Floppy swim platform

Given the age of your boat, I suspect the seals in the hydraulic rams need to be replaced. Hoses too.

Indeed the TNT manual doesn’t inspire any confidence…. We have a TNT on the Lazzara 110 I run after having a Nautical Structure on the 84 I ran before

the nautical structure had locks operated by smaller cylinders and much stiffer. The TNT doesn’t. One serious issue with the TNT is that should a hose fail, the platform will fall down. We had this happen a few years ago, luckily on the Bahamas bank and not in the Gulf Stream an hour earlier.

as a precaution when crossing we now run a heavy ratchet strap around the heavy bolts near the top of the arms to the cleats in the cockpit. Don’t think it would work for you though.

it seems to me that your problem is caused by the geometry of your installation. The brackets are really low on the transom causing the lift arms to be at an odd upward angle which will amplify movements. Ours are pretty much horizontal when the platform is up.

I wonder if you could thru bolt anchor points on both the platform and the fixed step and use SS turnbuckles as a locking mechanism when heading in rough water.

I don’t think either hoses or pistons are an issue here

in our case we had to add a second stage to the platform to raise it another foot as the prop wash between 9 and 15 knots was hitting the platform causing movement and vibration. Not your problem here as the 84 as a vertical transom unlike the 110 which keeps the prop wash from coming up closer to the transom

have you contacted TNT to see what they think of the installation ?

We had the same issue. The walker bay 15 didn’t fit on the 84’s sky deck.
The second stage isn’t cheap, $15k and it adds some extra movement even though it’s Pretty well built because of the additional leverage.

But looking at the geometry of your lift it doesn’t look like you could add a second stage.

I was looking at your pic#1.
If your tender was positioned forward, there would be less weight leveraging on the back edge of the platform.
Is there anyway to slide/position your tender forward when in ships operation? An extra set of rails up to or even on the fixed stern platform?
Then when docking or deploying, slide/re-position the tender further aft?

Nice sketching.
But we are not engineers.
In my opinion, look for another way to help your lift platform.
Even with these plates in place, that platform will still want to sag.

Screws and glue will not hold these in place.

Shear (sideways) tension (lifting and side to side) will destroy the original fiberglass.
Back bolting with heavy back matrix reinforcement will be required.
Either top or side mounted, Access from underneath / behind those surfaces must be opened up.
Lots of greatly expanded back plating matrix installed.
Then rebuilding the under box frame that was opened up in preforming these back-plating mods.
Then high strength bolts that would hold up to the sideways shear forces.

Further, that top plate with the pins in it, That plate will deform without a strong-back helping it.

I have witnessed list assemblies with a catch, next to the lift cylinder, that locks the platform up.
I would investigate this.
AND, some mechanism to reduce side to side movement.

I think something like that will work IF you can get inside the transom and install large back plates.

I would avoid the removable pins and instead have the transom plate overlap the platform plate and use two heavy bolts threading into the transom plate.