Marine ply decks - sheath in GRP or epoxy and paint?

Discussion in 'Fiberglass and Composite Boat Building' started by dbsharples, Nov 26, 2015.

  1. dbsharples
    Joined: Nov 2015
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    Location: UK

    dbsharples Junior Member

    Hi guys,

    New here, first post, so hello!

    I'm based in the UK and have just bought my first boat which I'm looking at as a project. I bought it a year ago and was meant to have it up and running within a couple of months but of course that didn't happen..

    I'm converting a 14' dory into a fishing boat with a side console and rear and front casting decks with hatches for tackle, fuel etc. First time I've done this so all the processes are new.

    I'm looking to build these decks out of 3/4" marine ply and my question is should I either;

    a. seal with West epoxy, paint and mix anti-skid into the final coat on the topside,

    or

    b. seal with epoxy, paint internally and lay GRP on the topside before flowcoating and anti-skid?

    I've been given different opinions here. I'm kind of hoping to avoid the GRP work as I haven't done it before and don't want to screw up.

    Any thoughts?

    Thanks!


    Edited: pics of dory to give an idea and a VERY rough sketch (pdf) to give an idea of what the proposed deck structure will look like. The picture taken inside is a mock up of what it will be like once the decks are in place. The console will need to be cut down to size as its a bit high..

    The dory is an italian made boat I believe, twin hulls.

    Just to elaborate on the plan for the deck, I want to have four hatches in the back and three in the front so the number of upright pieces of ply supporting the deck will be limited. These are the dotted lines in the sketch (I missed the cross support at the back in my rush to draw this). The rationale for the thicker ply for the deck itself is that there may be 50-60cm between supports so I was worried about the deck sagging.
     

    Attached Files:

    Last edited: Nov 27, 2015
  2. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Welcome to the Forum. Do you mean Dell Quay Dory? or similar say made by Dell Quay, Fletcher etc or what our US cousins might call a Grand Banks or French Dory?
    A picture would help or some sketches of the hull type.

    If it is a Dell Quay (or Boston Whaler) type, they are not really meant to have any superstructure, beyond a side console. This is uaually a factory option. Only the larger 17' version has any cabin option and that was a later modified more V hulled craft. However the small 13' versions should have one or two cross benches at least one of which hinges and contains a reasonable size hatch. Mostly this is for things such as flares, first aid kit, knife etc etc marine safety stuff, No reason why another sub seat hatch cannot be built into the second cross bench option. One thing to try and avoid is breaching the hull with a load of screws, this will ultimately lead to the internal foam becoming waterlogged. My prefernce if possible is to bond directly to the fibreglass having removed the gelcoat locally.

    If it is a flat bottomed French Dory type, you may have to do a few calculations for stability if it is intended for use with standing fishermen. Hard to say much without some drawings or some indication of what the boat is like. However 3/4" ply is way too heavy for topsides. 12mm or 1/2" is more than adequate for a floor you stand on with boots, mostly 9mm (3/8") is enough and verticals can be 6mm. These should have some framing (support battens) but should be easily strong enough. I won't comment on the finishing as I'm not sure of the layout from your desciption, but it will depend on how hard the surface is used ie scraping anchors, dragging bins etc or just boots, as to how tough the finish needs to be.

    This time of year is not too clever for epoxy work in the UK, the humidity is very high currently. If you can get a dry workplace with some temperature control you may be able to get this work done now, or you may just need to use the weather windows available through the winter. The minimum will be local tenting to help cure, though preventing or keeping blushing down will be very hard.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Most dories need to keep the weight as low as possible or stability issues can become a problem. Also, 3/4" plywood seems way over the top weight wise. I'd agree in that 1/2" will do, with 3/8" possible (normal) given sufficient support spacing and 1/4" "dressing", for any areas you're looking to cover. The supports I mention are usually 1x2's, typically on 12" to 16" spacing.

    A light sheathing does two things, seal and waterproof the surfaces and dramatically improve abrasion resistance. Areas that will receive high traffic can use a sheathing, other areas can live without it, if desired.

    All this does depend on how big this boat is and the intended use. Lastly, always assume there's going to be a blush, even if using a non-blush formulation, if working in non-environmentally controlled conditions.
     
  4. dbsharples
    Joined: Nov 2015
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    Location: UK

    dbsharples Junior Member

    Just adding some pics, will reply properly once I've thought it through..
     

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  5. dbsharples
    Joined: Nov 2015
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    Location: UK

    dbsharples Junior Member

    Another pic.. Can't seem to add multiples from my phone.
     

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  6. dbsharples
    Joined: Nov 2015
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    dbsharples Junior Member

    And this is a mock up of how it will look after front and rear decks are added plus seats. The console will be cut down to size before fitting.
     

