#3 Steel plate:
Steel plate figures you will see will be for example 10/6/4 and relate to 10mm bottom plate, 6mm hull sides and 4mm cabin.
Nowadays standard specification is 10/6/4 but steel thickness of 8/6/4mm and 6/5/3mm – if well looked after – will provide you with many years of service, so they should not be ruled out.
Wood and GRP (glass reinforce plastic) are also used on boats, mainly for the cabin. These are more commonly seen on older boats and are not as desirable as all steel, but provide a cheaper option.