fibreglass transom v plywood transom

Discussion in 'Materials' started by phillnjack, Apr 1, 2013.

  1. PAR
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    PAR Yacht Designer/Builder

    If the epoxy was gumming up your paper, it wasn't mixed properly or wasn't epoxy. Epoxy can tolerate being off a little with resin/hardener ratio, but not much. This is a common mistake, by folks that use polyester a lot, thinking they can vary the epoxy hardener amount for some reason. The usually result is a goo that doesn't cure or setup fully. Now this will gum up paper like crazy. If the epoxy says 3:1, then you better make it three to one. You can be off a little like 3.3 to 1, but much more and you're screwing the pooch.

    No, you don't need to fully wrap the plywood cores with fabric, if using epoxy. In fact, if using epoxy, use cloth, not mat, for the hull shell tabbing. Mat has very little strength, but is required with polyester, because of it's inherent weaknesses. Simply put, no mat with epoxy, just cloth.

    To protect the wood, you need a minimum of 2 coats of straight epoxy, before you bond and tab it to the hull shell. Special attention to the end grain, should be asked, as this is the place moisture can get in easiest. The same is true of any holes that get drilled, but you already planned on epoxy bonding these, so no worries there.

    Lastly, if you foam fill the voids inside the hull shell/liner combination, those seemingly thin skins will become quite strong. This is probably why the hull shell seems thin to you, as cored structures usually have about half the skin thickness, because it's split (some on the inside, some on the outside). This is weak, unless it's in a "sandwich" construction method, like this boat once was. Add some material if you want, but don't kill yourself.
  2. redreuben
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    redreuben redreuben

    The reason transoms rot is not because of polyester resin it is because when motors, boarding ladders, trim tabs and whatever are bolted on they are drilled and screwed straight into the wood core without adequate sealing first.
    They then swell up and burst from the inside, polyester has very little to do with it.
    Polyester will stick to wood remarkably well if the job is thorough and complete.
    No amount of epoxy will save you from stupidity or bad workmanship.
    1 person likes this.
  3. PAR
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    PAR Yacht Designer/Builder

    I'm not sure of your experience level Red, but polyester doesn't stick well to wood and this is well documented. It's true that coating breaches and poor workmanship are big culprits, but most everything else used on wood, sticks better then polyester. It's not the stick that's the major issue with polyester, but moisture vapor penetration. Again, polyester is at the low end of the scale compared to other resin systems, which causes moisture gain/lose in the wooden elements, eventually shearing the sheathings and coatings.
  4. redreuben
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    redreuben redreuben

    I've built and repaired poly boats for decades and am well aware of the vast superiority of epoxy.

    However poly has given millions of users very satisfactory product lives for the money spent.
    When you do replace transoms it is rarely the resin system that has failed, epoxy will not contain a 2" of wet plywood.
    Stringers and floors are another matter !
    Transoms usually fail well ahead of floors and stringers due to the penetrations.
  5. SamSam
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    SamSam Senior Member

    Is there a site that documents in % the difference in water permeability and adhesive properties between polyester and epoxy? Maybe that would also state the types and grades of polyester (iso-ortho) and epoxy (??-??) used?
  6. DGreenwood
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    DGreenwood Senior Member

    This is completely anecdotal and does not qualify as real data but still interesting.
    Some years back I was given the job of redecking a pro built 40' trimaran that had been sheathed with polyester. There weren't any bubbles or voids so I was skeptical that there was a need. I ground through to the wood around the deck edge and lifted the glass at the bow with a chisel. I grabbed the glass and lifted the whole deck in a single peel in seconds. The ply was very good except where the sheathing had been breached over the years. It came away clean and you could have laid on a new layer with very little prep. There were a few spots that had been repaired with epoxy. Which required serious grinding work to bring down to a surface to laminate over. The upshot is I was startled firstly how poorly poly bonded to ply and secondly how little that seemed to matter in keeping the deck functional. I still would always opt for epoxy but...
  7. redreuben
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    redreuben redreuben

    Our method was to make sure the ply was sanded to remove any contaminates from the plywood production, we then used to wipe with acetone with a bit of promoter in it.
    After cure this would tear chunks off the veneer to remove.
  8. PAR
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    PAR Yacht Designer/Builder

    Tensile modulus, compressive, strength hardness, stiffness, etc. are all available for each product line. A few sites attempt to make comparisons, but in many cases these tests are less than balanced or well executed.

    In regard to waterproofing, Red is correct and as noticed by DGreenwood, if done properly, can make a reasonable waterproof barrier. This isn't the usual problem with production builds, which are built to a price point, not a quality or technique level. In this regard, repairs, particularly by the DIY'er can make big inroads into polyester use on wood, because they're not paying themselves for labor and materials are a concern, but relatively inexpensive comparatively. The problem is peel strength, if you used sufficient fabric and resin over wood, even with good prep. Elongation modulus for polyester, with alternating mat/roving units, isn't sufficient to keep up with unstable wood's movement, so it needs to go down fairly heavy, often making the wood redundant. This coupled with moisture vapor permeability through polyester (about 10% - 11% for most brands) is high enough, to insure the wood will not stabilize, which continuously tests the peel strength (which is weak) of the resin, fabric, substrate interface.

