fibreglass transom v plywood transom

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

  1. phillnjack
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    phillnjack Junior Member

    I see what you're saying about the epoxy being better.
    'But if i can coat the whole lot of wood and encase it in fibreglass matt and resin before it even see's the boat, this must surely help keep any moisture from getting in.
    What i want to do is apply two layers of glass fibre to the wood and all around it.
    The second layer will not be put on until there has been a 24 hour drying time of the first layer. rubbed down and cleaned then re-glassed etc.
    then the wooden transom will be put in the boat after this is dry of course and the given a final layer to keep in place etc.
    The epoxy in the holes is just a final help, the engine top holes will also get the same treatment but they do not lie in the water like the bottom holes when at idle.

    The boats bilge bung at the bottom is what is going to be my most difficult to solve for water problems etc.

    for this i was thinking of having an area at the very bottom that has No wood within 3 inches of the bung and this to be a complete filled void of polyester and 450csm matt.
    The transom wood will be exterior grade ply that is considered "waterproof", i know its not waterproof but has high quality rot proof glue and is about the best i can get as far as ply goes.
    Would i be better of in varnishing the boards before laying the polyester resin and cloth over them ?
    i would give a 50-50 coat of white spirit and varnish first to act as preservative then 2 or three coats of varnish if this would be better, then when dry caot them in the poly etc.

    5 years does not seem much time for the bonding process to start wilting, but yes i agree that many mass produced boats are built very shoddy, and what you dont see is just terrible workmanship.
  2. whitepointer23

    whitepointer23 Previous Member

    I have built a solid glass transom on my little run about.what a job. You have to go slow otherwise the resin cooks. But i am glad i did it. I now have glass stringers and transom. No wood except ply on floor.
  3. phillnjack
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    phillnjack Junior Member

    What thickness is the transom and what horsepower can it take ? or what horsepower are you running ?
    i heard that glastron stopped using wood about 15 years ago in their sports boats, but wasnt sure what they used for a transom core ????

    i am also thinking would it be any benefit to use a solid pice of hardwood rather than ply
    or would this rot even sooner.

  4. whitepointer23

    whitepointer23 Previous Member

    Transom is 32 mm thick. Designed for 70 hp max. I think ply is stronger than hardwood unless you made a laminated hardwood core. A boat builder told me that they did not make solid glass transoms because it was slow and expensive.
  5. PAR
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    PAR Yacht Designer/Builder

    I know it "seems" reasonable, that a fully coated and skinned in mat covered hunk of plywood, might be water proof, but in polyester it's not, which is why I replace transoms, stringers, hull and deck cores and cockpit soles all the time. It's the moisture vapor ingress thing I mentioned. Polyester, even with some mat and roving tossed in, just doesn't "stabilize" wood. By stabilize I mean lock the moisture content below 17% (preferably below 15%), so eventually poly coated wood absorbs moisture and rots.

    No, varnish doesn't water proof anything and offer only 30% or so in moisture vapor penetration resistance. Varnish lets in lots of water, just ask your wife about water stains on the varnished coffee table, from a wet bottle of beer.

    Your lamination schedule is too slow. By this I mean you shouldn't wait until things are dry, before applying the next layers of goo and fabric. The best way is to do it all at once, though this can be impractical, so you do what you can, but before things completely cure, you apply more, so there's a chemical bond between the layers.

    Precoat the plywood with a few coats of resin. On the second coating of resin, use some CSM, which will help the bond to the hull shell. While this coat is still "green", bond it into the hull shell with more mat and resin. Next, "tab" the plywood to the hull shell with 'glass tape. Several layers of a moderate weight cloth is better then one or two layers of a heavy roving. My usual recommendation is biax, but polyester can't take advantage of the strength of biax, so it's just a waste of money, to use in a polyester laminate. Make sure the edges of the plywood have thickened resin smeared around them, so there's no hard edges or transitions from plywood to hull shell, before you apply the 'glass tape (or cut pieces of cloth and mat). Always alternate, mat (for the bond) then cloth or roving for strength in each layer. Mat to mat bonds are okay, but cloth to cloth or roving to roving, not so much.

    The biggest reason production boats fail in this regard, is they don't use enough resin or fabrics. You can fix this common shortcoming. In other words, if in doubt, use more goo and material.
  6. phillnjack
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    phillnjack Junior Member

    Ahh thats good to know about not having to wait between layers of resin and matt.
    its better for me to lay it up thicker all in one go to be honest due to having to get it all out of garage then put away etc.

    I have 450grm csm, will that be ok as i have lots of it ha ha
    i will be using a resin called Crystic 2-446PA ,its Lloyds registered for boat building and came with mekp catalyst.

