What is the correct number of darts for an origami boat? How do you choose?
A 1 dart is limited in the hull shapes it can provide. If you try and force the shape beyond these limits you will get buckling or similar unpleasant results. In general a 1 dart boat yields a slab sided round looking hull because the dart is underwater. However it is difficult to achieve a modern hull shape. Due to the limitation of conical projection a single dart works best in a hull with narrow ends and V shaped sections.
A 2 dart hull is the minimum number of darts to achieve a modern hull shape. With 2 darts you are able to increase the fullness of the ends to create U shaped sections. The disadvantage of the 2 dart design is that the upper dart is above water, spoiling the illusion of a round hull. A 2 dart hull is the easiest to build when looking for a modern hull form.
A 3 dart hull provides the modern hull shape possible with the 2 dart hull. The advantage of the 3 dart hull is that it maximizes the illusion of the fully round hull.
By adding a rub strake over the upper dart, the upper dart remains hidden. At the same time the rub strake protects the hull from damage from other boats and docks.
While a similar approach can be used on the 2 dart design, in general with 2 darts the upper dart is too low to carry a rub strake and looks out of place.