Exactly why and where- bulkhead

[Boatman]

I guess thats what it is called... bulkhead. I havent found but need the question of what the main purpose is if not just structure but where should I put on or them? This is for a cat hull. I know many put their flotation in the void space and I know to put a couple small holes for expanding and contracting air.

[Gerwin Brommer]

The main reason for me to use one ; lets say you crash the boat.
Nose cracks open : water will only fill the front of the boat.
The back stays dry and lets the boat float instead of sinking.

Or : your hatch comes off in a flip. If all goes well the air in the nosecompartiment keeps the boat at the surface.