From the Mariner''s Book of Days:
Less than 1 kt - smoke rises vertically
1-3 kts - smoke drifts
4-6 kts - wind felt on face
7-10 kts - light flag extends from pole
11-16 kts - wind raises dust, cinders, loose paper, etc.
17-21 kts - flag waves and snaps briskly
22-27 kts - wind whistles in rigging
28-33 kts - walking against wind is difficult
34-40 kts - wind impedes progress