30' cruising catamaran repair log

Discussion in 'Boatbuilding' started by jdory, Aug 7, 2015.

  1. jdory
    Joined: Aug 2015
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    jdory Junior Member

    I've got a KHSD 30' cruising cat I built in 1993 - 1994. I and a buddy did this quickly to get our feet into a brand new small boat crab fishery, then the plan was the buddy would buy me out and I would begin my 36' cruising cat. So we did not finish the boat - just got it floating to serve as a crab boat to get rich on. One month after launch, a very, very large barge was in the small boat harbor unattended and got loose during a relatively mild blow. It scooted across the harbor and banged into our boat, which was moored on a floating dock along with some less wide aluminum fishing boats. A friend said he heard the sound of splintering when it hit. Lots of crush damage and some holes in the starboard hull. It sat for 2 years as the lawyers duked it out about whether the barge company would pay us anything, then I built a boat shed over it and there it has sat for 20 years.

    Well, I finally mucked out all my junk from the boat shop and can get to the boat now - I retired and have time to tackle the repair. So I have started. I am hoping for some solid advice here and there. But let me state out right: It is the only boat I've built short of one wood kayak, and I had never been on a multihull before tackling the build. I know very little about sailing, but have done some longish trips on monos. There is no sailing community where I live so I can't run down to see how something was done or handled. So forgive my naivety.

    The build log, which I don't maintain anymore, can be found here: http://www.dorydesign.com/boat.html

    and all the pics together but not in very good order are here:
    https://www.flickr.com/photos/jdory/sets/72157625973914417
    with the damage on page 3.

    I want to honor the designer's desire to keep the cat light, so my struggle at the moment is how to repair some bulkhead damage. I could sister the bulkheads, but the damage is on both sides so unless I sandwiched them, the damage would be visible on one side. Or I could sister and on the other side add some bi-axial fiberglass or something. Other option is to scarf in new sections, which I tried and doesn't seem like that much more work then creating new bulkhead sections to sister. I'll upload some photos to that flickr page so I can post them here for specific conditions.

    thanks for reading, Jim
     
  2. jdory
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    jdory Junior Member

    Here's some photos of typical damage. One photo shows where I am trying a scarf. You can also see considerable damage to the stringers in some areas. I haven't quite figured out yet how to post images in a useful manner.
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  3. jdory
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    jdory Junior Member

    Here's some clamps used on gluing on the scarf section to the bulkhead. Also another damage pic of bulkheads.
     

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  4. groper
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    groper Senior Member

    instead of going to all that trouble, you could glass over the cracked ply. The plywood would then simply become a "core" and all the strength would be in the glass skins. Very quick and simple compared to cutting out damage and scarfing in new pieces....
     
  5. jdory
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    jdory Junior Member

    Thanks groper,

    I was considering that as an option as well. I have 24 oz. biaxial in stock, which seems like a sandwich of that would be sufficient. I also have 34 oz. tri-axial in stock but seems a bit overkill.
     
  6. groper
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    groper Senior Member

    Yes, all of the bulkheads in my 35ft cat have a single layer of 24oz double bias glass on each side of the foam core -that's all you need.

    Sand the broken edges down so the surface is fair, fill any gaps and then just glass over it - I would also recommend applying peel ply over it too when working inside as it leaves a smoother finish with less filling and sanding required afterwards. You can just paint straight over it after peeling it off if your not worried about a showroom finish... It comes up pretty good just like that if your neat with your glassing. Are you going to paint the rest of the interior?
     
  7. jdory
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    jdory Junior Member

    Will do. Sometimes a little kick like that gets me off the stump.

    I've been gluing up the broken stringers. Can't really rely on the glue joint now because it is too difficult to work glue down into all the cracks and crevices, though I think in many cases it is pretty good. To add some confidence I've wrapped the worst break in some 24 oz. bi-axial and the other questionable areas with two layers of 6 oz. woven cloth. Hoping that is enough.
     

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  8. jdory
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    jdory Junior Member

    Scarf in place and needing some cleanup, and bi-axial fiberglass patches on some other breaks. Will trim down width of bulkhead somewhat.
     

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  9. groper
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    groper Senior Member

    when you put those clamps and props in place, are you checking the outside of the hull to ensure its still fair? Looks like you could easily introduce some lumps and bumps if you use too much clamping pressure on those props...
     
  10. Charlyipad
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    Charlyipad Senior Member

    James, just curious- did the hull plywood crack or separate? did the balsa decks crack or flex away from the bond at the bulkheads?
     