    So, yes I've seen 30 year old transoms that were dry as a bone, just damaged, but this is the exception to the rule. More often than not, I've seen progressive moisture ingress, as soon as 5 years out of the manufacture. In fact, judging by the range of years I see, most production boats start to show signs at a fairly steady rate, regardless of brand (some are clearly better than others). Of course, it takes a while for the signs to become contentious enough, for someone to seek advice or repair, though at 15 years out of the manufacture, you can bet there are areas within the structure that are experiencing problems. As a result, most manufactures are moving to different, more expensive resin systems (vinyl and epoxy), as well as using more costly, but as more inert structural materials. They wouldn't be doing this, if they could get polyester to stay stuck to wood.
  9. redreuben
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    redreuben redreuben

    Agree 100% PAR but when you are working with an existing boat, particularly a production boat, switching resins is fraught with problems.
    Getting gelcoat to stick, matching the internal flocoat, will the next person who works on the boat realise that it is now a blend of materials and try and lay poly over the epoxy.
    Poly is bottom of the barrel in resin systems no question but when it has given you 20yrs and possible 20 more, I think you should repair like with like and keep it simple.
  10. PAR
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    PAR Yacht Designer/Builder

    This use to be true Red, but products are now in place where you can use epoxy over gel coat and even epoxy gel coat itself. Agreed, polyester doesn't stick well to epoxy, but with some planning, you can insure epoxy to poly bonds where necessary.

    I've seen very few gel coat attempts, other then very small ding patches, work out well for the DIY'er. Color matching, good flow, surface contamination, bonding, etc., in a guy's driveway, usually turns out less then desirable.

    Simply put, I've found that by the time someone needs a transom core, sole or stringer repairs, the gel coat is shot or nearly so anyway. This means paint for the DIY'er. Some can get away with spot gel coat along seams and such, but they usually stick out like a sore thumb, mostly because they discover what "fair" is after the gel coat goes down.
  11. Wynand N
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    Wynand N Retired Steelboatbuilder

    Check the maximum clearance of the mounting bracket on your outboard to get the max thickness you can go...

    When I was still in the boat building business, the GRP bass boats I produced we made the transom as thick as possible for added strength and was designed to take Yamaha/Tohatsu outboards (was agent) with the engine bracket clamp close to max opened.
    An unforeseen problem resulted due to certain other makes of engines having less clearance on their bracket clearances and could not fit my boats and I had to reduce the thickness of transom for other makes of engines to fit. Was quite costly to modified the deck mold at the transom to fit new transom and not to mention the embarrassment;)

  12. phillnjack
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    phillnjack Junior Member

    My transom thickness is 1 1/2 inches of wood inside the fibreglass.
    the total thickness on the outside is a fraction under 2 inches.
    The transom being built like it was was actually doomed to failer when i saw how it had been put in, the idea was fine if they had spent another 15 minutes finnishing it off and NOT skimped the size of ply put in.
    Even using the same thickness of ply,i would be putting back over 40% more wood content as the ply transom was not as wide as specified by the bloke who runs the factory where it was built !!!!!!!.

    Now i have spoken to this man and he was adamant that the transom had a full piece of ply from one side to the other and tabbed round the sides.
    the same with the inner shell, a full piece of ply to the shells full width then tabbed around the sides as well.

    I think 34 years is pretty good for the transom in one respect, but the transom should not ever rot if done correctly.

    Now im not concerned about future owners of the boat trying to do repairs, this will not be possible due to the way its going back together.
    im gonna do the job properly and its never comming apart again.

    There will be no delamination due to dirty moulding getting glass chucked in and resin poured over it.
    its gonna be good ,real good.
    The floor that goes down is going to have a couple of hatches put in so if i do find any water has got in then it can be swiftly removed with a sponge and cloth etc.

    The epoxy i hope will do the job its supposed to do.
    I have never worked with epoxy apart from tiny little tubes of this is new to me, ive only ever used polyester resin and matting.

    So this project is going to be more of a complete build than a repair.
    the transom is now completely ground down and ready, just a few tiny bits in the hull and then i can start building it all back up.

    the transom is rough to the touch as the fibres are on display and its all nice and clean.
    Now do i put a complete layer of 450csm on the whole transom with poly and let it dry brfore the new ply and epoxy goes on ?
    or just put the epoxy straight on the transom and wood ?

    sorry to ask so many questions, but i had planned everything out so i thought, but now my plans have been chucked in the bin and its all new to me again.

    my transom will be 59 inch wide 19 inch in the middle 15 inch at the sides.
    it is 1/2 ply, so how much resin should this take to do the job.

    i will have twice this amount to do as need 1 piece of wood on each shell.

    i dont to be skimping on it, i would much sooner do it right.

    i was thinking of getting 3 kilo of it wich is about 3 us quarts.
    would this be enough ?

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