Deck Advice Needed Please

Discussion in 'Boatbuilding' started by Stormbringer79, Feb 12, 2018.

  1. Stormbringer79
    Joined: Feb 2018
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    Location: Kent, UK

    Stormbringer79 Junior Member

    Hi all,

    I have recently become part owner of an Orkney Speedliner (we think) 16ft which has currently got a slatted wooden deck installed which is built from cross members and wooden planks. Problem being it is rotten so needs replacing. The hull is fine so just a case of installing a new floor and that is another of the jobs off the list.

    We want to use her as a fishing boat so do not really want a slatted floor as it will soon become tedious when we drop various bits of tackle that then fall down under the floor so we would like to install a flat floor. Our thoughts at this moment in time is to add extra cross members to ensure there is one every 6 inches so when a foot is down the weight will be distributed across at least two bearers, then on top of that have some marine ply that is expoy coated on the underneath and then layers of chopped strand matting on the upper side. This would be finished off with an anti slip covering.

    The biggest thing that is bothering us at the moment is the potential weight this could add to her so would like to use the thinnest sheet material we can which will then be reinforced by the fibreglass (I hope!!). Would it be feasible to use a 6mm sheet of marine ply and then build on top of this 3 layers of fibreglass (I am thinking 450 gram might be best???) to add the structural strength required?

    Thanks very much for any advice anyone has to offer.

    Keith
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Can you post some images, so we can see what you're up against?

    Your correct to be concerned about weight, though building stiff and light structures can be a challenge for the uninitiated.
     
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  3. jorgepease
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    jorgepease Senior Member

    Your much better off using sandwich foam core, way stiffer than ply and can be very light.
     
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  4. Stormbringer79
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    Stormbringer79 Junior Member

    Hi Jorgepease, thanks for the reply. Are there any particular brands you could recommend please?
     
  5. jorgepease
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    jorgepease Senior Member

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  6. Stormbringer79
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    Stormbringer79 Junior Member

    Fantastic!!!!! Thank you so much for your help mate, I really appreciate it :D
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I disagree in that low density and more importantly peel strength foam is the way to go. To make it stiff enough, you need a significant sheathing and/or core thickness. In either case, the costs (foam, fabrics, resin, etc.) will be considerably higher, than a lightly sheathed plywood build and the effort to make it fairly smooth also much higher, as a sandwich core composite. Do look up the cost of a single 4x8' sheet of 1/2" H-80 or similar, before you discount plywood.
     
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  8. leaky
    Joined: Sep 2008
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    leaky Senior Member

    Pictures would help..

    A little bit about cores, plywood, foam, etc.. Step 1 is do a weight calculation on how much your core really is going to weigh, most 16 ft boats would have 1/2 inch plywood decks if built in a conventional manner. Say you need 3.5 sheets of 1/2 inch to cover the deck @ 50 lbs a sheet - your core would makeup 175 lbs.

    Myself, my own observations based on what I have accomplished via 5 lb density foam core - I can shave about 1/3 of the total weight, when making a panel I feel is equivalently good for the function. When I say equivalently good it's a non scientific way of stating it will work for the application - but there is some cost, it's not going to be quite as durable as plywood and is not going to be quite as strong either, but it will not rot and you probably do not need it to be so durable or so strong, it just needs to fit the application. The point being - say you shaved 1/3 of the weight off the finished panels and a finished plywood 4X8 panel weighs 60 lbs with a thin glass skin top & bottom, 3.5 sheets then weighing in at 210 lbs - make it out of foam core and you probably save 70 lbs...

    Is 70 lbs a lot of weight - it's a good percentage, sure - but will you notice that difference? Probably not. But now you are paying ballpark of $150 to $400 for a 4X8 sheet of core (depending what type you choose, local availability etc..), and if you pick light foam that really shaves weight, then 75% of the weight of the finished panel is going to be in the fiberglass and resin - or another way of putting it, you are laying up a lot more fiberglass and resin, which also adds to the cost (and hugely to the effort too).

    (guys that know correct this if you think it's wrong please) to get a piece of light foam core feeling kinda like plywood, unless you are talking coosa ($$) which basically just shaves a little weight but otherwise behaves more like plywood- I figure a 4X8 sheet of 5 lb density foam, with a skin of 1708 top & bottom, ~6 yards of 50 inch fiberglass at 7 bucks a yard, 2 gallons of resin, and that's without a veil of some sort of mat, which you probably will want to add, that's gonna soak up more resin too. Ballpark that 4X8 sheet on the way cheap end runs you $150/core + $40/glass + $50 resin - $240 absolutely bare minimum (and I bet it's more like $400/materials), 3X the labor since you gotta build the panels then install them.

