Wind Speed vs. Power
- September 30th, 2010
- in Yachting
Our recent encounter with Hurricane Earl was for many sailors the first encounter with a really violent wind storm. Chaos and carnage ensued as boats tore from their moorings or snapped their dock lines. The force was such that the surging swell forced boats to strain against their lines with such power that half-inch thick aluminum cleats snapped like twigs. The pieces of metal fl ew through the air as fast as bullets. They can kill you. A snapping dock line will kill you, too, as it springs back under immense tension. A dock is a very dangerous place to be when the air is moving.
For this reason, it is essential that any preparations be made well ahead of an impending storm. It might seem feasible to adjust dock lines or re-tie anchor rodes when the storm builds, but the reality is that the wind is too powerful. It will knock you down. It will put your boat where you don't want it to be.
Estimates of wind speed varied considerably and increased in the telling post-Earl. Reliable instrument readings in Road Harbour indicated consistent high-50 knots with gusts reaching close to 70 knots. Other areas reported higher gusts—often the result of wind funneling down gulleys and over hilltops. When it comes to wind speed and power, it must be remembered that the power of the wind increases as a cube of the wind speed. If the wind doubles, the power increases eight times. So if a 20-knot wind doubles to 40 knots, the power increases 8 times. If that 20-knot wind quadruples to 80 knots, the power increases 64 times.
This uneven increase in power related to wind speed sometimes distorts the perception of the wind. When the boat seems to be shaking itself to pieces at double the power, the assumption many people make is that the doubling of wind power means a doubling of wind speed, but it isn't so. The wind speed may have increased by only 25% or 50% to produce the sensation of massively increased power.
Now this isn't the sort of story people want to hear over a cold greenie, so we'll just keep it between ourselves. But wait until we start talking about wave heights! That's bound to be fun