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    1 person likes this.
  7. dbsharples
    Joined: Nov 2015
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    Location: UK

    dbsharples Junior Member

    Thanks for the replies. I initially did a quick google search on what ply to use and ran out to buy the 3/4" stuff. Was probably a bit hasty there...

    As with all of these things questions raise more questions so apologies in advance!

    If the spacings/supports of the deck are far apart will thinner ply sag? The hatch walls will form the deck supports and the hatches are 40cm x 60cm approx.. I'll look to use thinner material and support battens for the walls/supports but anticipate I may need thicker ply for the deck itself? I may be wrong here of course..

    Also, agree on trying to avoid putting screws through the deck (although this had already been done in a few areas) but I'm concerned at removing gelcoat due to my lack of skill. Could the deck supports be bonded to sanded gelcoat with sikaflex or similar (3m 5200)?
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    3M 5200 is a sealant/adhesive and shouldn't be used as a glue, though often is on non-stressed elements. Primarily it's a bedding compound with aggressive adhesive properties, but shouldn't (again) be considered a prefered glue.

    Support spacing depends on several factors, with smaller, more closely spaced structure being stronger and lighter, than larger and more widely spaced arrangements.

    To get a good bond, the gelcoat needs to be ground away and epoxy (preferably) employed. There's absolutely nothing wrong with fasteners in the decking and it's often done. Adhesives can serve the same role, without the issues associated with fasteners, but fasteners tend to hold things in place, while the glue cures, so often are left in place rather than removed and have holes that needs to be sealed.

    Hatches and openings are bordered by "carlins" around the perimeter, usually also incorporating the lip the hatch lands on and also waterways, to remove anything trying to find it's way below.
     
  9. SukiSolo
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    SukiSolo Senior Member

    Thanks for posting the pictures, I now have a much better idea of the boat. I'm not sure she is what we in the UK call a Dory, as she looks like a shallow V hull not a cathedral hull type. There are several areas of concern with your proposed topside layout. Firstly the human weight is significantly far aft, it looks like one at the console and two behind on the seats? This is very far back, normal trim would be one (person) in the bow, one at the console and the third somewhere between. Assuming you have 20+ Hp and that hull is probably rated for 50, though a modern 25-30 would be more than adequate, the hull will plane. To plane it should not start from a bow in the sky attitude, it should be more level to let the shape provide the lift. She is probably a CAT C rated hull, the build plaque will tell you, and give the weight loading permissable which you must include your modifications (weight) as part of, she probably loses any (CE) rating after these mods but may be OK.

    Secondly you wish to use this for fishing, so I assume you wish to anchor the vessel rather than just drift. To get to the anchor feed over the bow, I would not want to be crawling over a flat deck (wet, at least in rain) with no security and then hauling in a CQR, Fishermans, whatever type of anchor and chain and rope you have. Not unusual to have 30-50+ meters of rope and a 3-4m length of chain. Much better to have access to the bow from 'within' for security and somewhere to store the anchor and chain btw do not forget to tie the end off inside somewhere solid....;)

    As PAR wisely says, you should aim at keeping the weight low, to my mind the rear seats are way too high, they should be below the sheerline by some distance. The way to get leg room on these foam filled hulls is to stretch your feet forward, you can't get 400mm down and keep good stability, unless you have a deeper hull.

    Also do not forget the routing of all the cabling to the engine. This is not insignificant, you have a Teleflex steering cable(?), two control rod cables (throttle and gear shift) plus a pair of battery leads. These need room to move and an easy arc to operate in if you want a decent service life. I suspect the rear seating as shown will impact on this requirement. You should mock this up for certain before going much further ie with the real cables and engine in place. Also don't forget the fuel tank and feed, assuming the normal 25ltr jobbie and piping. This and the battery (which should be covered) should be strapped down so they cannot be dislodged in rough conditions. 12+ knots in a decent short chop WILL move things around....

    One other drawback of all the aft weight is when you go astern in a seaway, hope you have (intended) a double transom to take the breakers from over the back... This is one area that even RIBs' suffer from, but a lot of genuine Dell Quay boats have this inner transom and a spare well to allow for this scenario without drenching the inner floor.

    I suspect a small Bimini type foldable spray hood is all you actually need, and a couple of bench seats with internal hatches. It's a bit tricky with these small hulls, they're not quite the small Shetland type. Useable but with care, a lot of similar stuff is used on the east coast around Southend and also parts of the south coast.

    Cutting back the gelcoat is easy with a multitool btw possibly a small angle grinder with care. Mask the edges with masking tape and work up to it. Orbital sanders can finish the job, even a little hand work with 60/80 grit.
     
  10. LP
    Joined: Jul 2005
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    LP Flying Boatman

    Please take PAR's advice on scantlings (material thicknesses and dimensions). The fastest way to kill any kind of performance is to over built with too heavy material thicknesses. The guy is a speed guru, among other things :D, and knows what he is talking about. You'll save 30-40% in material weight over your 3/4" ply if you go with the 3/8" ply. If there is any way for you to take the 3/4 back, do it. You won't regret it. You could be saving the weight of a loaded beer cooler or a niece of nephew that would otherwise need to be left behind.

