Engine mount support on a '85 Peterborough Monza Runabout-Need Help!

Pulled the Mercruiser 2.5L engine for a rebuild and decided to check the wood under the engine mount and found mush. Not sure what wood it is, as it was leaching black water back into the bilge it may have been a railroad tie? Pic below, had to handle it very gently not to have it come apart. Wish I had snapped a couple pics before removing it but didn't anticipate a issue with something as simple as this, surprise, surprise.



There was also some plywood under it that had rot, one on the port side all the way to the transom so I removed it and part of the starboard side until I found good wood. I then cut replacement pieces and built up a replacement for the railroad tie of laminated ply.



I glued up the support and epoxied all three pieces and installed the two pieces of ply. After I fill the gaps and throw a little fiberglass on the seamed piece, I'll be ready to install the beam. It was hard to tell but the one removed didn't appear to be glued down and had a nice piece of fiberglass securing it front to back. Must have been fairly strong as a 900lb. Mercruiser front feet were lagged into it with no help underneath. The beam wasn't seemingly sealed at all, just raw wood.

Now my questions, first, what is that support called? My burning question is how di I install the new support? Do I glue or epoxy the bottom to the replaced ply? Do I use some lesser holding adhesive (in case it ever needs to come out) like PL375 or Premium to fill in any voids and fiberglass it down? I did widen my replacement so I could fiberglass it to the stringers as well. The old one was 2" shy from them and was only glassed in front to back. Do I only lay glass from the top to the floor and sides and let it float?

I've spent hours searching with very limited success and nothing definitive. I'm a newbie to boating but generally can figure things out but this has me stumped.

Thanks
Bill

Pic of finished piece

Ayuh,.... The motor bed has to be firmly secured to the hull, to firmly support the motor,....

Thanks, what I originally thought. BTW, got you another Avatar

I installed the new support, glassed inplace and 2 layers of 1708 wrapped around the sides and up the adjacent stringers. Is two layers enough?

The wood shouldn't be considered part of the hull structure. Rather, it is a form over which sufficient glass and resin is laid in order to make a strong platform. Clearly the original bad hull design and poor factory layup allowed water to get into the weak structure and the weight of the engine hastened the crush.
You probably want at least 1/4 to 3/8 thick layup over everything in that area. Be certain that the bed is well tabbed to the hull at least 8 inches out all the way around. You can actually use styrofoam or the blue insulation foam that Home Depot carries as core material. It's a lot easier to form than wood. Manufacturers use wood because it's cheap, you can find people who know how to cut and form it pretty easy, and it's forgiving enough that bad layup and tabbing won't be a huge disater...at least for many years...long enough that the warranty will be over. Modern boat construction uses very little in the way of core, using preformed "egg crate" inner structures or other similar preforms.
To my eye it looks like your layup is very resin rich. You really want the very least amount of resin that will thoroughly wet out the fabric. You are using a resin roller?