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  1. #1
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    Oct 2010
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    Default Glueing the transom.

    Hi all. While since posting, however my putt putt build is progressing slowly. Station frames complete and about to commence the keel and stem post. Question. When I come to constructing the transom from three pieces of 200mm by 038mm Spotted Gum, to be glued on edge,should I dowel or biscuit join them?
    Cheers Dan.

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  3. #2
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    Biscuits are worthless on a boat. Are you sure you want to build a transom this way? I don't recollect your project, but thick, edge glued planks tend to leak without caulking seams or wedges. I wouldn't recommend wedge (spines) seams on a trailer borne boat. The wet/dry cycling will split the wedges. This leaves a carvel build, which is fine, though not the most user friendly type of transom.

    Maintenance on a carvel transom will be bothersome to most, especially in a powerboat (vibration). I'm assuming this is an inboard as a single skin edge glued/caulked transom can't be recommended for an outboard.

    The traditional planked transom is at least two layers of relatively thin planking. If epoxied, then no more then 19 mm if hardwood and 25 mm if soft. A typical double planked transom is two layers of 19 mm with staggered seams, so leaks are prevented. No edge treatments are necessary, just well fitted, tight seams, glued together, then glued or bedded and screwed to the second layer of planking.

    This type of transom works fairly well, but is no match to a composite plywood core, solid wood veneer transom. These look just like a tradisional single or double planked transom, except the inboard and exterior solid wood skins are glued and/or screwed to a plywood core. The plywood make the transom stable and the solid wood makes it look like it's supposed to. I've retro fitted dozens of this type of plywood, sometimes without the owners knowledge, with the results being nary a single leak. I consider this the only good way for an outboard transom as the plywood keeps the solid wood stable and transmits load better. If you did this on your transom, I'd recommend a 25 mm plywood core (two layers of 13mm would be better) with an interior and exterior 9 mm solid wood veneer. Of course all epoxied together. This would be much stronger and more stable then a single layer of thick, edge glued planking.

    It would be helpful to me at least if you could refresh us on your project and it's build method.

  4. #3
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    Also if you play with the veneer joints and the capping piece most will not realise that it is plywood.

    That is only if ply is a problem for you. There have been lots of these types of boat built with plywood transom - all depends on what the builder likes.

    MIK

  5. #4
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    I didn't go into all the details of the plywood core, veneer transom treatment, but that's one way to handle the edges Mik. The way I like is to not use veneers at all, but mill 19 mm boards to receive the plywood core.

    Try to picture this; I take a 19 mm plank and leaving about 75 mm on the outboard edges, where it lands against the planking unmolested, I plow out the middle so it's only 9 mm. On the top most board, there's usually a cutout for the transom. I leave this solid, so it looks like it's a regular plank, though if there's room I do knock out the middle for the plywood. The result is a set of hollowed out planks with solid edges where the planking fasteners will go into the transom and along the top and cutout areas. It looks just like a double planked transom, very traditional, but has a 25 mm plywood core to stabilize the assembly.

    I'll see if I have a drawing or pictures of a dissembled one to post. I started doing this several years ago when restoring a 50 year old lapstrake runabout. It had a double planked oak transom, but it leaked like a bottomless bucket. As I remade a new (plywood cored) transom with the plywood out to the edges of the transom's perimeter, I couldn't get good fastener grip in the end grain of the plywood, as I tried to screw the planking back on. There was a lot of "shape" to the transom and the fasteners where pulling out (well trying to). I didn't want to bond the fasteners because it would make removal and replacement very difficult in the future, so I remade the transom with the solid wood perimeter as I described above. The fasteners bit into solid oak and had no trouble staying put, plus they could be removed if repairs where needed. I've used this technique ever since. It's a little more work, but permits a bedded transom edge, rather then glued (future removal thingie) and solid faster landings.

  6. #5
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    Quote Originally Posted by PAR View Post
    I'd recommend a 25 mm plywood core (two layers of 13mm would be better) with an interior and exterior 9 mm solid wood veneer.
    Allrighty, why would two 13mm layers be better than one 25mm? (Aside from the obvious 1mm difference)

    I wood have (sorry, couldn't help myself) thought that a factory produced lamination superior to one produced by hand. Just learning here.

  7. #6
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    The factory produced transoms are done on budget and labor constraints, a back yard repair is less restricted. You get more grain long oriented veneers with two sheets of 13 mm then a single of 25, plus additional glue lines, one of which is epoxy. Further, if the grain is oriented at a slight angle to the other sheet (20 degrees), the cross grain strength (the weak link in plywood) is improved as well.

    Look at it this way, if you use two pieces of 13 mm, not great stuff, just 5 ply exterior grade, you have 6 longitudnal grain layers and 4 cross grain, plus the epoxy glue line. The same grade of exterior 25 mm will be 9 plys with 5 longitudnal layers and 4 cross grain, plus no additional epoxy glue line (which can have fabric added for even more strength).

    If you buy the good stuff, it'll be 11 plys thick at 25 mm, but the 13 mm will have 7 plys each.

  8. #7
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    That all makes sense, cheers!

  9. #8
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    Default Glueing the transom

    Thank you for your reply Par. The transom I wish to construct is not large. The boat I am building is a 16 foot open launch with a 5-7hp twin cylinder two stroke inboard Chapman motor as the power plant. I will have no trouble in constructing the transom as you have so wisely described by using Tassy Oak marine veneered plywood.
    Cheers Dan.

  10. #9
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    I should mention the best transom is a straight plywood laminate. A couple of 19 mm pieces glued and screwed together is about as easy as it gets. If you need to mechanically fasten the planks to the transom perimeter, then use some solid wood framing on the inside to receive the screws. No, it doesn't look quite as nice as a planked transom, but it doesn't leak either.

  11. #10
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    I used 22mm spotted gum tongue and groove flooring for the transom on the Whiting skiff Ibuilt 10 years ago and it's as good as new.On the stength of that I used t & g brushbox for my Yellowtail's transom,and with all the internal framing attached to it I expect no problems.Butt joining on the other hand is somethig I wouldn't be trying-it's a lot to ask even of epoxy.

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