Matchng the Previousl Transom Thickness

[woodman7975]

I am currently working on a transom and stringer replacement and I am getting the materials together to complete the transom once I finish sanding. The engine is a 4.3 Mercruiser with an alpha outdrive. The transom spec is 2-2-1/4" thick and the previous transom was made of 1" and 3/4" plywood with a thickness anywhere between 2" and 2-1/8". I am having issues finding good 1" plywood where I am at. The only thing that I can find is 1" subfloor sheeting with a million knots in it, so I will have to use 3/4" plywood.
I had previously planned on using poly resin peanut butter to attach to the existing exterior skin, 3 layers of 1708 bi-ax and 2 sheets of 3/4" plywood. However with the actual thickness of two sheets of plywood (1-7/16"), 3 layers of bi-ax (slightly over 1/8") and the exterior skin (slightly over 1/8") I am coming up about 5/16" short.
I would prefer not to sink the money and time into adding a bunch of extra layers of fiberglass, but I will if that is the best way to go. Would it hurt anything if I stuck an extra layer of 1/4" plywood into the transom, so I would have 2 layers of 3/4" and 1 layer of 1/4"? What is the best way to attack this?
Also, does anyone know a good way to sand the space between the swim step and the top of the transom. I only have about 1"-1-1/2" clearance in this location, but I will need something for the fiberglass on the top of transom to adhere to when I tab it in.

 

[GA_Boater]

You can make a horse collar out of say 1/4" exterior plywood and peanut butter it to the inner primary transom plywood. Cover it with cloth when you finish the inner transom to the 2 - 2 1/4" spec.

A picture or two could help on the swim step sanding question.

 

[woodman7975]

Thanks GA boater, that is probably what I will do. The transom is a 3-piece transom, so it will be easy to do this prior to install. It will not let me attach pictures while I am at work for some reason. I will try again when I get home. I did attach a link to a build by another boater on iBoats with the same boat.

One more question, would it do any good to build the transom in one solid piece rather than in three pieces like it was done in the factory? From the factory it came with one solid piece in the middle and the two "wings" on the side are separate pieces. I think that it would be easier to clamp the transom to the existing exterior skin it were built in three pieces, but it seems like the side pieces would not provide anything extra if it were built in three pieces.

 

[AShipShow]

Transom should be done with 2 layers of 3/4" ply, that will net you 1.5" of wood. Your outer hull fiberglass is typically around 1/8 to 1/4". Once the wood is installed with PB your probably gonna be close to 1 3/4 to 1 7/8"... Then two layers of 1708 on the inside and you should be right on the money of a hair over 2". Pretty much everyone on here uses 2 layers of 3/4" ply and ends up just right.

As far as the transom shape... Mine looked like yours and was 1 layer on the wings and 2 layers in the middle. I rebuilt mine with 2 full layers over the whole transom to add strength and to give me two layers of wood where the transom eyes pass through.

 

[steve_h7]

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[GA_Boater]

Purely my opinion - I would use two full width pieces.

The transom carries most of the weight of the motor and all the outdrive weight. Also all the driving force is against the transom. I feel full width does a better job at making a flex-free transom.

Maybe overkill, but I would feel more confident in the rebuild.

 

[steve_h7]

I make no claim to knowing which is the best route, but I had a 3 piece transom like you describe and chose to make a single piece out of multiple 1/4" plywood. Time will tell if I chose the right route but I thought I'd share how I did mine.

https://forums.iboats.com/forum/boa...tringers-deck-transom?p=10716987#post10716987

 

[Woodonglass]

Most 3/4 exterior grade plywood today is 23/32". And 1/2" is 15/32" Using these dimensions a Transom made for 2 pieces of 3/4 and One 1/2" sandwiched in the middle and the outer skin @ 1/4" would yield a transom of 2". this accounts for the PB use to adhere the wood to the outer skin. Two Layers of 1708 or 17 oz if using epoxy, will give you a near perfect transom dimension.of 2 1/8"