Balsa deck core: rotten or not?

Discussion in 'Boat Design' started by Swiftsure33, Nov 12, 2017.

  1. Swiftsure33
    Joined: Oct 2017
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    Swiftsure33 Junior Member

    Just bought a 60's sailboat with a balsa cored deck. Most of the deck is sound, but i started digging out the few soft spots. Where the fiberglass skin is delaminated, the old balsa has basically been reduced to powder, obviously rotten. My issue is that as I keep digging, the core still seems suspiciously soft even where it is not delaminated. It holds its shape but i can still dig it out with my fingers. Is this just the nature of balsa? The boat has been indoors for so long that there is no moisture so it is tough to tell how far the rot goes. If the balsa was wet at some point, but the skins are still well adhered, do I even need to worry about it? I plan to have substantial backing plates for all hardware and to fill any holes with epoxy before drilling. Any help would be appreciated, thanks.
     
  2. wet feet
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    wet feet Senior Member

    Did the pre-purchase survey fail to find the problem?
     
  3. Swiftsure33
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    Swiftsure33 Junior Member

    Boat cost less than a survey, I knew what I was in for. I'm capable of replacing core, just trying to figure out if I may need to do all of it, or just the soft spots.
     
  4. JSL
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    JSL Senior Member

    What's the boat brand/builder?
    Is the balsa core end grain ?(grain runs at right angles between FRP panels). Or is it flat gain (sounds impossible but I have seen it on 2 occasions).
     
  5. Swiftsure33
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    Swiftsure33 Junior Member

    1960 Rhodes Swifsture 33, end grain. My main question is is it normal to be able to pull out chunks by hand even where the glass is still well adhered?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Balsa is a pretty soft material, and though the laminate will toughen the surface a bit, if you dig below this, it's still a pretty soft material. Rotten cores are usually pretty easy to identify. Softness, color, smell, etc., all are "tells" as to what's up with the core. It's role in a sandwich composite is as a compression element, not so much as an individual structural element. It's part of an assembly and it's physical attributes shouldn't be valued as a separate piece of the puzzle.

    Given the age of this core, you'd be well advised to consider large sections of it suspect, though much of this is dependent on evaluation of what's there. I've found that cored structures of this era tend to get a "brittle resin", maybe work hardened type of characteristics, with age. This would cause the symptoms you've described, but (again) this is all dependent on a careful evaluation of what's actually there. Unlike many yachts of this era, this one was actually engineered, making the laminate and core thicknesses critical, in maintaining the strength and stiffness of the structure.
     
  7. Swiftsure33
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    Swiftsure33 Junior Member

    Thanks. Any recommendations for replacement core material? I'm going to put either plywood or high density foam in high load areas like under the cleats and windlass. But the soft spots all seem to be in the center of the foredeck where old holes and cracks let water in. Since there won't be any critical hardware there I'm planning on just going with new balsa and sealing well with thickened epoxy and re-covering with a polyester and 1708 biax laminate.
     
  8. PAR
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    PAR Yacht Designer/Builder

    You plan sounds fine, except for the "re-covering with a polyester and 1708 biax laminate" part. Polyester isn't going to bond well to epoxy, so maybe an all polyester or all epoxy repair would be better suited. I'm also not so convinced you'll need the biax, as it's likely it wasn't used previously. My approuch would be to save the outer skin (assumes you'll work from above), as best as you can, R&R the core and bond everything back in, repairing the seams along the cut lines as practical.
     
  9. Swiftsure33
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    Swiftsure33 Junior Member

    Thanks. Any personal experience with polyester resin bonding to share? There's some articles I've found that say that for small areas poly bonds fine assuming proper prep work. I'm guessing this would be a good place to try it out since if the bond fails I can just cut the new skin off and bond it back down with epoxy. Old skin was cracked and filled with improperly filled holes over the soft spots so I'm just going to lay up new glass.
     
  10. PAR
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    PAR Yacht Designer/Builder

    This is where having a look at what you're up against would be helpful. To me, preserving as much of the deck is wise. You save the raised textured areas, water ways, any landing pads, etc. You can even make a latex mold of unmolested textured areas, to use as a casting for repaired areas if necessary. Working with raw laminates is much more difficult, plus the cost of bulking up these areas to the previous thickness of the surrounding laminate. Post an image so we can see how much you have to do.
     
  11. Swiftsure33
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    Swiftsure33 Junior Member

    Won't be back to the boat until thanksgiving weekend but I'll get some pictures of my upcoming projects. One more question. I know epoxy gets damaged by UV, but I can't find much of a clear answer about polyester. I need to sand the whole deck and some of the hull is exposed. Will I be in trouble if I don't fair and paint it immediately? I know people leave their hulls bare for various amounts of time. Is this damaging to the laminate? Also, can anyone recommend a good uv stabilized poly resin for my core repairs, if such a thing exists? Gelcoat is left exposed to the sun on many boats and that's a poly base. If it's not a good idea to leave it exposed, can i just roll on some cheap primer to protect it from uv until I get around to doing to actual priming and painting? Thanks.
     
  12. ondarvr
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    ondarvr Senior Member

    A polyester laminate is significantly more UV resistant than epoxy, there's no need to protect it unless you plan to leave it for very long time. You do need to clean the surface well before you apply anything to it though, sanding is normally required, this removes any UV damage form sitting outside unprotected.
     
    Last edited: Nov 14, 2017
  13. PAR
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    PAR Yacht Designer/Builder

    UV inhibitors break down in time from (wait for it) UV. You can pour piles of it into any resin and eventually, the surface polymers will disassemble on a molecular level. The only options are paint (clear coats included) and gelcoat. Primer doesn't offer much UV protection and breaks down quickly. You can buff up gel coat, which is little more than really fine sanding, to restore the finish up to a point. Eventually even this will literally wear through this the gelcoat.
     

  14. Swiftsure33
    Joined: Oct 2017
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    Swiftsure33 Junior Member

    Thanks for everybody's replies. I think I have what I need to at least diagnose the extent of the issues and move from there.
     
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