Monday, October 12, 2009

Issuma: Anchor breaks loose in Force 7 near gale

Issuma is underway, having left Brazil for an expected 1 week off-shore journey south. Richard describes HERE, in his typically accurate, matter of fact way, how the newly adapted anchor broke loose as the winds unexpectedly built, and how the sharp edges of the steel shank shredded the lines he used to secure the beast.

Force 7, a "near gale" on the Beaufort scale is average winds 28-33 kts. But gusts can be expected to exceed the average by 20%. So 28-33 gusting to 40 KNOTS is the picture. Add 1/7 to get maximum gusts of 45 mph.

The sea state in Force 7 conditions is
"Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind", and on land whole trees sway in the wind".
Wave height and period is a function of depth, wind, and current. Shallow coastal seas have swift currents, producing large, steep waves, close together - particularly if wind opposes tide.