    I will make sure all the edges are nice and smooth and will be putting an extra layer of resin on the transom itself before to give a good surface bond.
    The transom on this boat is about 1/8 inch thick and needs a couple of repairs doing before the plywood and cloth go on it.
    The engine holes will be filled in as they are infact an inch too low, i know this because i made the mistake of drilling them a few months ago.
    So with a few repairs done this will be my plan of action.

    1/ Put complete layer of resin and cloth all across transom and around the corners etc.
    2/ have centre piece of ply already cut to shape and size and have cloth ready to go on it as well.
    3/cover the centre section in plenty of resin and place this in centre of transom and then straight away cover the section in matting making sure to tab the edges.
    4/ while all this is still wet get a complete layer of cloth on the whole transom again and fully wet it out with plenty of resin, complete smother full width transom board with resin and place on transom and then completely coverthe ply with last layer of cloth and make sure all edges are completely wet etc.
    Quickly go round all edges to make sure everything is well and truly wet and fully resined.
    5/Place a few more pieces of mat around the corners and bottom of the ply to make sure there is complete coverage everywhere.
    The clamps have already been put on by now to clamp it all to transom .
    i hope to have a helper (slave/gimp) with me at the time i do this to help get it all done in the one go.
    the complete transom on the blue done in the one go will be good for me ....
    once this is done and drying i have to do the same on the white superstructure all over again ha ha , and i also want to put a couple of layers of chopped strand matt on the bottom of the hull.
    Today i have been give a load of silicone carbide discs for my grinder so i have no excuses to not get the hull bottom as clean as the transom.
    once cleaned i will be going over everything with acetone to double clean it.

    once all this is dry i then have the job of lowering the top back ontothe hull, with this the two new transom s will meet each other as the white and the blue and the ply all make one close fitting unit.
    To do this i was thinking of applying wet resin to both parts of the transoms to help slide them together and maybe even help them bond while im putting it all back together with the engine bolt holes and ski hook holes etc being used with wood for clamping it all in nice and tight.
    at the front end i will put in the tow hitch eye and then go around the seam of the boat with resin and 4 inch tape and loads of clamps after its all wetted out !!
    now i hope that makes sense what im going to do.
    Once the top is on and the seam dry i have the job of bonding all the stringers from the white top to the hull bottom (blue).
    Once this has been done then its the foam process to be done.
    it sounds sos easy just seeing it here, but its a fair amount of work involved, but will be well worth it in the end.
    I plan on keeping this boat a good few years, and if i get 10-12 years from it i will be very happy.

    Now about the temperature to do the glassing work, what is the very lowest i can glass at ? i cannot get indoors with this as garage too small.
    at the moment its a definite no as today we had snow, yes april and we have overnight freezing temps its crazy.
    I will get all prep work complete in the next wekk then wait to the weather breaks.
    but like i say what is the minimum for the glass to definitely dry ?

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

    I don't work with polyester very often, so it's probably best if some of the other members chime in with recommendations. There's a fair bit of information about using polyester online and this might also be a good source.

    As long as you've ground the hull clean, just remove the dust. An acetone wipe isn't going to make anything better.

    I have to admit I don't like the idea of a blind bond on the transom core halves. This means only the first layer of plywood is actually tabbed to the hull, which isn't good. I'd treat the transom as a separate assembly, from the deck cap. You want the transom core (all of it) well attached to the hull shell, with several inches of tabbing all around. You can apply a layer of mat to the liner as you drop it in, which should insure it's stuck to the transom.
  8. phillnjack
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    phillnjack Junior Member

    I have found a place to get epoxy resin and will after some serious thought will be using epoxy resin instead of the polyester on the transom.
    Now regards putting the wood on the transom only and not the inner shell this looks like cannot be done that way.
    The inner shell has a piece of plywood bonded to it to give it the strength it needs as well.
    then when these two halves come together they make up the one thicknesss.
    its so very different to a normal transom that just has a cap over the top.
    This relies on the inner shell as the inner transom goes almost all the way to the hull bottom leaving just a small 1/2 inch gap.
    Looking from the seats when inside the boat the white inner transom is 19 inch deep.
    Now all of this is also strengthened up when the play attached to the inner shell has tabbing underneath it from the ply..
    ill have to have a good re-think and look how this can go back together by making the transom all one piece.
    It would be much easier to put it back together like Par has said above.

    im going to have a good look at this right now ha ha

  9. PAR
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    PAR Yacht Designer/Builder

    I just don't like the idea of no hull shell tabbing on half of the core. It's difficult to tell with pictures and text, but I have an idea of what you're working with. In person, I'm sure this would all make sense, but I'm a wee bit far to drive to.

