fibreglass transom v plywood transom

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

  1. phillnjack
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    phillnjack Junior Member

    Hi folks
    i am about to put in a brand new transom on my boat.
    Now the boat is very similar to a boston whaler 13ft and i have managed to
    get the hull away from the superstructure, just like the manufacturers say cannot be done ha ha .
    Anyway the transom wood was soaking wet and rotten so its time for a rebuild.
    The hull thickness in the middle of transom is around 1/2 inch thick and once you get say 20 inch from the middle its about 1/4 inch thick.

    On my boat there were 2 transom boards each made from 1/2 inch ply wood.
    1 piece gets stuck on the hull and glassed in and 1 piece goes to the suprestructure and gets glassed in as well.
    Duruing manufacture the makers of the boat skimp on the transom by only putting a 37 inch wide piece of ply in a 60 inch space, so i will be putting in as big a piece as possible on both hull and the superstructure.

    Now what i want to know is would i be better off by building the transom with just fibreglass and matt instead of plywood by 1/2 to make the hull 1 inch thick and the same with the supuerstructure instead of using plywood for this ?

    what is the strength difference in the fibreglass v plywood on the transom etc.

    Is there a valid reason why plywood is put in boat transoms from small boats to big boats ?
    is it purely to keep weight down or price down or another reason.

    if a fibreglass transom was solid 2 inch thick what horsepower could be put on this ?

    with just the plywood in the mid section the rating is 50hp, now what will the new rating go upto with a full transom and what would it be with a full fibreglass transom.

    pic of the hull below
    with almost everything out ready for the rebuild.
    also the hull bottom will have 2 extra layers of fibreglass going down and taken to and up the back of the transom for added strength.
    i have marked in red the original size of transom ply and marked in Green my new board size.
    hope this make sense :p


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  2. rasorinc
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    rasorinc Senior Member

    The hp rating is issued by the manufacturer and You can not change it. Think Insurance.
    You could beef up the transom to carry 300 hp if you want but the rest of the boat may fall apart. I would replace the transom as it was originally built changing only the length of the plywood to go side to side. Use epoxy for this and you will have a very strong transom. When trailering, an engine brace for when it is in the up position is a very good idea as bumps and rocks on the road can put great pressure on the transom when the engine is bouncing up and down. You could upgade by making the transom ply 3/4" instead of 1/2" but the rating remains the same. 13' with a 50hp on it should make you fly.
    Plywood is stronger and stiffer than glass.
  3. rasorinc
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    rasorinc Senior Member

    phillnjack, I should add I'm speaking of US law. You buy a truck that has a gross weight posted of 6'500lbs. and can trailer 5'000lbs. Nothing I add or do to it to hold more weight or pull a heavier load matters to law inforcement. It is basically the same with manufactured boats here and add in the coast guard as another law inforcement agency.
  4. missinginaction
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    missinginaction Senior Member

    Not to hijack the thread but I never knew that. 30 years ago we pulled a small V-8 out of a Buick that I owned and replaced it with a 455 CID engine. That's about 8 liters for those of you in the more mathematically advanced regions.;) Really livened up the car, of course we were young and gas was cheap. I sold that car a few years back and it's still out there. Always passed inspection and was a safe reliable ride. Am I a criminal?:confused:
  5. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    It doesnt mean you are criminal.... it just means if anybody gets hurt you could be sued. Where i live , when you change things you become the manufacturer. It dont matter who originally built it. Sometimes you got to turn away jobs because of the liability factor.
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Actually, you can get modifications certified, if done properly, but back to the original poster's concerns.

    A 1" thick transom is very light, so the engine must be quite small and portable. What is the max HP rating of the boat? If you don't know this, take the length of your boat and multiply by the transom beam and post this number here.

    I'm not sure of the construction method employed in your little boat, but the "superstructure" is probably just a liner and the transom would be best bonded to the hull completely, not both the liner and the hull in halves.

    Your idea of a full width transom is a good one and the most common way manufactures build them. Some try to save money using less material, but it usually bites them in the butt later.

    Plywood is used, because it's cheap, strong and relatively light weight. If you made a solid 1" thick laminate, especially from mat and roving, it will weight three times as much. You can use other materials, like foam, honeycomb and man made wood substitutes, but once you cost these things out, you'll quickly see why plywood is still king.

    The most important thing about a transom is it's stiffness and it's ability to transfer torque to the bottom of the boat. Since your boat delaminated, which is why you managed to separate the hull from the liner, you should strongly consider placing all of the transom material on the hull shell, not the liner.
  7. phillnjack
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    phillnjack Junior Member

    Here in the uk the orkney doy is rated for 50hp like i say, but i will not just be upgrading the transom i will also be upgrading the hull thicknessas as well and taking the fibreglass around the rear corners of the boat.

    As for using 2 sheets of 3/4 ply that is not possible, the space in between the two shells is just over 1 1/4 inch ans the spare 1/4 inch will be taken up by an extra layer of glass on the transom an extra layer of glass on the suprestructure and an extra layer of glass each side of the plys.

