What material for recore?

Discussion in 'Materials' started by lost soul, Mar 24, 2017.

  1. lost soul
    Joined: Dec 2012
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    lost soul Junior Member

    This is my first post so I dont know if this is appropriate, since it is about repair. Is that subject appropriate here?

    I have a 40 year old sailboat and the deck has a soft spot at the bow. I removed the skin from the bow to 3 feet back. There was plywood and balsa as a core material. The plywood was intact but wet and not bonded. Balsa was mostly gone.

    I have to decide on a 1/2 material to use for a new core. So many people use balsa or plywood. I am interested in foam. I have found videos and info mainly about balsa as a replacement core material. The standard line is that the balsa lasted 40 years......

    Anyone have an opinion as to what I might want to use?

    thanks!
     
  2. ondarvr
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    ondarvr Senior Member

    If everything is done correctly, balsa is cheaper and works very well, but one tiny leak and you're headed down the same road to a rotten core. Foam costs a bit more, but won't have the same issues.
     
  3. ToddD
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    ToddD OhBaby

    Corcell vs. Balsa

    I did the same repair on my sailboat and used 1/2" Corecell M80 sipe cut material instead of balsa and it laid up very nicely.

    Got the material from a place based in Toronto that has retail outlet in Buffalo. came rolled up in a tube so shipping was no issue.

    Good luck.
     
  4. Steve W
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    Steve W Senior Member

    I have built boats with both foam and balsa and they are both excellent core materials. I have also recored many decks and hulls and have used both materials also for this largely because the choice was up to others but for any of my own projects I always choose foam, typically H80 but sometimes the application calls for a denser core for example, maybe on the foredeck close to the bow it may be fitting intensive with a windlass, mooring cleat, chocks etc where it may be appropriate to use to use one of the core replacement foams such as coosa board, pensky board etc that are excellent for this, and they do not absorb water at all. You do need to use sparingly as it is 20 to 26lb density. Be aware that when using h80 foam for the rest of the area it can still absorb water, it just does not degrade like balsa, so you do still need to pot any fastener holes with thickened epoxy just like with balsa. I have a sample somewhere that I cut out of the cabin sole of a 1977 Lindenberg 26 sailboat that is all foam cored, hull (where it is cored), decks and furniture. This boat had sat for a long time with water over the cabin sole and there were a lot of unfilled screw holes in this particular area where teak slats had been installed without caulk and when I cut out for acess for installing transducers through the hull the jigsaw was throwing water in my face from the saturated foam core. When I removed the cutout, looking at the edge it was apparent it was scored foam and the kerfs were not filled and you can see through to the other edge on some kerfs. I put the piece aside and months later when I looked at it, it was totally dry, light and healthy, in fact you can not see any sign that it had ever been wet. If it had been balsa it would have been compost. I did not recognize the foam as ive always used klegecell or Airex pvc but I believe it to be urethane foam which was in use around this period, it does look like a more dense version of the pour foams I have used for buoyancy and fridge insulation. Whatever, it was impressive.

    Steve.
     
  5. gonzo
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    gonzo Senior Member

    The advantage of plywood is that it creates a fair surface to fiberglass over.
     
  6. missinginaction
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    missinginaction Senior Member

    First, you need to determine where the water is coming in and why. Are you planning on replacing an entire fore deck or just the section at the bow? If you're doing a fairly small area you might consider 1/2" Coosa composite. It's pricey but is easy to work with conventional woodworking tools, is light and finishes nicely. It doesn't absorb water either. You can find vendors that will ship you 1/3 of a sheet or thereabouts if you don't need too much.
     
  7. Steve W
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    Steve W Senior Member

    The problem with plywood is that it tends to delaminate when saturated over a long period. I have recored many decks that were cored with plywood and I can honestly say that I have never seen a rotten plywood core but many that are delaminated to the point that there is no evidence of there ever having been glue between the plies (of course there was) Of course no builder ever used BS1088 for a core but I'm sure they would have used at least exterior ply but it was not up to the task.
    The problem with coosa and similar products, as I mentioned a few posts ago is that it is not light, it is available in several grades, the lightest I think is around 20lb/ft3 whereas H80 foam is around 5lb/ft3, light balsa about the same and regular balsa around 10lb/ft3, so, use it sparingly where it is beneficial but not for whole re cores unless small. The other issues with both plywood and coosa is that it is stiff and doesn't conform to curves so needs to be cut into strips or squares and the most difficult area in a deck is the crown of a foredeck. When you cut plywood into squares for this purpose you expose a lot of end grain to water ingress so you had better be very sure you treat every fastener hole properly or you are back to square one.