    Epoxy does have a strength advantage in this regard, so maybe the two halves thing can work well, with lots of tabbing on the hull shell side, to compensate a little. I tend to make things a little tougher than necessary when these sort of issues come up.
  10. phillnjack
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    phillnjack Junior Member

    The ply on the hull/outer shell will be tabbed and also some mat and epoxy going round the sides of the hull and from ply to floor etc by around 12 inches.
    I plan of making 100% sure no water in hull is able to getto any of the ply from top bottom or sides.

    The inner shell will have ply almost to its ends and also be tabbed all round and extra layers going round the corners of inner shell by about 8 inches.

    Now before the inner and outer shells are pushed together and bolted and clamped, i am thinking of adding a layer of mat that is soaked in resin and also resin on both inner and outer shells allowing for a bond to take place when it drys in position !!!!!!.

    The amount of wood i am proposing to use is around 50% more, the makers told me this transom was full across etc,but they told a complete lie on that one, and that it was tabbed and also going around the corner.
    the very same with the inner shell, it was nothing like they said it was at all.

    The whole boat was built in a very shoddy and unprofessional way i think, but i have found this to be the case with most fibreglass boats when delved into to do repairs etc.

    I also plan on making the full length of the hull below the water line thicker from the inside to add some overall strength.
    the hull outer shell is only about 1/8th of an inch thick, about the same as all boats built using this type of construction all over the world.
    The expanding foam was the main strength,not the fibreglass !!!!!!!!!!!!!
    I just want to have a touch more strength in the hull for piece of mind if i decide to beach the boat etc.

    When i do put extra layers of glass on the hull i presume i can use the poly resin for this ?

    Also one more thing i meant to ask about is the coosa board type stuff ..!!!!!!!
    The stuff we have over here in the uk is recycled plastics, the board is very very strong and supposed to be rot proof water proof etc etc.
    Many people are using this for exterior buildings say its much better than plywood and stronger ???
    you can cut it like ply with same tools etc, you could put resin over it as one side has a dimple effect that would give a bond to resin i think.
    But ive never done a transom with plastic ... any ideas on this stuff ?

    or is it best to stick with marine ply and just do a traditional transom rebuild type thing.

    I am taking all answers onboard, as this boat can only be done the once.
    when i put this back together its never comming apart again by me 100%.

    Last edited: Apr 5, 2013
  11. phillnjack
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    phillnjack Junior Member

    These pics show the inner shell plywood was not as far across the inner shell as i propose to make the new ply, old wood 37 inch, new wood will be around 50 inch wide.
    Also a couple of pics showing the outer shell/hull plywood was only 37 inches wide and not glassed in at all.( and soaking wet and rotten through.
    these pics show a very poor construction altogether i think.
    Now i plan on the ply that attaches to the main hull/outer shell to go all the way across the transom leaving just a small space to fill with resin at the outer edges and to be tabbed around the corners of the hull as well.
    also note the transom wood had a void in the top due to the hull flairing out at the seam.
    this i propose to fill with mat and also tab properly to give a stronger bond.

    i hope these pics help show what i am doing with this project.


    Attached Files:

  12. tunnels

    tunnels Previous Member

    very well said and its true !!

    We get this debate about using wood in stringers engine beds and transoms every so often !!
    Ok wood has a limited life span once its put into a boat and covered with glass , water or moisture seems to be able to get in any where and the wood exspands and in poorly glassed situations can begin to delaminate the glass where its bonded to the hull so mosture gets in even quicker !!
    When glassing over wood a point to remember is the amount of glass used should be suitably strong enough so when the wood has started to rot and get soft and soggy then the covering glass will carry the load like it should have done from day one !!.
    In production boats the quality of the wood used leaves a lot to be desired . second grade coated with resin if your lucky and a spit of glass and thats your lot in most cases !!. :rolleyes:
    Myself i have a 1975 old power boat and was built with no wood any where under the floors ,it has a glass top hat frame fitter so just the transom was solid wood not plywood .
    I replaced the whole transom with a solid glass shaped transom because i fitted a much bigger out board , So now this boat will still be going well after im gone . :p
  13. phillnjack
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    phillnjack Junior Member

    Today done a bit more grinding and a lot of looking as well.
    then after having a long look, i then sat down and done some thinking.
    The thinking i found a bit stressfull,especially as the sun was out and it would of been a boat test day if it was back together already ha ha .
    Before i knew it i had attacked the boat again with the grinder and made something nice
    "DUST" not just ordinary dust, this dust was staying nice and airborn with millions of little glass particles drifting off towards a neighbours garden who i realy dont like , so that was good.
    I have come across a few places where previous hooligan owners had decided to splodge down a few chunks of epoxy into the hull rather than do a proper repair on the holes that were originaly used to put on a stainless steel keel band/protector.
    any of these types of boat that has them fitted from new is doomed to get a soggy load of foam that will not let all the water out.
    The bands are always just screwed on or riveted once the boat is fully built.
    If i decide to put back the keel band i will be putting capitve nuts and going over the top of each individual nut/bolt with fibreglass etc.
    But at the moment im thinking to just leave it off as ive done the external repairs to this area a while back and its been good and not letting in any water at all.
    its from the original keel band that water had got in between the two shells and caused the transom to get wet and rotted away in the first place. so maybe putting that back on would not be a good idea, might just get keel guard.