    I am not concerned with things like insurance rules as here in the uk we do not have to insure a boat or licence a boat on the sea.
    The engine i will use is a 1995 3 cyllinder evinrude 60hp..this engine comes in 3 sizes from new 50hp 60hp and 70hp all the identical weight and diamentions externally.
    I just want a much stronger transom that could take more power than i will use.
    I like to have more strength as a safety margin.
    I have already used the 60hp evinrude and yes it does fly, and goes very well indeed.
    I just wanted to see if there is a way of making it a lot stronger by maybe making a solid transom , but if the plywood is better for this then i will stick to the plywood and just rebuild using that.

    What is the reason i should not use polyester resin ?
    the entire boat was built using polyester resin and its lasted 34 years so far.
    in the uk epoxy is over 4 times the price of poly

    thanks for all the input.

  8. michael pierzga
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    michael pierzga Senior Member

    Your boat was cored construction, not solid glass. How will you replace the core before fitting the superstructure skin ?

    Also....outboard rating has little to do with the strength of the transom and everything to do with the hulls ability to float on its designed lines and remain seaworthy under power

    One and a half to two inches is a typical transom thickness.

    one and half inces of glass resin would be heavy and expensive.

    Use a plywood core.

    Somehow you must transfer the load from the engine into the structure of the boat.

    Knees or longitudinals are typical.
  9. phillnjack
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    phillnjack Junior Member

    To PAR

    the transom ply is what is 1 inch thick , with 2 x 1/2 making up the 1 inch.
    the fibreglass transom is 1/2 thick and the fibreglass inner hull/liner is 1/2 thick making a grand total of 2 inch thick transom.

    Now im not a boat buildrer and dont mean to sound crazy but there is a lot of boats out there rated at way over 150hp with 2 inch transoms.

    If you look at the average 13ft speedboat in the uk it would be rated at around 90hp.

    There are a lot of ribs nowdays rated at over 100hp that are just 1 1/2 inch transoms as well. but these being short in width at transom might have something to do with that.
    My hull is 13ft 3 inch long, it is 70 inch wide at the widest point and is mainly between 60 inch and 70 inch all the way along.
    its the same shape as a classic boston whaler 13ft but a lot heavier construction, and higher freeboard,longshaft transom.
    The inner hull is unlike the whaler as it has beams running across down in the hull as stringers.
    these get fibreglassed to the hull for reinforcement, the inner hull is not just a liner by a long way, its built of heavier material than the hull and makes the whole construction a lot more rigid.
    I will put a few pics to show how this is achieved.
    the stringer built into the inner hull once glassed to the outer hull are every 2 foot along the boat untill the bow goes up, so 4 in total.
    These are aprox 7 inch wide at the bottoms with 2 inch lips..
    then 2 part foam filles all the voids between the inner and outer hulls.
    the entire boat is much stronger and more rigid than say a 13ft speedboat.
    many makers of these type of dorys do not use a stringer system and rate the boats similary apart from one that i know who rates at 60hp and 1 that rates to 30hp (that boat is ulta lightweight and flimsey).
    Boston whalers are rated at 40hp and have no stringers.
    Dell quay dory were rated at 50hp and no stringers at all and just foam with a very thin inner hull....
    I just want to know that what i am going to do is going to be stronger than original, and safe.
    Bouyancy will be put back in the boat with new foam ,and a bit more of it as well.

    hope this all makes sense, from calculations the boat should be able to take
    65hp if my math is right, and that would be without any extra strength being put in place.
    This is also a very old equasion i am going by and not a new modern one.
    its probably what was used on wooden boats for engine recommendations.


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  10. phillnjack
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    phillnjack Junior Member

    to michael pierzga.
    the boat will be put back together then have the foam put back in very easy.
    i plan on having a 4 speaker stereo so will make 4 large holes high up in the inner hull.
    the boat will be tipped over on one side and raised say at the front slightly then foam poured in or sprayed in the back right hole. when the foam as expanded and started to come out of this hole i will then move onto the front hole on same side.
    then the boat will be tilted forward to help get the front side cavity filled.
    then simply do the same the other side.
    Unlike the whaler my boat has a floor that can be unscrewed from new leaving 4 huge cavity's under the floor, these have their own seperate foam pillows in each cavity.
    The boat uses aproximately 22 cu ft of closed cell foam from new, my boat will have over 30ft of closed cell foam when finnished as i plan on putting foam in every place possible.
    the re-foaming is not a hard thing to do, many people put new foam in their boats when re-building.
    Unlike the whaler my boat does not totaly relay on the foam for its strength.
    the floor that unscrews is around 1/2 thick solid fibreglass and extra beams built in for strength.
    so i will not be weakening the structure in any way even if i left the foam out of the boat.

