Fetch is a measurement of distance that indicates how far wind has traveled over open water.
The distance wind travels over water before meeting an obstacle, like a shore line or reef, is the fetch of the wind.
Why is Wind Fetch Important?
Wind fetch is important to understand in a maritime environment because of the effect wind has on the formation of waves.
Wind and waves are closely related. When wind blows over water friction pulls the surface water along in the same direction. The water gains energy from the wind and waves form because the water is being compressed by the wind.
Once a wave accumulates enough energy and grows to a certain size it will bump into the wave in front of it which will cause it to gain height. By gaining height a wave exposes its surface to the wind and gains more energy.
This cycle continues to produce larger waves as long as the wind blows in the same direction and there are no obstacles to stop the waves.
A long fetch of the wind will produce larger waves and meteorologists are able to predict wave action using wind forecasts. Tides and currents can also add or subtract energy from waves but wind is the driving force behind waves.
Wind Fetch for Mariners
When navigating or piloting a mariner needs to be aware of immediate conditions and of the potential conditions that could develop into a dangerous situation.
A close eye should be kept on wind direction and areas that have a long potential fetch of the wind. In these areas a wind shift which causes the fetch to lengthen will very likely cause wave height and frequency to increase.
A long wind fetch combined with a long duration wind event can cause wind and wave challenges for mariners including rouge waves, difficult maneuvering, and shifting sand bars.
Day to day decisions involving wind fetch include navigation, and setting scope while anchoring.
If a wind is blowing from east to west across a body of water and there are no obstacles, the fetch of the wind is equal to the east- west distance of the body of water.