    Now im definitely going to be putting down an extra layer of fibreglas chopped strand mat on the hull of this boat to make stronger etc,
    The question is can i just use polyester resin for this ?
    The boat was originaly made completely from poly and i have the fibreglass hull realy realy clean and right back to strands etc its that clean.
    The cloth im going use is 450 gram stuff (about 16 oz) so pretty thick and will do the job of strengthening the hull bottom enough i think.
    The piece to be layed down will be 10 feet long and 39 inch wide all going down in one piece (i hope).

    on monday i will be ordering my epoxy and resin and whatever else im gonna need to finnish the job , and same with ordering the ply.
    But first im going to try a dry run with some scraps of thicker ply to see if its possible to make atleast one of the 12 mm(1/2 inch) boards a 18mm (3/4inch) board.
    The hull transom got a real good going over with the grinder today and i can confirm its clean, infact like brand new, no dirt or any traces of previous gunk on it.
    Ive cleaned the bottom edge all around the transom to make sure i get a real good bond there as well.
    Tommorow i hope to get the rest of the hull finnished and ready for glassing.
    Then i have the horrible task of cleaning up the inner shell bottom and its stringer system of all the old crap on it that had delaminated due to being put down dirty,very very dirty.

    I am realy looking forward to getting this boat back on the water and giving it a real good test.

    but still a long way off just yet, any idea about what to use inside the hull as in poly resin will be good

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

    Reinforce the inside of the hull with polyester first, particularly if you use epoxy on other areas. This is because epoxy sticks well to polyester, but polyester doesn't stick well to epoxy. I'm not sure you'll gain much strength with just a light layer of mat and poly on the inside of the hull, but if it makes you feel better . . .

    The end result on this boat will be a far sight better then the original, mostly because you care and will employ reasonable techniques and sufficient materials. You're not building to a price point, like the manufacture, so she'll be one tough *** (technical term) boat, when you're done.

  15. phillnjack
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    phillnjack Junior Member

    Thanks par.
    the reason behind thickening up the hull bottom is that its just so thin.
    if i put down 3 layers it would be double the thickness it is now its that thin.
    i would say its about 3mm definitely not 5mm as i put a 5mm piece of steel next to the boat lip and the steel was a lot thicker !!!!!!
    Maybe ill put 2 layers down to beef it up a bit.
    I am glad you say about the poly down first, i never knew about that, it makes good sense then to do the hull first then the transom as the transom will be tabbed onto the polyester and glass in the middle 3 feet of the back of the hull bottom.

    Is there a minimum temperature to use the epoxy ? i have no choice but to do this work outside, my garage is too small to allow me to work in there with the boat.

    Maybe i need to put down about 3 layers then ha ha
    I am not worried about adding extra weight in the hull, as it will be all over the bottom not just the back end.
    the boat planes at about 15 mph and was a pretty light ride before, but i have taken out so much fibreglass that wasnt doing anything at all as it was just put down on dirt and never really bonded.
    with the wet foam and wet wood and fibreglass rubbish, i will of taken out about 100 kilo's (220 pounds) in weight of pure rubbish in total ,but will only be putting back about 40 kilo in total.

    I am hoping i can get the thicker wood in the gap between the hull shells, but im not thinking its definite just yet.
    The price of the materials to do this are very expensive in the uk, but this is nothing compared to the price of a new dory of same make.

    The next few days will make it all start looking better, the inner shell stringers need to be cleaned up big time, and a few repairs on them as well.
    Being as they are hollow, i might try and fill those with some closed cell foam to stiffen them up as well.
    they are sort top hat construction and spread out as they go down to the hull.
    these are a pretty strong construction and make the hull a very rigid hull (if glassed in correctly).

    I did find out today that epoxy resin realy buggers up sanding disc's fast, there was a big dollop of it in the middle of the hull that used to hold a transducer in place.
    but when i say a dollop of i mean a huge lump about twice the size of a big fist.
    it must of cost a fortune to put in this cray amount of epoxy to hold a little transducer ha ha , but it was very hard getting it out of the hull.
    No glass with it just epoxy, and it was cracked like toffee !!!!!!!!!

    Now earlier on in this topic i was talking of wrapping the transom boards in the cloth completely, is this the right thing to do ?
    or do i just coat the boards with the epoxy and make real good wide sections of mat when i tab it in ?
    I dont mind using plenty of cloth and originaly had planned to wrap everything in it.

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