How thick is the hull?

[Ru88ell]

Zephyr has a NASA Marine Duet depth/speed fitted, but I'm not convinced the depth works properly. In Porthmadog harbour, in 1.8m of water it was reading 3m - and that was with the offset at 1.3m

The sensor has been mounted on the centreline of the hull just in front of the bulkhead in the forepeak locker. I'm surmising that this is right where the hull is at it's thickest - but how thick?