Having rolled this issue over and over, I agree with LFH about headroom. If you know that the only places you can stand fully upright are in hatchways, you will will like a cat, not bang your head and be ready to move as the boat does to keep yourself from falling. If you are fully upright, you tend to lose the concentration that makes you aware of all the movements of the boat and the sea. Nettie K had just enough room on the center line for me to stand erect, the rest of the time it was "watch yer head, there!"