be grounded for as serious a rot problem at 30cm.."

    What is the purpose of the beam?

    Purely out of interest, if the beam weren't there at all, or it failed, and the forestay was attached to each hull by a bridle how badly would it impact the structural integrity of the boat? Would it cause a catastrophic failure to the structure, or would it have no impact at all?
     
  11. fallguy
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    fallguy Senior Member

    Ryan-respectfully, that is a rabbit hole.

    It would be inappropriate for me to comment and is not the general purpose of the post, so I won't. All I said is that rot in another beam of the boat and you should not be sailing her. A 30cm section of rot is fairly significant. Consider it similar to cutting the beam in half. Would you do that on the next beam back and go out? I think not and that was my comment. Frankly, I am a little unnerved about the forestay and perhaps you ought to ground her if that could fail.
     
  12. Ryan Glister
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    Ryan Glister Junior Member

    Thanks Fallguy, i respect that response and completely understand where you are coming from.

    That also clears up what you meant by grounding as I was a little confused.

    Ill be pulling her out of the water some time in the next few months and will follow up with photos of the repairs.

    Cheers

    Ryan
     
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  13. fallguy
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    fallguy Senior Member

    Post back here with follow ups.

    Make sure the beam is supported and perhaps even with some laminated timber 50x150 because it could easily lose shape with as much as it is rotten. Probably clamp the timbers on before you cut it unless the dimension is simple squares. You may find the forestay and the net are keeping it from deflecting and you ought to string test it as well to look for sag already. If it is already sagged a bit or pulled up by forestay tension? then you'll need to build it back straight which can be done jacks and shims I'd say. If the forestay has pulled it up; stop sailing.

    To string test, just get 3 timbers of same thickness and draw the string singing tight across two of the timbers on the ends and see if the 3rd board is thicker or thinner than the gap in the middle. It may require bigger boards and a tape measure to get over the rollers, etc.

    Is the beam glassed to the hull?

    Kind regards.
     
  14. Ryan Glister
    Joined: Mar 2021
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    Ryan Glister Junior Member

    I'm heading down to the boat tomorrow and will carry out the string test and have a closer look to see how it is attached to the boat. I can take some photos as well. I'm also planning on visiting a local boat yard to find out when I can get in.

    The beam seems as solid as a rock/stiff as a board, which I found surprising given the amount of rot in the section. I've jumped up and down on it, and dragged the anchor out by putting the boat in reverse and there seems to be no flex in it at all when I do this.

    Thanks

    Ryan
     
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  15. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Ryan,

    Anyone who tells you the rear crossbeam in not critical structure is looking to get you killed.
    If you believe them then just cut it off at the ama gunwales.
    I'd be suprised if you got away from the dock before you get complete failure of the boat.

    The description of replacing all the wood and scarfing the plywood will work fine, just add a few layers of glass over the patch to be safe at the scarfing.
     
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