Materials & prep for wood outboard mounting plate

What wood do folks recommend when replacing the wood mounting plate on exterior side of the transom, and what kind of preparation is required? This is in an old Alumacraft 14-footer. The transom board is solid and dry, but the exterior plate is in tough shape. It’s not a picture of my specific boat, but the attached shows what I’m talking about in general.

For hardware do I just want stainless steel? I’m planning to thru-bolt and will use 5200 to seal around all bolt holes.

Thanks!

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   richlite_transomplate__0008-e1432132360960.jpg
   

What are the dimensions?

I may have some 3/4″ marine ply leftover from my project. It was in the to be burned pile last week though.

I was gonna make it a little wider to accommodate placement of the trolling motor further to the port side. Thinking approx 24 by 12 inches?

In the past, I’ve used 3/4 inch brown or green treated boards, squared the edges with a jointer, and then glued up a pice as big as required using polyurethane glue.

My feeling is that anything that’s “plywood” is prone to delamination at some point. Solid wood can’t delaminate.

Yes, I would use stainless for all fasteners. And if you through-bolt anything, make sure there is a locking nut on the other side.

Grouse

I’d have no issues using marine grade plywood. It is prone to delamination however, you’ll get 10 to 15 years out of it.

TheFamousGrouse wrote:

In the past, I’ve used 3/4 inch brown or green treated boards, squared the edges with a jointer, and then glued up a pice as big as required using polyurethane glue.

My feeling is that anything that’s “plywood” is prone to delamination at some point. Solid wood can’t delaminate.

Yes, I would use stainless for all fasteners. And if you through-bolt anything, make sure there is a locking nut on the other side.

Grouse

Thanks for the thoughts, Grouse — I would probably be cool with just gluing it up, but the old plate is through-bolted so the holes are already there.

By gluing up, I meant applying glue on the edges of boards and clamping them together to make a piece as wide (or tall, in this case) as is required.

The deck boards I’ve used in the past were 5 1/2 inchs, so to make a transom plate 12 inches high, I edge glued 3 x 4 inch pieces. I also get all fancy and use a round-over router bit to round the edges of the side that not against the transom.

On a nicer boat, I have also done oak, which is also tremendously durable but will need to be varnished or poly coated. Same process, edge glue boards together and clamp.

Some may worry about the glue, but actually with modern glues, the glue is stronger than the wood, so if the board ever does split, it won’t be along the glue line.

You can now buy brown treated boards for decks that have a very similar color to the old transom boards on Alumacrafts.

Grouse

Oh, I get it now. I’m such an amateur when it’s come to this stuff!

I’ll probably go a simpler route and opt for marine ply. If I end up getting a sheet I’ll cut a few of these and have the extras ready to go.

One point about marine plywood that’s worth discussing here.

The glues used to bind the layers of marine plywood are water resistant, but marine plywood is not meant to be exposed in exterior applications without protection or a protective finish like an outdoor polyurethane.

It’s not the same as treated plywood that does not require a finish to be weather resistant.

Personally, on a deal like this, I’d be tempted to just use treated. Much cheaper.

Grouse

beware of going too thick. My old small boat, there was only room for about 3/8″ thick before the outboard was real hard to get on with the clamps opened all the way. It is also old aluma craft.

I coated in epoxy and it seems to be holding up ok. If you want to be paranoid about water, over size the holes a lot and fill with epoxy/sawdust and redrill the holes proper size. I put a little bit of fiberglass cloth where the motor clamps to so it doesn’t eat into the wood. There are a lot of things you could do but really only interesting if you enjoy doing them.

If you use CCA treated wood it will make the aluminum corrode from the copper in the treatment. Put them together get them wet and the aluminum hull becomes the sacrificial metal.

Completely cover it with a thin layer of silicone sealer. It will outlast the boat.

chubby wrote:

If you use CCA treated wood it will make the aluminum corrode from the copper in the treatment. Put them together get them wet and the aluminum hull becomes the sacrificial metal.

I believe you mean ACQ, CCA hasn’t been used since the early 2000’s.
But you do make a good point. I’m not sure if ACQ affects aluminum though.

Leftovers got burnt up man. Have some 1/2″ exterior I could lop a chunk off if needed.

I would stay away from any pressure treated material. A chemical reaction between the treating and the aluminum is always possible. The aluminum usually looses.

I have use either “balic birch” or marine plywood. These materials have fewer voids than any other plywood panels.

The tin boat forum recommends a coating of spar varnish, linseed oil, and turpentine. Refereed to as “Old Timer Formula”. Check the tin boat forum on the internet for the mix ratio.

Spar is more flexible than epoxy and has some UV protection in it.

nhamm wrote:

Leftovers got burnt up man. Have some 1/2″ exterior I could lop a chunk off if needed.

Djshannon slipped his comments in ahead of mine but I agree 100%

I’d opt for the exterior grade of plywood and apply several coats of spar varnish to every surface and cut edge prior to mounting the wood. I’d drill the attaching holes before doing the spar varnish as well. Personally I’d cut an aluminum plate to cover the wood even though its been coated well with the varnish.

Some of the newer treated woods are wet when you get them. My concern in using them would be how or what reactions from that moisture/chemicals with the boat’s aluminum be.

As far as extending the plate for a trolling motor to mount on, no problem in that at all and maybe even a good idea.

You ask me most of the prep is overkill. Coat that ply several times and slap it on. Most of forums are geared towards saltwater usage. Guys just got to go out and fish sometimes….

You give me the dimensions I might even cut it out for ya in shop. Been sitting inside few weeks now so it’s plenty dry. Even got some spar varnish I never used you’re welcome to have. Might need end of week to get up there and do it though.

Truly appreciate all the feeeback.

@nhamm I’ll shoot you a PM later today. Gotta fish right now!