transom question

recently purchased a 14 foot semi v. the serial number indicates it was made in new york and registered with the state / coast guard as a home made boat. find it very odd, they did not add a drain plug, but thanks to this site i will be able to add one myself.

i do have a concern when traveling with the motor attached. when driving down the road, i can see the motor (evinrude 6 hp) bouncing around some in the back. not too much, but i never saw this with other boats i've had. its really more moving back and forth flexing the aluminum. is this something i should be concerned about? i'm worried it will weaken the aluminum and cause a tear or rivet to go, etc.

the stern is all aluminum (seems thin) with one center bracket and the builder used 1 x 4 boards for the transom. i'm thinking about removing the boards and covering the whole stern with one piece of plywood. just attach it right to the aluminum. there is a lip around the stern where it is riveted on the outside. if i fit a piece of plywood into this lip, will it change the dynamics of the boat? should i be concerned about the additional weight on the stern? if it is ok to do this, what thickness should i use for plywood? and should i attach on the outside or inside? will it make a difference?

it was previously used as a row boat and i added the seats, shelf for tm, etc. the tag says the boat is rated for 650 lbs and a 9.9 hp motor. i'm just not sure the current 1 x 4 board configuration is the best way to go. it just seems almost too flexible. the motor also sits at a slight angle because of the board thickness. doesn't seem to be a big deal, but something i noticed.

any thoughts are greatly appreciated. thanks.

I'd add plywood. The shape of the transom lends itself more to putting the plywood on the outside. Place the wood so the grain is running side to side. Provided the lip is big enough (looks to be that it is), I would probably use 3/4" ply. No need to run all the way down. Just past the motor clamp should be adequate.

so do a piece of plywood from one side to the other and go down just below the motor mount. what would be the best way to attach it? thanks.

I would go a coupld inches past the bottom of the motor mount. Think about an outboard, the prop spins and pushes in at the bottom of the motor bracket and pulls out at the top. If you stop it too short it could start to weaken the aluminum.

I can't tell from the pics, but it looks like the motor is just clamped at the top and no bolts through at the bottom? If so that I'd say you can drill a couple 1/4 inch holes at the tol and just through bolt the transom in. I'd personally add a couple at the bottom and just make sure you seal them real good.

As far as strength, the further down you go, the stronger it will be, as huntinfool said. But, then you start submerging the plywood, and adding weight, so it is a crapshoot.

Good coat of spar varnish should keep the wood fine for a number of years. I'd just through bolt with 1/4 inch stainless bolts, sealing them with 3M 5200.

the motor is just clamped on. i will definitely make sure i go far enough down without going to the bottom. i can't imagine the plywood will weigh too much more than the boards that are already there. but i definitely don't want to over weight it.