When does a fetch become long enough to be a problem?

[Kelpie]

Idle curiosity here. Is there some rule of thumb, or a means of working out, what size of waves can build up over a certain length of water? I presume other factors like height of the surrounding land, water depth, etc will play a part. But in general, is a 'long fetch' one of five miles? Ten? Fifty?

 

[Frankie-H]

Idle curiosity here. Is there some rule of thumb, or a means of working out, what size of waves can build up over a certain length of water? I presume other factors like height of the surrounding land, water depth, etc will play a part. But in general, is a 'long fetch' one of five miles? Ten? Fifty?

It all really depends on where it has come from.

 

[Skysail]

A fetch can be 500 miles, or thousands in the Pacific; see here, bottom of page, for a graph of wave height against fetch:

http://www.btinternet.com/~keith.bater/wave_height.htm

 

[Kelpie]

I was thinking more in the context of an anchorage, and how big a fetch is considered acceptable, although it probably depends on local conditions. Interesting link though, thanks.

 

[gasdave]

Is this not a bit like "how long is a piece of string"?

The fetch is relevant when considering anchorage conditions, but there are also other factors which may be even more relevant on any day - depth, shelving angle, current wind direction and strength, wind direction and strength over previous days. Any of these could "trump" a relatively short fetch when determining conditions.

 

[WilliamUK]

If you ever come across it, have a read of Surf Science by Butt, Russell and Grigg.
As the name suggests, it's geared towards the science of waves as it matters to surfers wanting to predict where a storm is likely to provide surfable breakers, but it does a fantastic job of pulling all the relevant factors (fetch, weather and bathymetry being the biggies) together in a very easy to understand package.

It does a great job of explaining how a swell is affected by things like points, bays and the likes too.

Though a lot of it isn't directly relevant to sailing, I can only imagine that knowing where things are going to be rough on a given swell and what the local conditions will do to it would help quite a lot when deciding where you *don't* want to be.

Unless you're into a bit of surfing or have money burning a hole in your pocket I probably wouldn't suggest paying full price for it, but if you can get a cheap copy somewhere it may well make for some useful reading.

 

[theoldsalt]

I was thinking more in the context of an anchorage, and how big a fetch is considered acceptable, although it probably depends on local conditions. Interesting link though, thanks.

If you are worried about fetch at anchor then you are probably on a lee shore and for an overnight stay should be avoided whenever possible for safety as well as comfort.

 

[rob2]

Our club moorings are on the eastern side of Southampton Water and with the predominant Westerlies are somewhat bouncier than those on the western bank, that's with a fetch of a mile or so. A northerly wind is cold but doesn't have as much effect even in wind over tide as a southerly coming all the way up the Water. When it does get lively, often the limiting factor is whether one can lauch and recover the dinghy on the exposed beach.

Rob.

 

[Searush]

Remember Lakey's pics of rough water on Windermere? Not generally much room for a long fetch on a lake. Half a mile can be too much for comfort with wind over a tide or river current.

Having a twin keeler I usually go as close to a weather shore as I can when anchoring. Anything over 50mtrs fetch might mean I can't fill my glass to the brim.

 

[WilliamUK]

Anything over 50mtrs fetch might mean I can't fill my glass to the brim.

Not so much...
With timing you can fill it to the brim... you just can't KEEP it that way for long.

 

[greenalien]

Too long a fetch is most likely to be a problem when you anchor in a nice sheltered bay, with the wind blowing offshore - and then overnight, the wind changes direction 180 degrees...

 

[Orla]

This was Dunstaffnage marina in May last year, approx 300m - 400m of open water...... 2m to 4m depth

 

[srm]

A fetch of about 100 metres in Lerwick marina produced waves big enough to swamp and sink a sea angling boat that was stern on to it. My 42 ft 11 ton displacement yacht on the lee side of the pontoon had the wind about 20 degrees off the bow and was being heeled until the toe rail was almost on the finger. Waves were being refracted around the end of the pontoon and breaking over our stern.
The town was a dangerous place with flying objects, and it was almost impossible to stand up on the marina, crawling being a far more stable posture. - Wind speeds 80 plus knots gusting around 100 knots.
Any answer to your question depends on the wind strength.

 

[Lakesailor]

On Windermere it depends on the wind direction. NW is bad for us as it can give nearly 4 miles of fetch. There are no currents and no swell to exaggerate the fetch but it can get sharp and choppy.

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[guernseyman]

Another interesting book on waves is The Wavewatcher's Companion. It's about all sorts of waves - electromagnetic &etc - but there's a lot on sea waves.