Flattening transom skin before installing core.

[captsuperfly]

I'm rehabbing a 23 Seacraft and am getting ready to install a new transom. It's an I/O to Bracketed Outboard conversion. Old core is removed, previous holes are filled with epoxy/1708 layers and flush with the inside of the skin.

With the old core removed, the skin itself has a little bow across the middle. It's easy to push back into shape. I'm about to lay 3-4 4 layers of 1708 across the entire inside skin before setting my Coosa and I don't want to reenforce the bow as I lay the new glass, I want to get it flat.

Two choices I'm considering:

1) I have a 1/2 inch ply rough transom template that I can place along the outside of the transom to brace it like a giant, wooden, transom shaped washer. I'd put 2X4's across it to keep it true and drill through the skin and use small bolts pull the skin against the plywood and 2X4s. That will get it flat. Then I'd glass over the screws with the layers of 1708 and cut out the screws and close the holes after installing the Coosa.

Downside is I hate extra holes and I worry about the 1708 laying flat against the screw heads -I might create a bubble around them but I guess I'd just grind that out too.

2) Instead of drilling holes, I screw the plywood to the 2X4s (which would extend past the sides of the boat. I can through bolt eyes on the 2X4s and use ratchet straps to pull the whole thing tight the transom.
Upside is no holes. Downside is that I can see getting the whole thing to pull tight being difficult.

Any better ideas, I'm all for hearing them.

​​​​​​​thanks!

[dirtwheelsfl]

Does it go in or out??

[captsuperfly]

It bows out a bit in the center above where the outdrive was. You can pop it back by putting a knee on it. The skin is surprisingly thin up top. One of the reasons I'm adding the extra 1708.

When I had to transport it recently I put the same plywood template across the inside and bolted it to some scrap pieces on the other side through the existing holes and it pulled nice and flat.

If I were coring it as is, I'd just let it pull flat when I installed the core and pulled that in - but since I'm laying these layers of glass first, I'm worried I'd be stiffening the skin and building in that bow and creating a problem when I install the core.

[strick]

How thin is it? industry standard back then was about 3/8" thick out side skin...you may be making more work for your self then is necessary then simply just bonding the core to the existing transom...that being said if you feel it necessary to lay down the extra glass then you could get a couple straight 2/4's placed on edge and counter sink some holes so you can get some screws into the old skin to keep it flat then do like you said and grind them flush with the inside skin...maybe put a little wax on then just to be sure they will back out easily...you may find that after removing the braces that you still have a bow and then you can do the whole thing over again when adding the core...

strick

[dirtwheelsfl]

A few layers of glass aint gonna lock the bow in, but itll probably be easier to get it out beforehand...

If you really wanna avoid screws and holes you can just put a few dabs of epoxy on the 2x's and glue it right to the boat, pop em off later.

If you do use screws go from the outside and cut/grind em flat on the inside like strick said.

Youll need some something back there for bedding the core regardless...

[captsuperfly]

Thanks! New solution. I'm going to back the trailer closer to my garden wall and brace a 2X4 against the plywood and cross 2X4s. It seems to make it flat and I can leave it there while I glass and it cures.

[captsuperfly]

So, the bracing didn't really work. problem is when I put pressure in one direction, the transom contorts in another.

I removed the braces and added one layer of 1708 last night while it cooled down a little. Even that one layer has really stiffened up the skin. I can still pop the shape back but def want to flatten it before adding more glass.

[Capt Terry]

About a year ago I completed my transom restoration. I removed ALL the wood from the transom without removing the fwd or aft sheets of fiberglass, excepting some chainsaw scars. I used 5/8” plywood with two 2” x 4” horizontal pieces for bracing the full width of transom. No screws into the fiberglass sheets, but well clamped and braced to the trailer, splashwell and whatever I could clamp to or push from, all before the Seacast pour. With the plywood firmly clamped the gap at the left and right sides of the plywood to the transom was maybe 3/16” of so making me wonder if the transom had ever been truly planar. In any case the aft sheet is flat for about 2’ either side of ETEC centerline. BTW I braced the fwd sheet with appropriate sized plywood as well

[captsuperfly]

Originally Posted by Capt Terry

About a year ago I completed my transom restoration. I removed ALL the wood from the transom without removing the fwd or aft sheets of fiberglass, excepting some chainsaw scars. I used 5/8” plywood with two 2” x 4” horizontal pieces for bracing the full width of transom. No screws into the fiberglass sheets, but well clamped and braced to the trailer, splashwell and whatever I could clamp to or push from, all before the Seacast pour. With the plywood firmly clamped the gap at the left and right sides of the plywood to the transom was maybe 3/16” of so making me wonder if the transom had ever been truly planar. In any case the aft sheet is flat for about 2’ either side of ETEC centerline. BTW I braced the fwd sheet with appropriate sized plywood as well

It's interesting. I was wondering if it was ever really flat from side to side. Most of my bow is top to bottom.

I picked up some six inch carriage bolts and a sheet of 3/4 ply. Current plan is to drill through the skin and through bolt against the ply. The head of the carriage bolts are low profile, so they will be on the inside of the skin and I will glass over them. (If I need extra bracing I can add 2X4 on edge across top to bottom outside the plywood). Before installing the core, I will cut out the carriage bolts with a plug cutter and probably use these holes in the skin to thru-bolt the core against the skin.

[strick]

Well I guess thats one way of doing it but I really would like to see how much of a bow you have already...most of us usually bed the core to the existing skin and then thru bolt to bring the skin flush with the core...It may not be perfect but it will be close and then any defects in the skin are fixed later with fairing and blocking. When thru bolting the core to the hull make sure you wax the heck out of the bolts or you will not be able to pound them out after it cures...

strick

strick