Transom materials

Discussion in 'Materials' started by Nasher, Dec 7, 2014.

  1. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    I've been mucking about with and refurbishing boats, particularly RIBs, for years now, and have performed some major surgery replacing transoms etc in the past with great success.

    The 6.5M RIB I've had for a few years now definitely has some water ingress into the ply in the transom, which dates back to damage caused by the previous owner, and was the reason I picked it up so cheap before repairing it.

    It's still strong, but as it's a keeper sooner or later I was planning on cutting it open and replacing the ply. As mentioned I've done this successfully in the past so the method and work involved doesn't worry me - Too much:D

    However I've been thinking lately, is there an alternative to Ply?
    The RIB currently has a 200HP outboard which is due for replacement in the next few years, but means the transom needs to be strong. I might even convert it to an inboard diesel of @200HP so all these options need consideration.

    So, what are my options with regard to an alternative to ply?

    Thanks

    Nasher.
     
  2. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Steel or aluminum plate. They are making a lot of drift boats out of one piece plastic 1/2" thick. I cannot access my favorites now so can not post a link telling exactly what it is called but it is tough. The tabbing and the method should guide you as to the best materials for doing this. 200 hp needs a strong transom.
     
  3. Nasher
    Joined: Apr 2005
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    Nasher Junior Member

    Thanks rasorinc.

    I'm currently using a 15mm thick Aluminium plate bolted to the transom to raise the engine as it's an XL shaft and the transom was originally made for a Long shaft. The Engine was another bargain I picked up so had to adapt the boat to use it.

    I was actually thinking more or a material I could put inside the transom and encapsulate in Fibreglass, just like the original Ply is.

    Nasher.
     

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  4. rasorinc
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    rasorinc Senior Member

    Found the link http://www.boulderboatworks.com/drift_boats.php
    It is a polyethylen hull You can buy this stuff on line in many lengths and thickness.
    I believe it can be heat welded and all fasteners have to be pre drilled. it is heavy and strong
    and unaffected by water.
     
  5. SukiSolo
    Joined: Dec 2012
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    Location: Hampshire UK

    SukiSolo Senior Member

    Personally, I can't see why a quarter sawn piece of densish timber cannot be superior to ply. Say Doug Fir, Ash or similar. I may have to do a similar job on a Boston Whaler/Dell Quay Dory as I will be cutting down the transom by 5" to suit a short shaft motor. We'll see what's inside and take it from there, but if it is saturated or soft foam it will be replaced by solid timber. Also possible worth doing and end grain laminate if you can ie a really solid end grain balsa type to help take compression. If you have al plate each side to take the thrust it ought to be enough unless pushing the horsepower.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Timber was the common choice, before ply came into popularity after WWII. The usual problem was leaks and cross grain stiffness and strength. Double planked helped solve some of this and indeed they are stiffer longitudinally, but they still leak, work fasteners loose, etc.

    There's a number of products that can be employed, but each will have a set of good and bad to consider. Cost/strength/stiffness are the usual deciding factors. You can get materials that rival and exceed plywood in stiffness and strength, but cost several times more. You can find materials that are similar in price, but simply not as strong.

    In the end, plywood is used because it's cost/strength/stiffness ratio, plus workability make it the reasonable choice. If you want inert, try Coosa board, though a 4'x8' sheet (3/4" thick) will run $300 and the transom will need to be thicker than this for all but very small outboards (10 HP or less). You can use PVC foam core, but check those prices, plus laminate materials and goo. The same is true of other cores materials.

    I've had folks bring me boats with aluminum and steel transom plates. I don't touch them any more, mostly because what happens when they've been there for a while or why they where installed in the first place. Often, JimBob and his half brother/uncle JoeLarry decide instead of repairing their sister/step mother's Bayliner transom, they just glue and screw a plate of alloy on it's butt. The end result is a pile of mush, between two thin layers of laminate, being crushed by the through bolts in the alloy plate. Yeah, I've seen enough of these to not want to see another.
     
