The tendancy is to try and use full-length strips. It seems logical that one strip that is long enough to span the full length of the boat would be easier than piecing together several shorter strips. For some parts of the boat this is true, but not always. You would also think that avoiding joints in a strip would creating a stronger boat. This is not something you need to worry about.
Short strips are actually easier to work with sometimes.
A 18' strip needs to be handled with some care. It is hard to move around a shop. A 9' strip is alot easier to handle. When you get to the bottom of the boat you need to taper both ends of a strip to fit into the remaining space. The shaping of the taper requires some precision fitting for tight joints at both ends.