I have never used lumber longer than 10'. I do, as John suggests, scarf the strip at the sheer just to get a good look at the curve. I clamp the two strips to be scarfed to a straight line. Very hard to lay out a good shear with a crooked strip. From then on it is mismatched, butted joints. They all disappear as the boat is finished.
You also asked if you needed various lengths - no you don't. You won't be far into your project before you have a wide selection of lengths. You will spend some time sorting lengths to get the best use of available material for each particular strip. I try to keep my butt joints 30" apart so I can lay approx. 7 vertical strips before having to repeating the pattern.
One of many good suggestions I have gleaned from this site was to cut a 3 degree taped on the edges of all strips. This works excellent, makes a lot of curves without having to plane anything. When cutting the strips I use a felt marker to put a dot on the narrow side of the strips. Saves a lot of time trying to decide which is which. Also, the "dot" will always go on the inside just in case the ink soaks in to deep to sand off.