Transom Core Material

[Aaron23]

I am restoring an 84 Cobalt 18DV with Merc sterndrive. I'm replacing part of the rotten transom, which was originally made with three layers of plywood. There is a layer of 3/8" (I think - I haven't measured it) plywood across the whole lower transom, with two additional 3/4" layers in the center - the inner one smaller than the outer.

As you can see in the pictures, I have cut out the three layer section and the two layer section back to the outer skin, leaving the solid and dry 3/8" plywood on either side. I plan to seal the remaining wood with penetrating epoxy and reconstruct the transom using epoxy and ....

This is where my questions start. From my research and reading this forum I think everyone agrees that the "right" way to do this would be to duplicate the original construction using well sealed plywood. My questions are:

1. I have seen Marine grade and MDO plywood recommended and cheap housing grade plywood discouraged, but what about the cabinet grade hardwood plywood like birch ($43 at Home Depot)? Is this an acceptable alternative? (I just happen to have a bunch lying around, but I won't use if it's not a good idea).

2. Is there another "right" way to do this using a material that is unaffected by water and rot? I have heard rigid, closed cell foams mentioned as core materials, is there a "correct" foam to use here - if so, where would I find it? What about Nida-Core, does it have the compression resistance necessary for mounting the gimbal housing? I would prefer not to use Seacast, as I would have to build a form of some sort, plus it makes me nervous.

I plan to keep this boat for a long, long, time, and I never want to do this particular repair again. Thanks in advance for any suggestions or info.

Aaron

[PAR]

Cabinet grade ply is of very good construction, but the questions are what adhesive is used for the layup and how rot resistant are the lumbers used in the layup. The adhesive must be waterproof and the lumber in the layers should have known rot resistance qualities.

Generally, you get what you pay for in lumber and sheet goods. I think most quality plywood are using water proof glues now. This leaves the interior layers to be questioned. Birch isn't very rot resistant and the interior layers of mystery "meat" may well be of much less quality, hence the price.

Quality plywood will have a nice thick outer layer with even thickness inner layers. Most cabinet grades have a paper thin outer layer and inconsistent quality inner layers, making them un suitable for exterior use in the marine environment. Typically, the more layers the better.

Sapele, an African species of mahogany with a wider grain, works and finishes well, also has good rot resistance.

Okoume, is another African species (not mahogany) with lighter weight and slightly less costly, but with lower rot resistance.

Philippine mahogany, this includes the lauans which have low rot resistance and the meranti species, which has excellent rot resistance.

Douglas fur and western larch, these are the least costly of the bunch, what MDO is made from and have reasonable rot resistance, though will check and require filling and sheathing to slow down this process. I wouldn't use MDO inside a transom. Most don't have enough layers of sufficiently dense wood to work well there and you'll have to sand off the rosin face(s) to get the wood to wood bond needed for the laminate.

So, basically check out the construction. You'll look for panels with no voids, no over lapping layers, even layer spacing and a good rot resistant species used throughout the panel.

[D'ARTOIS]

Before the epoxy-era, that began at the late '80s to become popular and accepted among leisure craft builders, it was common practice to use marine grade plywood as core material in FRP constructions. Not knowing (at that time) that very slowly, the glass strands in the composite transferred moisture and oxygene. Contrary to what one used to believe, FRP is not watertight!

So it can take 20 years ore more, before the internal decay becomes visible and that the polyester skin peels from the underlying plywood. Fiberglass bonds beautifully on wood using polyester resin, but it will not last eternally.

Now the repair: to take the whole transom out is a hell of a job, as the whole repaire will be - depending how you look at it. Do you have all facilities at hand, or are you just a diy - it depends mostly on your skill. In your case I would take out the transom, as you may rest assured that the decay of the plywood is pretty wide spread around the opening of the Z-Drive.

To remount the Z-Drive unit you need to have provided the transom with sufficient body to take up the forces that are unloaded on that part of your boat. Using a soft pvc core without sufficient strength to take up the loadforces from the engine etc. doesn't make things better so you might use
a similar set up to the originial although this time carried out with an epoxy resin instead of a polyester type resin. A couple of years ago I have helped a friend with carrying out a similar job on his Chris Craft.
Probably I will forget here and there a few things but you might have also response from other members who might have better proposals, more suitable to your needs.

[JR-Shine]

Our transom foam is from DIAB, its called Renicell (E240 2"). Its not quite as expensive as H130 in the same thickness. If you do use wood, make sure to do the repair with epoxy.

Joel

[Aaron23]

Thanks for all the tech info on the plywood and foam.

Based on the price of the foam, I think i'll be going with marine plywood. Yes, i'm using epoxy, and as long as I keep everything sealed this should last a good long time.

Thanks to everyone.