  11. jdory
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    jdory Junior Member

    Groper - thanks for the caution on the props and clamps. Actually though, the hull was sprung in a little and I used those to push it back to align the edges of the bulkheads. It only amounted to about a 1/4" of movement at the worst, so far. I'll keep a watch on the fairness. Also for re-gluing up broken stringers I'm not using a lot of pressure, just enough to close the cracks - if it isn't in need of a push outward.

    Charlyipad: There are some areas in the hull skin (two layers of luan plywood) that did separate and some areas that cracked. I'm grinding those out on the inside and adding some bi-axial cloth. On the outside where some holes were punched, I'll scarf in some new luan laminations. There is an outboard crack near the keel toward the bow that I'll grind away the delaminated part and see what to do there.. maybe some more glass if it isn't all the way through. Otherwise I'll have to decide.

    There is some crushing right at the shoulder of some of the bulkheads where I had used some duracore strips and cloth. Bulkheads there will be delt with and it looks like there may be some delamination of the glass from the duracore in some areas. I'll remove all that and replace with glass, depending on what I find. I'll try to post some photos. Don't find any deck delam yet, except it kind of separated a bit from the sheer timber up at one bow. I think I can just re-cove it there. The composite bulkheads seemed to be very stout (1/4" plywood laminations with balsa or foam between and 3 or 4 mm luan laminations with balsa or foam between) and did not show any damage, except perhaps in one area hit really hard. But pretty limited. Will post pics as I work towards them.
     
  12. jdory
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    jdory Junior Member

    Charlyipad,

    You asked about hull cracks. Besides some inside skin cracks, here's one about 5 or 6' aft of the bow probably just at the line of triax in the keel area. I did drill a half inch or so hole at the crack to see where delamination might be.



    And also right at the bow, on both sides are some cracks. These amazed me as there are multiple layers of cloth here - must have been quite the stress on them. These I'll grind a bit and add some more fiberglass.
     

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  13. Charlyipad
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    Charlyipad Senior Member

    Doesn't look too bad. Hope you are back in the water soon.

    I had a couple of splits at foldup and found a few voids when the panels were first bagged. I tried the resin injection method, with syringes or ketchup bottles, but found that in my case a quicker easier and cleaner fix was just to take a sharp razor knife and cut out "footballs" where the problem was. If it was delaminated there, the football would come right out. If it held, then at least I knew where the good adhesion line was,so I would keep cutting until I could extract the skin, then use the piece as a template and cut a new piece, mix some thickened goo, and press it in place. If you have an external layer of glass cloth already applied, that might not be so easy on the outside, but may be better to go at it from the interior. If you only have two layers of ply it would be so much the easier.

    On one hull I had a bad split at the bow and slightly below the keel pour, it was a messier fix. On the exterior I had to remove a lot of material and re- fair. but still not too bad in the big scheme of things.
     
  14. jdory
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    jdory Junior Member

    How did you determine you had a problem with delamination or voids? I can see it would be easy if there appears an unfair area or bubble, but I don't see any of that in my case. I find areas around the impacts where some layer has delaminated - not necessarily at the glue joint but one of the factory laminations.

    When the barge company sent a marine surveyor he went around tapping the hulls with a hammer and marked areas all over the place as delaminated. I watched and was very skeptical - he didn't seem to take into account the different sound caused by stringers/bulkheads versus the areas not supported by structure. He seemed like a drinker and not totally with it, but it did introduce a lot of uncertainty in my mind. So far as I cut out areas around the impacts, I get to good areas fairly quickly - not far from the hole. Unfortunately his pencil marks wore off long ago so I can't go drilling holes everywhere searching for trouble.
     

  15. Charlyipad
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    Charlyipad Senior Member

    I used a hammer also. It is much easier though to find delamination before the stringers and bulkheads are glued in, while the whole panel is still in the wobbly stage. There are thermal image detectors out there, but I don't know anything about them. maybe someone here will chime in. I wouldn't stress too much about it though , if you tap lightly on the outside of the hull and there is a delam you should hear it stringer or no, especially if you only have two layers.

    edit, Note to prospective cylinder molders:
    Many of the voids that I had were near or at the scarfs, where they overlapped the panel below. If I had to ever do it again I would have a dedicated man, at the lamination stage, with a wide putty knife and some thickened resin mix, and as we rolled on the neat resin in prep for the bagging, let him go around and skim coat all the scarfs just before the panels were laid up on the mould- kind of a quality control roustabout...
     
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