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:

fuel:
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!
Greywolf:
How else do you expect to keep the water out?
Fly Inverted:
Because it hurts less than being keelhauled.