I had a 28' and looked at putting a door in the transom. I think it would work fine with reinforcement around the cutout. I never did it, sold the boat.

Diamond plate is lousy for a swim platform, at least in Florida. The metal stays hot, and the little triangles are rough on bathing suits. I have the stock aluminum platform on my 32', and I use a rubbery antiskid paint that stays fairly cool, and is easy on suits. That compressed cork stuff would be better.

I don't suppose there is a limit on width - it should be comfortable to stand on, and wide enough to hold a good folding ladder. My platform is a pain when maneuvering near pilings - I think that is your limit.

Someone has already solved the kicker/platform problem elsewhere on this site, and very elegantly.

You might want to consider aluminum or stainless frames with teak or starboard platform. Teak is expensive but wonderful, starboard is cheaper and low maintenance. There are a lot of swim platforms floating around in Florida: I see that www.fpmarine.com has a teak platform 69" x 24-1/2" for $375, and stainless brackets for $150, might work. I think I would make the brackets and then do my own platform out of 1" starboard, also available surplus.

Wild idea: A friend suggested that I extend the hull back three feet, with a platform on top, like the modern European designs. Better performance, more flotation, extra storage. I think he is my friend...

Aluminum would be the best choice for a swim platform, especially if you are going to mount a kicker, but it involves welding and painting. Don't know where you are located, but Menards, a big box home improvement store in the great lakes area, carries a deck board that is a close relative to teak. It's 1-1/4" thick and should do the job. It's span rated so you can determine how many stainless brackets you would need for a swim platform.

Notching the transom on a Marinette for a walk-through would be a lot easier and safer than with a wood or fiberglass boat. You can survey the transom from the hatch in the cockpit. You will see just 2 channels running vertically from the bottom to near the top of the transom. You would be better off locating the door on the centerline between those stiffeners. Somewhere on this site there is a great picture of a cross section of the deck-to-hull welded joint. From that picture you can easily see why these boats are so stiff and strong. It's not just stitch welded, it's continuous welded. That makes these hulls monolithic. This creates lots of redundancy. Removing 18" or so down to near deck height in the middle of the transom should not have any measurable effect on the stiffness or strength of the hull especially if you welded in a horizontal channel below the cut.

p.s. I've bought a few of things from flounder pounder over the last couple of years and have had very good experiences.

Jc000
I dont know what year your Express is , but i'm not a fan of major chopping of the marinettes. Swim platforms are great , aluminum are the coolest in my mind . This is my third marinette , and smallest . Try not to over do and junk them up . There is something in there Lines that makes them special. My thoughts ,but doesn't mean a thing .

T.N.X. Mark
trash66