  7. Nasher
    Joined: Apr 2005
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    Nasher Junior Member

    Thanks PAR

    As mentioned it’s still strong at the moment.
    I withdraw the through-transom bolts that are holding the plate on once a year, inspect them for dampness and prod around inside the hole with the bent end of a scriber to see if the ply is still firm, them refit them with some sealant. I also have large Stainless washers on the inside to spread the load and there doesn’t appear to be any compression yet.

    I am however aware that the bottom 3in of the ply was wet when I first brought the boat which I repaired as best I could with the intension of replacing the transom completely at some stage.
    Like you I’ve seen some real bodge jobs where soggy sponge like ply has been covered by all sorts of things.

    My interest in other materials stems from the amount of Ply I’ve seen that’s rotted over the years, but as you suggest it’s probably the best around on balance.

    My normal procedure to replace the Ply in a RIB transom is as follows.
    Bearing in mind there is usually no support at the ends of the transom where it may just have flanges to glue the tubes to, and the method below makes it sound much easier than it actually is.
    Remove any metal cap and cut the fibreglass skin off the top of the transom.
    Remove any Knees or other form of bracing between transom and deck/stringers inside the hull.
    Cut the inside fibreglass skin leaving a few inches around the outside and lever it off the Ply where it’s still attached.
    Cut/Break/Smash etc the existing ply out, and clean up the inner face of the outside skin.
    Laminate up the required thickness of Ply in the best marine grade Ply I can find at the time.
    Cut the Ply laminate to the shape required to drop it in from the top where the top of the transom was cut off.
    Prime the new Ply and inside of existing hull laminate with a first coat of resin and leave until set.
    If the shape of the transom is awkward, like around the tubes, small upright sections can be shaped and pushed into the sides of the opening to fill and voids that can’t be reached when dropping the main piece in from the top. These are obviously primed and ‘glued-in’ with resin.
    Where possible I create large Tongue and groove joints between the main and side pieces for strength.
    The majority of the inner face of the ply doesn’t need to be coated as the majority of the inner skin is still missing at this stage.
    Smother the same surfaces with another coat of resin and push down into the void from above.
    I often screw temporary handles to the inner face of the transom to help with holding it .
    Remove any temporary handles, fabricate any knees required for strength, and build up 4 layers of matt and resin overlapping the flange left around the inside when the inner skin was cut out.
    Tidy up and recap the transom top.

    Nasher.
     
  8. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    I don't see anyone addressing the most important issue in maintaining a strong and dry plywood inserted transom. That is that all holes, for any purpose, must be fully protected from water ingress. By far, the best way to do that is to drill an oversize hole, fill it with epoxy and re-drill for the fastener. Especially important for motor mounting holes as the epoxy bushing will provide added compressive strength along with water proofing. A transom built and maintained that way should never cause trouble.

    Same thing for cockpit soles where people are prone to mount stuff to the sole with self tapping screws and forget about the inevitable leakage. I once removed over 500 lbs of water from the "sealed" double bottom a 16' center console that had accumulated there from that cause.
     
  9. Nasher
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    Nasher Junior Member

    I always add some sort of sealant to through-transom bolts, Sikaflex or similar.

    Nasher
     
  10. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Could not agree more Tom, and not just for ply/timber but a lot of foam cores too. Even better to put glass filler in the epoxy to help the compressive strength after ensuring neat epoxy saturates the grain.
     
  11. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    I fully agree that in an ideal world this is the way to go, but I'd always add some sort of filler like loose strands to give the plug some strength in compression.

    Oh yes, I've patched several RIB decks where no thought had been given to water ingress around screws put directly into a deck.
    I've found that a small amount of water works it's way into the Ply and rots it from the inside out underneath the flowcoat. It's a big problem before you even know it's happening.

    It's often easier to cut the whole deck out and start again.

    Nasher.
     

  12. tom28571
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    tom28571 Senior Member

    Yes, I always do that for any hole that needs compression strength. Also good for fairlead bearings to route sail controls on small boats. Not for sheet leads of course, where low friction is needed.
     
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