Replacing A Rotten Transom

[intheboatshed]

I started to prepare the transom of Token Gesture for a revarnish, but have realised the wood just below the top pintle is completely rotten. So much so I dont need a chisel to remove it. I wasn't really surprised, although the degree of rot has come as a bit of a shock - no idea how the transom has stayed on this long. I should think the vanish was holding it together!


I could replace the rotten wood (no idea how far this goes yet), or I was wondering about cutting out the whole of the transom and replacing it with an aluminium cross beam to provide rigidity and to attach the top rudder pintle to.

Never having done this to a boat before, can someone recommend what size of extrusion I would need to use, and should I get a channel or angle extrusion? Bearing in mind the most weight it will probably have to carry is me trying to get back into the boat after a capsize!

Thanks in advance.

Tim

[jimc (Guest)]

Joining metal to wood is rarely a very satisfactory exercise, and aluminium is particularly difficult to glue in place, so the smart money is that wood will provide a better result. If you want to go all hi tech high strength and exotic then carbon or glass over a cedar core is a very viable option. Provided you have a decent plane and workbench then a low hassle way of getting a suitable piece of cedar is to see what the local wood merchant has in bits of cedar - often they have special sections for some variety of garden shed, fence or furniture you can plane down. Its a very expensive way of buying cedar per cubic metre but it means you can pick the lightest and most knot free piece and not have to organise delivery.

[intheboatshed]

Thanks for the advice - what is the minimum size of cedar core I would need, to obtain the necessary strength.
Tim

[JimC (Guest)]

what is the minimum size of cedar core I would need, to obtain the necessary strength.

Its more a question of how much carbon you wrap round it...  But you need to think of the area as a whole.  Even on a double bottom boat you almost certainly don't want an open transom because too often water comes in roll tacking and in light airs. OK it goes out again, but it still has an effect. Then if your current transom is a solid piece of wood then its bracing the skin all round, and that still has to be dealt with... And a wooden boat needs different treatment to a foam/glass one. My estimates of required strength could be *very* inaccurate because I'm used to boats with bigger rudder loads, but were I doing it I guess I'd probably put two layers of unidirectional carbon and one of weave over a 30mm square piece of cedar, and put two layers of extra weave where the fittings will go because of crush loads from the bolts and because the holes actually weaken it anyway. Then have a 3mm ply transom with drain hole(s)/slots in the centre - I tend to a triangular drain in the centre so the deeper the water the more it lets out. I'd use triangular section strip wood to fillet the joins between transom and hull skin. You could maybe even use what's left of the original transom for this provided those areas aren't rotten and just cut it back 3mm so that the ply fits flush, depends whether its solid wood or ply. You have to see everything really to say. But the folks who know about Nationals can give you a better idea, better yet someone who knows your boat and how its put together.
 

[JimC (Guest)]

...better yet someone who knows your boat and how its put together.
 

And you know, re-reading your description, I think if the rest of the transom is really sound and its just local rot from the fastenings, I think I'd be most inclined just to cut out the existing transom well beyond the rot and graft some new timber in there, especially if my wood bashing skills were sufficient to make the contrasting pieces of wood something of a decorative feature...There's a lot to be said for not disturbing the main structural areas at the stern.

[intheboatshed]

Thanks for the advice Jim.

I will cut away until I get to solid wood, and see how much is left!


I suspect you are right about looking to replace like for like, rather than changing the design.


Tim