Density of foam required for deck replacement in small runabout

Discussion in 'Fiberglass and Composite Boat Building' started by Tom Mckinney, Jan 11, 2021.

  1. Tom Mckinney
    Joined: Oct 2016
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    Tom Mckinney Junior Member

    I am rebuilding an old glasspar. It is about 13 ft long and about 4 feet beam at the waterline. I have removed the old stringers and need to redeck the boat.

    My plan is to use pourable structural foam to fill the hull to the level of the old deck and create a surface for a deck, then fiberglass over that. There would not be any stringers as the whole deck would be one big composite structure. The plan is to use a 4 lb pourable foam to fill the bottom 1 to 3 inches and shape a deck, then fiberglass over that.

    I dont think I need stringers since the load is carried by the whole structure. But I could be wrong

    Anyone have experience or comments?
     
  2. TANSL
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    TANSL Senior Member

    Probably, should be checked.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    In theory, in such a small boat, it could work well enough, the big problem will be in getting an even flat surface to glass over, that foam rises like a loaf of bread. Unevenly !
     
  4. Tom Mckinney
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    Tom Mckinney Junior Member

    I hope you arent trying to be as rude as that comment sounds. Why else would I be asking the question on here? I want to see if I am wrong.

    Perhaps my question wasn't clear, so maybe I could have done better as well. In any event I would ask you double check your own rhetoric as it really sounds offensive to me.
     
  5. Tom Mckinney
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    Tom Mckinney Junior Member

    i was going to over fill qnd shave it it down.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    It really depends on how the hull is arranged. Since the answer can be..yes and can be no.
    Since the strength, i.e how easy it is to fail or break, is dictated by the stiffness.

    A drawing or picture would help significantly ...since i'm not familiar with this type of boat and what is or is not there structurally.
     
  7. TANSL
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    TANSL Senior Member

    Sorry, I can't know how my comment sounds to you, but I didn't mean to be rude. It is simply a concrete answer to a very specific question. There is no more data and, therefore, I do not think another answer is prudent. With all due respect, but I sincerely believe that you would do well to check if that deck can withstand the loads it will be subjected to, without any stiffener. If you remove the stiffeners you will probably have to put much more material on the panel which will possibly make a heavier structure with similar strength.
     
  8. Ike
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    Ike Senior Member

    The one big flaw to your plan is, if the boat swamped it would roll over and float upside down, dumping you and any one else on board, in the water. Boats of that size usually have foam distributed around the boat in such a way that if the boat swamps it floats right side up. What is generically called level flotation. Filling the bottom all the way across almost guarantees it will roll over. Boat manufactures usually leave a space open down the middle of the boat. This helps to balance the boat when swamped. As far as using foam under the deck, that would work fine if it is structural foam, but if you use the usual pour foam it would not be very rigid and not very strong. I suggest you use sheet foam (which you can buy at any home improvement store) which is a lot more durable than pour foam, stiffer, and you won't have to carve it down. You can put some cuts in it so it will bend and assume the curve of the deck. see Level Flotation Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html It's about regulations for new boats, but can be used by anyone to install flotation in their boat.

    Oh, to answer your question, usually boat builders use 2lb density foam (one cubic foot weighs 2lbs) for flotation, but for structural they use denser, more rigid foams, 4lb or 6 lb.
     
    bajansailor likes this.
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    If the level is the same as the old sole, and that was sealed up, there is no difference so far as the boat turning turtle if swamped. Structurally it should be OK, but you still have to provide whatever the old structure did.
     
  10. fallguy
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    fallguy Senior Member

    There is a significant flaw in the logic here.

    pour foam does not result in a reasonably fair surface

    Not sure I agree with the issues of flotation capsized, etc.

    You need to rethink the deck construction or you will regret the decision.

    For alternatives, you can build a deck in three different materials; generally.

    Structural marine foam
    Plywood
    Solid glass

    Each of them have varying degrees of difficulty. The easiest one is plywood. The best one is foam. Solid glass is not great and not easy.

    As others have said, a picture that shows the space will help.

    you are implying building in fiberglass which will be extremely difficult as presented
     
  11. Tom Mckinney
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    Tom Mckinney Junior Member

    Didn't realize my question was confusing, but I see that I left out information so apologies all around.

    I'm not filling a hull with foam. The original deck barely rose above the hull surface. The deck to hull space on this boat is barely 1 inch deep. At the centerline it may be 3 inches deep in places. it is a very shallow v hull.

    Currently the hull is a single skin shell, all the reinforcement stringers (and the deck) have been removed

    Rather than replace the structural members, I want to make the hull and the deck into a single foam cored sandwich panel about 1 inch thick. The foam would be, as noted already, a 4lb density structural foam that is used in deck repairs. It just happens to be pourable. I would pour it in small batches and layers and over pour it then shape it down with tools to create a fair surface and then fiberglass that surface. This would make it one big cored structure. I was trying to ask if the foam density was sufficient . I finally was able to figure out that divinycell is about 5lbs/ cu ft. so I was wondering if the 20% decrease in density would be a problem or would the thickness of the core make up for the lower density?. Furthermore I wasn't sure if pouring the foam onto the hull would provide sufficient bonding between the skin and core. Should I pour it on a wet layup that can cure under the foam and bond the foam and hull skin?

    was going to cover the foam with a single layer of 1708.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Don't use pour foam.
    Typically it is "open cell" and the only thing that keeps the water out of the foam is the skin that develops on the outside after rising.
    But you said you are going to cut that off.

    Major failure.

    Most pour foams are also not very rigid, which is typically better for a foam sandwich.

    Best to use expensive, closed cell sheet foams sold for foam sandwich.

    There are real and specific reasons for stiffeners (and different reasons for stiffeners in different places) . Bad idea to remove them without an engineering analysis of the results, as Tansl was saying.

    Of course you could just do it by eye and report the results. No one ever reports back when they do this. Do you think it was because the job was a success?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Higher density pour foam is not open cell by any means, I can see what he intends to do here, and it should work OK , if he puts a decent layer of glass on it.
     
  14. fallguy
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    fallguy Senior Member

    I agree with Mr E to an extent.

    But it certainly depends on the horsepower plan. Skipping the stringers and their tabbing for some 4 or 5# foam is not really going to get you in any Boston Whaler magazine wave jumping photos. The boat won't be quite as strong I'd say. It is a generalization I make, subject to zero analysis.

    So, I would add back into the plan the stringers. Then use 4# pour foam and glass. Just so you know. A single layer of 4# foam with 1708 is just at the edge of sufficient. Make sure to use some thickened resin on any low spots in the foam or it'll delam easy.

    As for your question on the foam bonding to the substrate, I would precoat a clean hull inside skin with epoxy 12 hours before the foam work which should offer a chem bond, but no danger of puddling and exotherm with the pour.
     

  15. ondarvr
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    ondarvr Senior Member

    Foam bonds to just about anything quite well, you won't have a problem with it bonding to the hull, and it's closed cell.

    Strong enough, maybe.
     
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