Replacing the wood on the new yacht. The 50 year old transom is 1-1/2" thick. There was a 1/2" plywood support on the outside of the transom for the motor to rest on. Question I have is there was also a 3/8" thick piece on the inside where the motor would clamp to. This, to me, looks like it was an after thought, maybe added to a weakened transom, but I'm not sure. Rather than being thru bolted with stainless hardware it was screwed in with run of the mill wood screws

 

Am I correct in my assumption or do I need to add this piece as well?

Our old Alumacraft had something like this.  I always assumed it was just some sacrificial part for the motor clamps to dig into.

If that's the case, what wood would be the best option? It will get painted of course. I'm not going to drop another 75.00 on marine plywood for what would be the equivalent of a 1×4×12"

Pressure-treated 1x4 - $4 for a 4' length

https://www.homedepot.com/p/WeatherShield-1-in-x-4-in-x-4-ft-Appearance-Grade-Cedar-Tone-Pressure-Treated-Board-314585/303606839

If you have enough 1/2" plywood leftover, just use that. You should have more than enough room to get the motor on. 

7 minutes ago, Jigfishn10 said:

If you have enough 1/2" plywood leftover, just use that. You should have more than enough room to get the motor on. 

It's 3/4". I used 2 pieces sandwiched together for the transom and 3/4" for the outside support.  I'll have to check the spacing on the clamps to see if 3" will work.

27 minutes ago, MN Fisher said:

Pressure-treated 1x4 - $4 for a 4' length

https://www.homedepot.com/p/WeatherShield-1-in-x-4-in-x-4-ft-Appearance-Grade-Cedar-Tone-Pressure-Treated-Board-314585/303606839

Although it wouldn't be in contact with the aluminum, i haven't heard anything positive about using pressure treated wood anywhere near an aluminum hull.

1 minute ago, slonezp said:

It's 3/4". I used 2 pieces sandwiched together for the transom and 3/4" for the outside support.  I'll have to check the spacing on the clamps to see if 3" will work.

Check your PM.

You can also get like 12" aluminum flat bar stock from a hardware store. You're basically looking to distribute the load of the motor clamp and protecting the transom like @J Francho mentions in his post.

I went with .125  aluminum. Last forever. 

I just 86ed that piece on mine, but I put a mud motor on and the clamps fit better without it