Does Sealing Make Transom Thicker?

Hey Guys,
I'm in the process of replacing the transom on my 1984 Grumman 16CC aluminum boat. I've done a lot of reading on the different ways to seal the transom but I can't find any information on whether the various techniques affect the size of the wood. My transom is 2 inches thick and is made up of 2 three-quarter inch sheets with a sheet of half-inch sandwiched in-between. It has to drop down between two sheets of aluminum (the boat has a splash well), sits in a "U" channel and it's a snug fit all around.

If I put numerous coats of epoxy resin all over the wood is it going to increase the overall size of the transom? Should I allow for this when I construct the new transom - maybe use a five-eighths sheet of ply instead of 2 three-quarter and overall make the transom slightly smaller?

All info is going to be greatly appreciated!
Colin

 

The first coat of epoxy on raw plywood will not noticeably increase thickness (maybe about .1 mill), but subsequent coats will. You'll pick up about 3 - 4 mils of thickness on each side with three coats, which amounts to about a human hair's thickness on each side.

If you're tight or expecting to be tight, plane or sand a small amount off, between the two sheets, before you glue them up. I've done many transoms and never had to shave this small an amount, but each boat is different.

 

Thanks Par, that's exactly what I wanted to know
Colin

 

seapea, also mate, when you get to your final fitting, slip the plywood down with wet epoxy glue it will stick it to the alloy "reasonably" well, certainly enought to assist creating a one mass structure....but the big thing is, when you drill to make attachment bolt holes, then get a nail, bend it over a bit and run it thru the holes, inside the alloy so that you smash out a certain amount of plywood to increase the hole size in the ply section. Fill the holes in again with epoxy glue and then drill out again with the original bit to create a new hole in the glue, this will ensure that there are no more rotten plywood laminates in years to come.

 

Thanks Landlubber. I hadn't heard about that trick with the nail. OK, I need to make sure I understand what your saying - I want the wood to be attached to the aluminum? I was thinking the opposite and trying to make it easier for water to escape should it get between the wood and aluminum and was considering drilling weep holes in the bottom of the "U" shaped shelf the transom sits in.

 

You don't need to back cut the plywood, just bond the holes properly. Log onto systemthree.com and westsystem.com for more information about fastener bonding.

 

PAR,

It makes for a certain sealed hole thru the epoxy if the plywood is cut out , filled then redrilled. I have been fitting transom parts, engines, transducers, trim tabs etc, this way for over 40 years now, and never had a failure, so happy to recommend it as being a good system. If by chance the sealant of a bolt leaked, then the water would not be getting into the transom this way.

 

I understand your technique, which is nothing more then a modified fastener bond. For example if you have a 1/2" shank and you drill a 5/8" hole, then fill with thickened epoxy (after wetout) and let this cure, you don't need the back cut as the end grain is sealed. Where the seal is located isn't as important as the amount of goo around the fastener shank, when redrilled for the fastener. In short, if you're drilling through cured goo, then it doesn't mater if it's backcut or not, so long as it's a thick enough epoxy "bushing".

 

so long as it's a thick enough epoxy "bushing".


....yep agree entirely. To each his own, but the end result MUST be as stated...the "bushing"

 

It's the only word I could think of that fairly describes the end result. The process of bonding a fastener hole leaves the substrate with a fastener bushing. The bushing serves to seal the exposed end grain and surrounding wood, while also providing a hardened crush zone under the fastener head. Any moisture that manages to get past the bedding, will only contact the fastener shank or epoxy "bushing", so . . .

As far as "thick enough" the rule is generally accepted as at least 30% larger in diameter than the fastener shank, though if you go bigger, you can't go wrong.

 

Par,
I went to systemthree.com (http://www.systemthree.com/reslibrary/literature/EB_Section_07.pdf) as you suggested and read the following:
"Don’t put stainless steel bolts in any epoxy resin if
the application will be around water. Stainless steel works only
in the presence of sufficient oxygen. The epoxy will deprive it of
oxygen causing crevice corrosion in the presence of an electrolyte
like seawater. Stainless steel fastener failure occurs where the bolt
emerges from the epoxy resin."

My transom is held on with stainless bolts and the engine is also mounted with stainless. Is there something I'm missing here?
Colin

 

It's been my experience that if the fasteners are buried in epoxy bonded holes, there isn't a corrosion problem. Of course exposed fastener heads can see some corrosion, but they can be coated too.