I believe the hull plate thickness on the FPB65 is 12mm.
Can you tell me what you used for the FPB83 and Beowulf?
What was the deck plate thickness?
The FPB 64 has 12 mm plate from the bottom to roughly 150mm above the waterline. From there on she is 8mm. Deck is 5mm.
The FPB 83 has 6mm in the topsides and the bottom varies from 6mm to 12 in some places. Her framing and plating stiffness is twice what the Lloyds Special Service rule calls for. Deck plating is 4mm.
Beowulf varied from 4mm in the topsides aft to between 6mm and 10 on the bottom. Deck plating was 3mm.
You cannot look at plating thickness on its own. It has to be related to framing. All three boats have relatively close framing systems.