Albert,
You can use whatever thickness of wood you wish! If you have the myHobbyStore plans you will see a set of body lines from which you can calculate the shapes of the 'planks' to suit the wood you have available.
There is an article on working with hull lines in the current Model Boats Plan Features Special which you should still be able to find in branches of W.H. Smith or order directly from MyHobbyStore: **LINK**
Colin