pour foam, 14ft low sheer flats skiff ??

Discussion in 'Fiberglass and Composite Boat Building' started by Mark C. Schreiter, Aug 11, 2021.

  1. Mark C. Schreiter
    Joined: Nov 2020
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    Location: Tampa, Fl

    Mark C. Schreiter Junior Member

    In the next month or hopefully less ill be attaching the deck to the hull. I am attempting to build a self bailing, unsinkable boat. I raised the sole above the waterline and have a drain hole in the transom where ill install some sort of scupper/one way valve. the idea is to avoid a bilge pump. Yes it will be a very wet ride but it will be wet with or without as it is a small low sheer boat. I'm fine with that.

    that being said I plan to pour foam in the entire bilge area, have storage in the bow that will be elevated high enough so any water will drain to the sole and do the same in the stern. on the bow and the stern below the deck on the port and starboard side will be sealed cavities. would it be a worth while idea to fill the cavities with pour foam? I don't think the weight will be an issue and i probably wouldn't benefit to much from the reserved buoyancy. I guess my thoughts would be to give at best a little more support for the decks. Im open to thoughts and suggestions.

    thanks

    -Mark
     
  2. gonzo
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    gonzo Senior Member

    Foam does not give you reserve buoyancy. A sealed tank will provide more buoyancy. The foam would help if you hit something and make a large hole in the hull though. However, if you have several separated sealed cavities, they should keep the boat afloat.
     
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  3. bajansailor
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    bajansailor Marine Surveyor

    Mark, did you design the boat yourself?
    Can you post some photos showing what you have built so far?

    Re 'pour' foam, be aware that it might not be closed cell foam - in which case if it does get water in the compartment, the foam will soak it up like a sponge.
    Also be aware that your 'reserve' buoyancy is actually less if the compartments are filled with foam, compared to if they are filled with air, as the foam will increase the weight of the boat. This is assuming of course that the compartments filled with air are not breached, and that the compartments filled with foam have closed cell foam.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Raising the sole above the waterline in a 14 foot boat does hurt stability, OK if you have an inherently stable boat and not venturing into open water perhaps. Just be careful you don't "trap" the expanding foam such that it can't expand freely, or you may get cracking sounds and bulging panels !
     
  5. jehardiman
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    jehardiman Senior Member

    As Gonzo and Bajansailor said, air tanks work better than foam. Additionally, foam low in the hull turns a survivable swamping into a dangerous clinging to an overturned awash hull. Foam should never be added low for emergency flotation purposes. If you want to add foam (closed cell only) for emergency flotation of an open boat, it should be added high along the sheer.
     
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I've seen several boats that allegedly had "buoyancy tanks" sink. If the hull splits where the tank(s) is, for example. Swamping is a big chance of upsetting a small boat that has a self-draining cockpit, unless there is plenty of buoyancy on each side above foot level, inside the boat. There are good reasons why few smaller trailer boats are self-drainers.
     
  7. Mark C. Schreiter
    Joined: Nov 2020
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    Mark C. Schreiter Junior Member

    it is sandwich core construction. 2 layers of 1208 foam and 2 more layers of 1208. I fish in shallow areas with tons of oyster beds so having the hulls filled would give me piece of mind fo sure.

    my concern with air cavities is the expansion of the air inside due to heat. I built a hollow wooden cedar surfboard and had to put a vent in it to relieve the air pressure from the changing temperatures expanding and contracting the air trapped inside. a couple frames ended up separating, my guess was do to the expansion and contraction.

    the sole will only be raised about an inch (or so I thought, more on that below). pretty flat on the bottom as well. whatever the term is escapes me but from keel to deck at the widest area is about 15 inches and the draft is supposed to be 4 - 5 inches (I doubt that I will achieve this though as im hand laying up the fiberglass). stability is pretty important but i wont be going out in heavy chop or into open water, just crossing bays to get into estuaries and then either trolling around or push poleing it.

    I drew up a quick pic of locations where I was planning on pouring foam but im waiting for it to be "approved" before i can post it.

    in the meantime, i made some quick and admittedly dirty calculations. im sure im off but I'm hoping that im close.
    the plans give a dry hull weight of 190lbs, which i don't believe will be true with my build. given my that my raw materials weighed in at a whopping 400lbs. accounting for waste and oversight and the use of heavier cloth and wet layup, I'm estimating the hull weight to be closer to 300lbs. with a PPI of 200lbs this would give me a draft of 5.5 inches (bare hull). add a motor (150lbs) plus my weight and gear and accessories, add maybe another 300lbs and the draft rises to maybe 7.75 inches. now with all the cavities sealed either with air or foam and provided they are in fact sealed, calculating the open spaces, including storage areas and the sole, I calculated the cubic feet of said areas and multiplied it by the weight of water and that added up to be about 200lbs.

