noise insulation - help needed

[Birdseye]

i have a prout cat (with a single central engine and a catamaran drive leg) and will be insulating the bay with foam over the winter. however, i already know that a significant amount of noise is structure born, most (i suspect) from the drive leg which is bolted to the transom (ie the prop beat).

the drive leg is fastened to the transom using stainless steel bolts through the aluminium mounting plate. the joint is watertight, since the drive shaft runs through the transom and through the centre of this mounting plate

anybody know of an insulating material which i can sandwich between the flat aluminium mounting plate of the drive leg and the transom but which will physically withstand the transmission of 35hp of thrust from the prop, through the mounting plate of the leg and into the boat. in other words, i need physically strong sound insulating material which wont crush. it also needs to be sealable against water.

i thought of sheet lead, or hard rubber, but suspect the former would need to be thick and the latter would transmit vibration.

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[Avocet]

Is there a chance of standing the whole assembly back off the transom on four or more car engine mountings and just sealing the hole that's left with a flat sheet of PVC or something? I can't quite picture the setup in my mind's eye but is it (effectively) like an outboard on a transom (i.e. most of the load pushes against it and only the outdrive's weight acts downwards?

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[Birdseye]

you're not far off. the transom is within a foot of the water at that point. there's a hole in the transom where the drive passes through, from the gearbox to the outdrive leg. so if i put the mounting plate of the leg onto car type rubber mounts, there would be big gaps in between the plate and the transom and water would get into the boat.

imagine the drive leg as an outboard with the motor removed, and replaced by a drive shaft coming out at right angles ie horizontal.

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