Re: Transom replacement on my 12' Sears tinny
I'd mimic what you took out: same size & same thickness plywood. Use an exterior grade A/C ply, douglas fir 5 layer ply if you can get it, marine plywood (non-pressure treated), MDO, Auraco (may be hard to find 1 of 2 production facilities had a serious fire), in a pinch underlayment plywood, among other choices. Depending on the maximum width your 9.8 Merc's transom clamps will fit over, plus some room to tighten the clamps & allow for the thickness of the hull, will make selecting which thickness ply you need fairly easy. If the absolute widest asjustment on the clampsprovides a 1.5" space, the maximum thickness ply you'll want to use is 5/8", 1/2" would probably be sufficient for both as well. At 5/8" they will be slightly thinner then 1 3/8" when doubled, add some for the hull thickness, the coats of sealer you've applied, and to be able to actually tighten the transom clamps. If you have any 'good', void free, waterproof glue plywood already in both 5/8" & 1/2" thicknesses, I'd probably use 5/8" on the exterior & 1/2" on the interior. Thru bolt them and call it good at @1.25" total installed thickness. This will give you a little more room to slide the motor on & off, and allow for some expansion of the plywood. You might also make a slightly larger plywood piece on the interior, then what was there, that would then follow the size of the pc on the exterior, perhaps just down until it's @ 1/4" above the start of the green interior paint.
Cut the pcs, sand & prep as necessary for the product you've chosen, as last I remember you were looking for Spar Varnish..
Clamp the plywood onto the transom 1 pc at a time & mark out the 6 mounting thru bolt holes. Drill the thru holes and start sealing the wood, multiple coats each side, and pay particular attention to the plywood's edges & thru holes. The edges & thru bolts are the likely start of water intrusion into your wood.
Let how & where you store this boat, and whether you store it w/ the OB installed or removed, dictate whether you thru bolt the OB to the boat (recommended for loss while under way & theft). That thru bolt will need to have it's hole drilled & well sealed. You can also use the 3M 4200/5200 to seal the bolts into the thru holes on installation of the bolts. Just dip them into 4200 & push them thru their bolt location while rotating the bolt (helps spread the sealant out inside the bolt hole. Clean the threads & surrounding surfaces before you tighten the nut & again after....
If you are going to prime & paint the rest of the boat, do that 1st, apply the plywood to the transom after.
On a boat this size w/ this size of motor, I wouldn't adhere the plywood to the inside or outside faces of the aluminum transom. Check the plywood regularly for cracking of your sealer along the grain of the ply, and on the ply edges. Touchup & re-seal as often as necessary to prevent water intrusion. Regularly check the tightness of the thru bolts for both the plywood & the motor.
Depending on the 'holes' and water leaking, there are a few ways to deal w/ that. I think you've got a handle on that from the original boat, w/ the torn out rivets & a damaged hull, that you flipped to get this 1.