    so if the boat was completely swamped in all the open areas it would have a draft of around 10 inches, if im looking at it correctly?? therefore making it unsinkable, of course considering nothing catastrophic happens.

    additionally, this would mean that if I wanted to keep out all water and make it self draining/bailing under normal conditions i would actually have to raise the sole almost 2 inches to obtain my goal. this might be pushing things a bit, especially considering that the frames are already built to the drawing specs.

    so at this point I think making it self bailing would actually make things much more complicated and if I just throw in a bilge pump I would have an easier time building the storage spaces and the boat in general.

    need to put a little more thought into this one.

    -Mark
     
  8. ondarvr
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    ondarvr Senior Member

    On the foam part, any foam you'd be buying for a boat will be closed cell, I'm not sure where all the comments on open cell come from.

    Watertight compartment in a boat typically means once Water gets in it can't get out. Filling them with foam helps, but compounds the problem when Water does get in.

    I make all supposedly Watertight compartments with access hatches and drain plugs. I try to leave the drain plugs out when the boat is on dry land.

    Leaving foam out of an area at the keel and adding foam higher on the hull sides helps swamped stability a great deal.
     
  9. gonzo
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    gonzo Senior Member

    Even the two part foam that is supposed to be closed cells eventually soaks up water. It is common to see that in older boats.
     
  10. ondarvr
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    ondarvr Senior Member

    Yes, it breaks down over time and does a very good imitation of a sponge. In areas that aren't likely to see water until you're swamped it's not as bad as in places that will most likely get wet in normal use.
     
  11. Mark C. Schreiter
    Joined: Nov 2020
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    Mark C. Schreiter Junior Member

    below is a rough crappy image of what I was thinking. the red areas are where id potentially foam with of course closed cell 2 part, marine grade, bla bla bla foam. I understand that the foam has issues but most of those issues would be far down the road provided the cavities are sealed well enough. I think I have to weigh the boat to get the HIN in Florida, I could be wrong but id plan to weigh it regardless and at least Id have a baseline to determine if there is any water intrusion in the future.

    Maybe I could add some sort of PVC tubing at the bottoms of each compartment that would run to the bilge at the transom? this way they would be vented and hopefully water, if breached could escape through it.

    I don't know...... I guess the chambers are small enough that the chances of enough of them to be breached and filled with water to the point of sinking would be slim if not impossible. and if it got to that point im sure whatever could've caused that much damage probably would've already taken me out with it.

    I do really like the sound and feel of a foam filled deck and sole and the plans very vaguely state that you can foam under the entire sole, so at least there's that.

     
  12. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re foaming your buoyancy compartments or not - I will just throw this in to the equation.

    The Mirror dinghy must be one of the most successful plywood kit boats ever, with over 70,000 built - and I very much doubt that any of them every had positive foam floatation added inside their buoyancy tanks.
    We used to have one in the 70's, and we didn't have any issues with the air inside the buoyancy compartments heating up and pressurising the tanks. Each tank had a drain hole, with a simple cork in it - if the pressure got too much the cork would have just blown out, but I don't recall this ever happening.

    Mirror Sailing Dinghy Wooden Hull Kit https://www.tridentuk.com/gb/product-mirror-wooden-hull-kit-tbkt.html

    Mirror (dinghy) - Wikipedia https://en.wikipedia.org/wiki/Mirror_(dinghy)
     
  13. Mark C. Schreiter
    Joined: Nov 2020
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    Location: Tampa, Fl

    Mark C. Schreiter Junior Member

    Let's say hypothetically I install drains in all the compartments that drain to the bilge and foam everything but the bilge area. what are the worst case issues? water intrusion and saturation? in such a small boat how much of anything would be really noticeably affected?

    This weekend i finished all the layups for the hull which is 2 layers of 1208 on each side of 5lbs density foam and I found it to be very firm but it did flex under my weight (the diet starts today). I've been thinking about adding some additional supports between the stringer's of doing some type of corications on the undersides of the deck but I keep feel that the addition of foam will give that little kick of support i feel I need and also quiet down the hull a little.
     
  14. bajansailor
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    bajansailor Marine Surveyor

    Can you tell us please what the design is, and ideally give a link to it?
    And maybe post some photos showing the hull construction so far?
    How thick is the foam core of the hull and deck?
     

  15. gonzo
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    gonzo Senior Member

    You need to either completely seal the compartments, or drain them and not install foam. If you think water will get in, sooner or later the foam will start absorbing water and add weight to the boat. The other option is air compartments with a drain which you can check periodically.
     
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