    Paul, what is your 1/4" dressing that you are talking about?

    SukiSolo's advice is well founded also.

    So much experience offered here and it's all given so willingly.

    Good luck with your project, no matter the choices you make.
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Picture a set of 1x2's arranged around the perimeter of a seat box, that is covered over. 1/8" (prefered) or 1/4" would be the dressing to hide the "stick" framing.
     
  12. dbsharples
    Joined: Nov 2015
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    dbsharples Junior Member

    Thanks again for all the replies. I am hugely grateful and only wish I'd thought to ask these questions sooner!

    It is a cathedral hull dory, sorry, you probably couldn't tell from the pics so here is another one that shows the hull in more detail.

    Just to add some more info around intended usage, it will be mainly a river and estuary boat so not intended to take much swell. The layout I am trying to achieve is similar to the bass boats or flats skiffs used in the US with the decks raised to gunwale height fore and aft and strong enough to support a standing fisherman. Before I stripped off the existing hardware there was a flat area at the front that came halfway up the gunwales and standing on that didn't negatively affect stability too much.

    The boat is powered by a 25HP yamaha.

    If I used the 1/2" (or 3/8") ply for the top of the deck are you confident it wouldn't flex? Especially the hatch lids which will be supported only around the rim and need to be strong enough to be stood on? The hatch lids will be up to 30cmx50cm and i'm concerned they won't be rigid enough..

    So if I understand correctly I should look to use 1 x 2s as the actual deck support and then box in internally with 1/8 or 1/4" ply? That makes a lot of sense if so and will also provide a nice support for the hatch lids.

    Any recommendation on what type of timber I should look at for these supports?
     

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  13. LP
    Joined: Jul 2005
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    LP Flying Boatman

    I made the sole of my runabout from 1/2" ply. The framing is on 2' centers and I epoxied 1/2" ply stiffeners longitudinally (counter to the framing) on about 16" centers on the underside. The stiffeners are about 2" wide with a 45 degree chamfer all of the way around. The sole was designed to be removable, so it is only installed with screws meaning it gains little stiffening from the attachment to the underlying structure. I find it be be a rigid installation.

    It would have been better to use solid timber for the stiffeners as only half the grain in the ply is working for you in this application. A 1/2" ply deck with 1X2 (.75x1.5) stiffeners on 16" centers will give you a firm deck. 3/8" ply with the same stiffening on 12" centers should give you close to the same, but I can't speak from experience on the thinner ply. A little bit of camber would help too, but I think a casting deck is best left flat.

    As far as the access panels go, a doubler all of the way around the opening will stiffen the deck locally and gives the access panel a place to live. Supported all of the way around its perimeter, the panel itself should be sufficiently sturdy. If the panel is large enough it may require a stiffener, but not likely in the size that you mention.

    If you are working with epoxy and plan to encapsulate all of your work, you can use just about any kind of straight grained, knot free to mostly knot free wood. Spruces, pines, or firs should work fine. Lighter (soft) woods would be preferred. Hardwoods could be used, but at a weight penalty. The challenge in adding structure to a small boat to do it with as little weight gain as possible.
     
  14. SukiSolo
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    SukiSolo Senior Member

    For timber I'd likely use WR Cedar, European Spruce, or maybe Doug Fir for the heavy duty bits. You want reasonable quarter sawn Spruce or Pine, but make sure it is really dry, most places here sell it at too high a humidity for epoxy shaething without some extra drying. A couple of weeks inside the house should do it, if not too thick!.

    Just I beam down the C/L for the front deck and put side supports to the 'carlin' or inner gunwhale. Depends on how much standing on it you want to do, and if 9mm or 12mm for the deck. So between 300mm and 400mm spacing should be fine. You can dry run this, so test it. It will be stiffer when all in place, and glued up. I'd probably use between 50 to 60mm depth by 20-22 for the side bracing, hogged to save weight.

    LP is right about using doublers around any hatches, well worth the trouble. Radius as many edges for openings to stop any potential stress raisers cracking. Epoxy with at least three coats to ensure waterproofing.
     

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

    All good advice here. The framing stock should be a moderate density softwood, whatever is available to you and selected for grain orientation and defects of course. I don't think the hull side cleats need to be anything other than the same framing stock, though it's not going to hurt anything other than weight. I radius all structural element exposed corners as a matter of course, but if they contact a perpendicular, especially the hull shell, I'll tab/fillet it or bevel it (as LP has mentioned), which mitigates the hard point loading issue a great deal.

    The double framing around deck openings is standard fare. The outer piece is a facing or trimer, while the inner piece is the carlin. Think of these as jack and king studs in window or door framing in a house.
     
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