What is Fetch?

The definition of the word 'fetch' is simple:

The distance that wind travels over open water.

The first time we come across this word it is natural to ponder why anyone would feel the need to name such a thing. And even if we concede that somebody decided to define the concept and then name it, we still might question what value it offers.

However, spend enough time in a boat at sea and the reason becomes apparent.

Wave size is determined by three main factors:

The strength of the wind.

The length of time it has been blowing.

The distance of open water it has been blowing over: the fetch.

If any of these three variables increases then the size of waves will also increase. The reason that understanding fetch is so valuable is that the first two are intuitive and feel logical, if not obvious. Fifty foot waves after a Force 11 violent storm would not surprise anyone. Equally, a gusty force 6 breeze that has not relented for days on end would lead to expectations of choppy seas for most.

But a change in something less obvious and often invisible - the fetch - can have as big an impact as either of these factors. You cannot get big waves in a small pond, however hard or long the wind blows. But you can get very big waves if a modest wind blows over water uninterrupted for hundreds of miles.

From 'How to Read Water':

"One of the biggest differences between beginner sailors and more experienced ones is that early on it is easy to read too much into the forecasted wind alone. If the Met Office forecast a Force 5, a new sailor might think, I’ve been out in a 6 before and it wasn’t all that bad, so this will be fine. A wiser sailor will think, where’s this 5 coming from?"

It is possible to sip cool drinks and sunbathe on a yacht in a Force 5 in the sheltered Solent. Or to grip the tiller with white knuckles in a Force 5 in the exposed North Atlantic.

Although fetch is of most concern to those bobbing about on the open ocean, it can be understood, observed and enjoyed by those who never set foot on a boat.

Look at the image below of shallow water on a beach.

The breeze is coming from the right of the picture.

If you look closely at the nearest water, you'll be able to see how there are no ripples at the right hand edge and how they grow steadily bigger as we look towards the left hand side.

Now imagine that you are an ant on a tiny twig of a vessel, bouncing around on these ripples. You are a conscientious ant and checked the forecast before embarking and it promised steady light winds. And yet suddenly the importance of fetch becomes apparent.

The wind has not strengthened, the time it has been blowing has not changed, but still you'd be very keen indeed to get to the right hand side of this water, where the fetch is minimal.

By using your understanding of fetch, you should be able to work out the direction the wind is coming from in the top image.

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