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cliveshep
Thailand
1324 Posts |
 Posted - 24 Sep 2013 : 18:18:10
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Mike,using screws and penny washers is a good idea, I'd want to use say 6mm ply only if replacing the old back lay-up panel, cover it in "sticky-back-plastic" - the sort our kids covered their school books in, strictly speaking it should be an acetate film but it works pretty well too.
Putt a nice layer of CSM over your new work, same over the panel, slap it into place and screw the ply over it to pull it well in.
The engineering of a transom is pretty basic but still adheres to old well-established principles. Basically the outward moment imposed by the motor at the top of the transom tries to twist it off against the bottom part, the structure resists this by the stiffness of the transom and the triangulation at its ends imparted by the motor well sides. For this to work the transom must be well bonded to the motor well sides. The motor well also reduces the effective clear span and therefore deflection element of the transom.
The outer skin assists only in that at the hull sides and bottom it is bonded to the stiffener in the transom that runs pretty much top to bottom and side to side so that bottom and sides of the transom are anchored by the outer skin turning to sides or bottom. The centre of the outer skin does very little in engineering terms and indeed is too flimsy to offer much apart from keeping the water out.
I don't know how much of the original 100hp motor allowance you intend to use, I suspect not much, but you might consider putting a couple of threaded rods with nuts and penny washers through the top of the new transom on the outside of the motor well sides, setting them into counter-bored holes, and glassing them to the motor well sides in the two side lockers before you glass the outer skin back on and cover the heads. The idea being to ensure a good triangulation with the motor well sides. I'd then glass in a couple of vertical gusset or "knee" stiffeners under the motor well down to the hull bottom to resist any tendency for the repaired transom to break away at the bottom to move inwards.
The cost of boating is insignificant compared to costs of a young wife and two teenage kids! |
Edited by - cliveshep on 24 Sep 2013 21:20:11 |
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mikeb
90 Posts |
Posted - 24 Sep 2013 : 20:48:15
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Clive, I won't be using all the 100 HP capacity - just 60 or so!  That's why I want to make sure it's as strong as possible. Frankly, looking at the original construction, I wouldn't want to put 100 horses of load through it, even if it wasn't rotten. It looks a bit under engineered for that kind of power. If my repair is as strong as the original, then I'll be happy (otherwise I'll be swimming back to shore).
What I've done is, using combination mat, looped it straight over the top of the transom and down the inside to the bottom of the outboard well, in a kind of 'U' shape. I also plan to put a few layers down one side of the existing outboard well, capping across the top of the transom and then lap it in up the otherside of the outboard well moulding, like this:
Due to the extra material inside the outboard well, the transom will end up a little thicker than original. Looking from the side, the cross section of the layers will be something like this:
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cliveshep
Thailand
1324 Posts |
Posted - 24 Sep 2013 : 20:53:19
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I just edited my post to add a sketch - you posted too fast for me!
Here it is again:
The cost of boating is insignificant compared to costs of a young wife and two teenage kids! |
Edited by - cliveshep on 24 Sep 2013 21:19:29 |
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mikeb
90 Posts |
Posted - 24 Sep 2013 : 23:31:20
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Thanks Clive, your diagram is slightly higher standard than my sketch. 
I'm pretty confident of the strength of the new transom area around the outboard well/middle of the stern, but the long bolts bonded through to the side of the outboard well moulding is a good shout. Is it sufficient to just glass them in, will that hold onto the threads well enough?
Anyway, my main concern is how the forces are transmitted away from that area to the sides of the stern, worst fear is that the middle of the stern rips itself off! As you can see from the photo's I've had to make the plywood layer in three sections to get it to fit, so the middle is wedged in behind the taper edges of the two sides. It is all slotted in behind what is left of the outer skin, so it should transfer the loads to the corners well enough.
Those gussets that you depict - is that function not already provided by the vertical and horizontal lengths of the bench seats either side? |
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cliveshep
Thailand
1324 Posts |
Posted - 25 Sep 2013 : 11:31:44
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If you've got bench seats then so long as the vertical sides that run parallel to the keel are glassed in both to the hull as well as your new transom then yes, they take the place of the knees or gussets. That would have been the original construction plan. They transmit loads along the length of the hull which is a good thing.
I'd anchor the rod with a few strategic nuts spaced up the length and glass it all in to get maximum strength especially as you have had to make the transom up in pieces as you say. If it is out of sight you can go for strength rather than beauty.
The cost of boating is insignificant compared to costs of a young wife and two teenage kids! |
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mikeb
90 Posts |
Posted - 15 Oct 2013 : 22:15:27
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Made a bit of progress today - it's starting to look a bit like a complete boat again, outer skin is reattached. Still quite a bit of work to do though, just hope the weather stays warm for a bit longer yet.
I told the missus that the strips of wood are permanent, but I will cut off the overhangs. I don't think she was impressed!
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ken cattell
United Kingdom
192 Posts |
Posted - 21 Oct 2013 : 00:31:39
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| Been following this project since the first proposed cuts to the transom were posted. To see it comming close to completion is a brilliant reflection of Mikes determination and the contributors to this site. A really interesting topic and a repair job that looks 1st class. Well done to all concerned I've really enjoyed following this. Thanks. |
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mikeb
90 Posts |
Posted - 12 Nov 2014 : 21:51:25
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Well, it's been a while, a long while in fact. She hasn't been touched for nearly 12 months, but I've now found a bit of time to pick it up again.
This is what it looked like in October 2013 with the old outer layer bonded back in place:
A channel was ground all around the edges, then glassed together. I then used top coat to cover it all up and give it a new outer layer. Applied with a sponge roller, it produced a nice artex effect:
That was November 2013. I've since sanded it flat and filled the low bits, which was an iterative process, but now I think it's mostly levelled off and ready to try and polish it to a reasonable finish.
So now I've turned my attention to other areas of the boat that need work, and the list is getting bigger, and bigger!
I went round the gunwale and removed the rubbing strake, but was left with loads of rusty screws stuck in there:
I could have just chopped them off flush with the angle grinder, but I thought that would lead to orange rust stains emerging beneath the new rubbing strake fairly quickly. So I ground them back into the surface, through the thickness of the gelcoat and then filled all the craters. That all needs sanding back to a reasonably flat surface for the new strake.
There is quite a bit of rotten wood around the cabin bulk head which I'm trying to replace the worst without tearing out the entire bulk head and starting again. Also need to make a new door for the toilet compartment as that had been converted into a closet without a door. And the wiring was a bit ropey, to put it politely, so the whole lot needs rewiring, including the engine loom (which was chopped by thieves). |
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