    Steve.
     
  8. missinginaction
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    missinginaction Senior Member

    You have a lot more posts than I do Steve and it sounds like you have experience composite materials.

    The OP may not want to spend the money on something like Coosa.

    Last summer I purchased some 1/2 inch Coosa to strengthen the transom on my Silverton. The boat's a v-drive and since the transom was never designed to carry any appreciable loads it was quite thin, about 3/16 inch thick.

    I used marine ply above the waterline and Coosa below laminating two layers of one half inch thick material onto the inside of the transom and reinforcing with fiberglass cloth.. Working with both materials I found the Coosa to be considerably lighter than the plywood. I didn't weigh it but I could tell just by handling it. Neither material was particularly difficult to bend (in the 1/2 inch thickness - 3/4 would have been a much different situation) There was a curve to the transom and I used a hydraulic ram braced against inside supports to press the plywood or Coosa onto the existing transom material. I don't know how much crown the OP has in his deck so it's impossible to say weather Coosa would be an option. We'd need more information.

    It worked out fine for me and I now have a solid transom that carries a much larger swim platform and a Yamaha T 9.9 thruster.

    Regards, MIA
     
  9. Steve W
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    Steve W Senior Member

    Yep, coosa, and presumably its competitors ( ive only worked with coosa) are great materials and at 20lbs/ft3 it is lighter than most plywood but is still heavy being 4 times the density of H80 foam and light balsa which, when dealing with a realativly small area may seem insignificant but keeping weight down is always important on any boat. As far as workability is concerned you don't get to choose your core thickness, thats already been determined by the original builder, it may well be 1/2" but coosa is too stiff to bend over most deck crowns, especially if it is not a large piece (its always easier to bend a long piece of anything than a short piece, all other things being equal) and you are often laying a core over a very flimsy inner skin which is often not airtight enough to hold a vacuum for vacuum bagging and you can't always shore it up enough either because of inner liners etc so its easier to use a core with no springback which is why contour core is easiest to work with. Many boats were cored with plywood cut up into squares and the same can be done for recoring with coosa. Obviously vacuum bagging any core in is preferable but its not always possible.

    Steve.
     
  10. lost soul
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    lost soul Junior Member

    There should be a picture attached.

    The fore deck removed is about 3 X 3. The glove thumb is where the windless was bolted. See the 4 bolt holes? There were cleats and bow rail mounts also.

    The other holes were for chain pipes. I have the anchor locker separated into two sections. not sure where I will reinstall the chain pipes, but I will keep the manual windless position. It is in a good place and it straddles a bulkhead.

    The original deck was mostly cored with plywood. The balsa started about 10 inches ahead of the aft cut. Interestingly the plywood was intact, but was wet and not bonded to the FG. I had to chisel it out.

    I am concerned with mounting the cleats and windless. What about the compression forces? I did not have issues with the windless before and the core was gone for years.

    I like the idea of foam because of the problems with wood. I am still undecided and if I use foam, I have to decided on which foam to use.

    It says that my photo is too big. I will try a resize.

    thanks guys!
     
  11. lost soul
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    lost soul Junior Member

    see how this works.

    BTW that big hole was there, lol. The bottom skin is thin. I will need to laminate a layer or 2 from below. A job that wont be fun. I have not ground it from blew yet. That might be the worst part.

    I also have a cockpit hatch that I am recoring. It was also plywood cored. Again the plywood was intact and not bonded. It will be my first try at recore at home on the work bench. Easy-peasy?
     

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  12. gonzo
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    gonzo Senior Member

    You can laminate over the hole from the top. Use plastic sheeting (large garbage bag) and screw 1/4 plywood from under with drywall screws. The screws will pull out after.
     
  13. lost soul
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    lost soul Junior Member

    Gonzo I thought of that. But would t hat not create a bump in the lower skin. I worry it would cause the new core to push up.
     
  14. Steve W
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    Steve W Senior Member

    Even if you use foam you should use use a higher density in areas where items are through bolted, this is where coosa and its competitors shine. Gonzo is right, it is quite common to repair the inner skin from the top. While it does create a bump you can easily fair it after it is bonded in.

    Steve.
     

  15. Jim Allen
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    Jim Allen Junior Member

    Would you recommend using coosa with a covering of basalt material? I love coosa and wouldn't use anything else but I also am using basalt more and more and want to know your recommendation on it. Thank you.
     
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