Why do ships have planks?

Best Answer:

fuel: A plank is a wide thick board of length needed. It is the best way to cover a area with fewer seams to seal.

Other answer:

Well some body has to sail it or helm the boat and with some who know sod all about boats Two Thick Short Planks Do The Job Nicely!
How else do you expect to keep the water out?
Fly Inverted:
Because it hurts less than being keelhauled.