Calling all experts - deck / stringers / bulkeads / foam

Discussion in 'Boat Design' started by Franknstein, May 26, 2021.

  1. Franknstein
    Joined: May 2021
    Posts: 2
    Likes: 2, Points: 3
    Location: United Kingdom

    Franknstein New Member

    Hi all
    New to the forum.

    I have a 1987 GRP hull made by Shetland in the UK which I've had since 2016 and been rebuilding.

    In 2017 I ripped out the sodden deck, and layed a new 19mm marine ply deck and glassed it to the hull without removing the original foam. This boat originally had no bulkheads, it has three stringers - one on keel and one on each of the chines, made from PVC pipe and glassed in. The deck was supported by 3 lengths of hardwood planks loose-laid across the upper two stringers and then poly foam poured in. In my opinion the under deck construction originally leaves much to be desired.

    Anyways, my boat was listing a little last summer, so I ripped out the newly laid deck and decided to remove all the foam. Interestingly this boat had no drainage design, so I've used PVC pipe cut lengthways in half and running them from bow to stern alongside all the stringers to facilitate drainage to the rear bilge and made a hole with a bung. I've since sorted the source of the water getting in the hull.

    I've now made custom bulkheads and extended the heights of the stringers and tabbed them all into each other and to the hull.

    I was hoping to drop the old deck (still perfect condition) back down in place once I've re-foamed, and screw it down to the bulkheads - but I was hoping to not glass the deck back in place but just screw it back down, then sikaflex the gap all round. Planning on adding some plywood lips beneath the original deck-lip all round so it will be screwed down all the way around with screws about 4 inches apart. Additionally screws down into the three bulkheads. Reason for wanting to just screw it down is in-case I get some issue down there that needs repair, it will be a lot easier to access if I just need to unscrew the deck instead of cutting it out!

    I also made a mould for a below deck cooler box - see pics.

    My questions really would be:
    1. Will screwing the deck down be strong enough? Or do I have to glass the deck back in?
    2. I removed all the original foam, but was considering laying cellophane all over the construction work, pouring the foam, and when set, pull the foam out and remove the cellophane. This would mean I'd have perfectly formed foam blocks in each compartment but be totally removable for access. My concern is in the original design the foam was used as a medium to create strength and rigidity to the hull, but if the foam isn't adhered it may be a step in the wrong direction? Although surely my bulkheads and stringer mods should be ample?

    See what you think
    thanks

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  2. fredrosse
    Joined: Jan 2005
    Posts: 399
    Likes: 55, Points: 38, Legacy Rep: 56
    Location: Philadelphia PA

    fredrosse USACE Steam

    The elastic modulus of foam is low enough that the foam in the original design did not provide very significant strength to the basic hull form. The foam however can, and does provide significant support for relatively large flat areas of the hull.

    So in conclusion, foam is not needed to reinforce your new stringers and bulkheads, that part of the structure is good without foam support. Large flat areas may need support of foam, especially if the fiberglass hull thickness is very thin, and your proposed foam process will supply this type of support, so you are OK with your proposed plan.

    Nice professional work you have been doing, congrats for that.
     
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  3. Franknstein
    Joined: May 2021
    Posts: 2
    Likes: 2, Points: 3
    Location: United Kingdom

    Franknstein New Member

    Thanks for the time you took to reply to me. I appreciate it.
    I've had someone suggest I should perhaps add another two bulkheads. They're currently 50cm / 20in. apart. What are your thoughts on this? Don't really want to loft out another two if I don't have to, we may have some good weather soon!
    And also, simply screwing the deck/sole down in your opinion would be adequate or would you suggest glassing it back on?
     
  4. fallguy
    Joined: Dec 2016
    Posts: 4,482
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    Location: usa

    fallguy Senior Member

    Foam is needed to keep her from sinking. And enough. Not sure why you'd cellophane. Just use painter's plastic if you insist on a barrier. It ain't coming out once it molds to the hull and other shapes.
     
  5. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    One option is to use large heavy duty garbage type bags, put the foam it that and leave it all in place.

    This does a better job of protecting the foam from water, and still allows for easy removal if needed.

    Water will always find its way into the bilge, getting it out easily after use is a good thing.
     
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  6. fredrosse
    Joined: Jan 2005
    Posts: 399
    Likes: 55, Points: 38, Legacy Rep: 56
    Location: Philadelphia PA

    fredrosse USACE Steam

    Sorry to have taken so long for another reply. I have been super busy, converting my electric boat to a steamer, hopefully getting into the water this coming Saturday. Cramming a 300,000 BTU boiler, steam engine, piping, plus electric feed pump, electric oil burner, generator, batteries, piping and heat exchangers into a 14 ft boat is rather daunting.

    I really cannot provide a firm answer to your new questions, except to say that almost all boats can be overloaded without prudent operation, and additional reinforcement (such as glassing the deck in place) the helps here. However it appears that what you have done is adequate, but that is just my very mild opinion. As a registered professional engineer I would be duty bound to perform several calculations to allow providing a firm response here.
     

  7. OneWayTraffic
    Joined: Jun 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: South Island, NZ

    OneWayTraffic Junior Member

    While I couldn't tell you if screwing down the deck will be strong enough I can tell you for a fact that gluing it to cleats on the frames and stringers, and then glassing the perimeter will be much stronger.

    It's the basis of the boat I am building. I just asked a somewhat similar question involving foam, like you I wanted some foam in there, but I also want drainage.
     
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