    Anyway - that's the stuff to think through on this. My 1973 16 foot center console has a 1/2 inch plywood deck with, 2 part epoxy, 1 layer of 6 ounce cloth on the bottom, 2 layers on the top, worked great in that application and is holding up perfectly after some years now. The 32 foot I'm building now I'm going for "no wood" construction and so far have done lots with corecell and some coosa, certainly are benefits to build this way, and I think it's a good way to build a new boat today, but the labor and cost is astronomically higher with such materials - with light core it takes about 5 hours of labor by the time I setup, build, and prep a panel so it's ready to be cut & installed versus taking a piece of plywood off a rack.

    Jon
     
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  9. leaky
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    leaky Senior Member

    ya what he said :)
     
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  10. Stormbringer79
    Joined: Feb 2018
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    Stormbringer79 Junior Member

    Thanks for all your input guys, I am really grateful.

    Plywood is looking quite favourable again taking everything that has been said into consideration. I will price up options for both and see where that takes me. Saying plywood is the route I take would 1/2 inch be the minimum you guys would recommend I use? As I said before, there will be bearers spaced approx every 6 inches underneath so this should ensure that anyone of any weight should end up with their feet spanning at least two bears at any given time. Taking this into account if I was to use 1/4 inch ply how many layers of fibre glass would be needed and what sort or weight matting would be best to use?

    Sorry for all the questions I have never tackled anything like this before :)

    Thanks again for all your help and advice.
     
  11. Stormbringer79
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    Stormbringer79 Junior Member

    Hi Jon, I will be up working on the boat on Wednesday so will try to get some pictures of the deck in it's current state and hopefully we will get to removing it to show you what we are left to work with and will then post them up.
     
  12. leaky
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    leaky Senior Member

    Take a piece of plywood that's somewhere in the ballpark of what you want, put a couple 2X4's under it, and stand on it... Add glass it gets stiffer, fasten it down it gets stiffer.

    My 16 foot has 4 longitudinal deck supports/stringers, two kinda where stringers would be normally w/ a 20 inch gap, 2 set close to the edge of the hull, with a similar 20 inch span. The deck in the stern area is 3 panels that are about 8 feet X 20 inches - the longitudinal supports are 3 inches wide on top where the panels set, so the edges of the panels join there and are fastened mechanically via lag bolts into the pressure treated wood that makes up that shelf. 1/2 inch plywood, with the glass that's on it, fastened how it is, is rock solid in those dimensions. Of course I seal the bolts so I'm not rotting my wood out with the holes. There are other ways to do this but I wanted the deck removable for repairs and to mitigate occasional "squatters" that like to move in and create messes.

    How thick it's gotta be depends on a lot of things - how much glass is there, if it's fastened into what's underneath it (so standing on it essentially creates a tensile strength situation not just how much weight it can hold), what sorts of holes you cut into it for hatches underneath - the strongest thing is a deck with no hatches well fastened down. With 2 part epoxy and plywood not so much glass is required - a little on bottom to make it water proof, a little extra on top as you want to make it a bit more robust for foot traffic and such. Simple water based deck paint with nonskid finished it off - has been great, even after at least 4 years (you might find my post to see how long back I did it) the paint is perfect.

    Jon
     
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  13. leaky
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    leaky Senior Member

    I guess I should add - if you said you wanted to do something without wood, to make it last forever, or you were building a race boat where every pound counts - foam cores have their place. Coosa is great stuff to work with, like plywood that is lighter which simply an never rot or soak up water in practical terms - however it's a whole lot of $$ for not much benefit on an old fishing boat that is just getting a repair to make it functional, a complete gut & refurbishment expensive materials may start making more sense but you gotta do the math - pound for pound and dollar for dollar what works. Light density foam is both costly and labor intensive - has it's benefits but has it's drawbacks.
     
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  14. Stormbringer79
    Joined: Feb 2018
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    Stormbringer79 Junior Member

    Sounds perfect to me Jon!!!! I'll go hunt back for your post tomorrow. Cheers mate :)
     

  15. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Given your support spacing, 1/4" plywood will work fine for half the weight of 1/2". The amount of 'glass required to significantly improve the stiffness of plywood is considerably more than a light sheathing. Simply put, if you put a 10 ounce (400 GSM) sheathing on 1/2" plywood, there's a measurable difference, but not very much in the big picture, so this type of sheathing is for abrasion resistance, not stiffness.
     
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