Hi there Ade as has been mentioned – there are numerous materials which can be used to plank a model. It boils down to your own preference as to what you can work with that suits you. One of the favourite materials used by people in the 'olden days' is Obechi and – its still used today – it is slightly tougher than balsa wood – it sands and works easily though. There is also lime wood which is used for planking – a finer grain and slightly harder than Obechi and balsa wood. Tend to pre-drill it if you are going to nail it – and then, we come on to the hard timbers such as Mahogany and I have known ash and oak to be used in models. These can be a very difficult timber to work with in small scales. Personally, I tend to work with Obechi and/or lime for building hulls. I did a build for the MV Troyberg / Duberg where I double planked the hull and this was following the procedure of planking that the magazine who did the Titanic/Bismark/Hood builds where they used a very short plank which was obviously to accommodate what could be put through a letterbox, and they did it in two layers on the hull.
I followed this but only deviated where they jointed all their planks on one frame – but I staggered my planks similar to brickwork in a wall to increase the strength.
If you have a look at my albums – you will see pics of the build for the M