  11. phillnjack
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    phillnjack Junior Member

    found out today that the transom is not 1/2 thick fibreglass at all. its about 1/8th thick of glass and there was another pice of wood that was laminated against that.
    so in the centre 20 inch the ply is altogether 1.5 inch thick, the inner and outer fibreglass makes a total of just fractionaly over 2 inches in total.
  12. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Looking at the first photo, I'd like to know what the story is on that 15 foot tall guy standing behind the clothesline and why does the garage roof look tufted?
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    For the engine you're putting on, the boat should have at least 1.5" of total transom thickness. It would be better if it was thicker.

    Plywood is stiffer and cheaper then laminate, so if you have to bulk up the transom thickness, you're better with plywood.

    Polyester doesn't stick well to wood. Your boat is an exception to this rule, having survived so well, but trust me, I replace transom cores all the time and poly just doesn't stick or seal wood, so the wood gets wet, the poly releases and the wood rots.

    I think the shape of your deck cap has saved your transom, plus all the foam, leaving few places for moisture to collect.

    Lots of folks put big engines on boats (transoms) not rated for them. Fortunately, we tend to work in healthy safety margins, which get tested by these owners.

    You seem to have things well in hand. Use a big and thick a transom as you can. It'll help spread loads over a wider, larger area, which is good.
  14. phillnjack
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    phillnjack Junior Member

    Ahh 1.5inch total thickness !!!
    I have way over that already if including the hull and inner hull thickness at the transom.
    the wood alone is 1.5 inch plus then the fibreglass on each part of the hull making up a total of just over 2 inches.

    As for polyester resin not sticking to wood !!!!!!!!!!!
    so all the boats out there made of polyester are all falling apart are they ?
    Every boat made with polyester is realy running around with loose stringers, bulkheads, transom, engine mounts and so on ???

    I am going to be putting an extra layers of polyester on the bare transom hull on the inside, then i will be putting the wood, then that will have a complete layer of fibreglass matt all over it encasing it completely, no gaps its going to be completely encased.
    Now the only way this can get water into the wood will be through the transom bolts.
    Where the bolts go through the transom will be sealed inside the transom wood with epoxy resin.
    these holes will be 1/2 inch through the outer skins of fibreglass, but 3/4 inch through the wood.
    They will be filled with epoxy resin and then re-drilled out to 1/2 inch and also have sealer when the bolts go through to hold the engine.

    Now if this dont stop 99.9% of water getting through to the wood then nothing at all will stop it.
    Wood that is completely encased will last a long time, this boat is not left in the water its a trailer boat.
    This is about the best i can do to make the transom stronger and safer than original.
    The original wood was exposed in many places to allow any water in the hull to wet the wood and thus rot it out.
    I will be using proper industrial plywood of exterior grade and waterproof.
    The engine will be bolted on and using my large stainless steel plates to spread any engine loads across the transom rather than just small washers etc.
    i look at it as if the load is spread then the bolts cannot burst through should any accident happen like hit a rock or bounce down off a wave etc.

    phot below shows the type of plates i use for transom savers, they are 24 inch long 3 inch tall and 1/4 thick 316L.
    Ok so maybe a bit over the top but they not only look good when shaped but realy do spread the load.
    They now just need to have a bit of shape put to them for making them a bit more pleasing on the eye.
    plus they have to be drilled for the ski eye's to go through as well.

    there is not much more i can realy do to make the boats arse end any stronger.
    So maybe you can see why i am doing this re-build and strengthening it up a bit.
    plus so many people say this type of boat cannot be taken apart i just had to try and do a restoration on it.


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

    Phill, yes, polyester will let in moisture and no polyester will not stop 99.9% (not even epoxy does this) of moisture vapor ingress. In fact, it's much more like in the high 80% range, depending on formulation (epoxy is in the high 90% range). This isn't an opinion, but well documented fact. And yes, most production boats start to delaminate their wooden elements within 5 years of purchase. With good care, careful design and sufficient resin application techniques, these wooden structures can last a long time, but this just isn't the case with production built boats. They skimp on resin, use poor application techniques and the labor is usually quite poor, especially in out of sight locations (whee the newbie gets trained).

    Your boat seems to be an exception (lucky you) and you do have the option to use enough resin and fabrics, plus good application techniques, so your chances of success are high. Epoxying the mounting holes is a very good idea and one we call bonding the fastener holes. Epoxy will prevent moisture from getting at the core. No it doesn't stop all the moisture, but it's a high enough percentage, that the moisture content of the wood doesn't reach the "problem threshold", which is about 17%.

    If you can keep the wood's moisture content below this, you'll be okay. Polyester at best, will permit a 10% rise in moisture content, so if you start with stable and dry wood, say at 10% (pretty dry stuff) moisture content and it's coated with polyester, the poly will let another 10%+ into the wood and this means, the wood is now over the magic 17% number and you can expect rot and other issues. If you use the same piece of wood and coat it with epoxy, the moisture content will also rise, but only a few percent, so you'll still be below the magic number and the wood retains stiffness and doesn't rot. This is why epoxy is preferred on wood, plus it's a lot stronger and bonds a lot better too.
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