Staying Safe Where Big Waves Meet Shallow Water

While a large swell rarely causes more than discomfort (and involuntary feeding of the fish) in deep water, in shallow water it can produce dangerous, breaking waves capable of capsizing even relatively big boats. So what does this mean for boaters?

Clatsop Spit South Jetty

Wind generates waves, but waves often outlive the wind that spawned them to become swells. Since the prevailing winds and most storm winds in the temperate latitudes come from the west (with the notable exception of the East Coast's infamous nor'easters), the West Coast of the U.S. is normally a lee shore, open to the full fetch of the Pacific, while the East Coast is, most of the time, a protected shore. It should be no surprise, then, that the list of the 19 biggest wave sites in the world, includes six locations along the West Coast. Hawaii can claim only four and Australia just two.

To get a feel for the difference in swell between the left coast and the right, take a look at the NOAA's National Data Buoy Center. Click on one buoy on each coast and read the significant wave height (the average of the top one-third of all waves over the interval). To be a bit more scientific about it, Seaworthy averaged all of the significant wave heights taken at one-hour intervals throughout 2011 from Station 41623 off Mendocino, California and Station 44009 off Cape May, NJ. Waves average 3.7 feet off Cape May, less than half the 8.4-foot average off Mendocino. The table summarizes data farther offshore, from Buoy 46006 located 600nm southwest of Portland and Buoy 44004 located 150nm east of Cape Hatteras. Combined wind and swell is never less than 3 feet off the West Coast and exceeds 12 feet almost a quarter of the time.

Comparison of Significant Wave Heights* on East and West Coasts

BuoyLocationPercent of time waves < 3 feetPercent of time waves > 12 feet
44004~150 miles east of Cape Hatteras7%10%
46006~600 miles SW of Portland0%23%

*Average of biggest one-third of all waves recorded during time interval

First, when navigating in a swell, do not enter water shallower than 2.5 to 3 times the total swell height. The US Sailing panel that investigated the loss of the Sidney 38, Low Speed Chase, and the death of five of her crew in the Farallones Race found that the vessel crossed a 28-foot shoal with a forecast for swell of 12 to 15 feet and wind waves of 3 to 7 feet. In that depth, combined sea and swell that size will produce a breaking wave capable of capsizing a 38-foot boat several times an hour. Be prudent and steer wide.

Second, when choosing an anchorage to weather extreme swells (in excess of 20 feet), look for protection from the swell direction and avoid anchoring in shallow water. As this article illustrates, very large swells will refract around points of land, changing speed, wavelength and direction. Anchoring in water depths 2.5 to three times the combined swell and wave height (and carrying enough rode to do so) is good insurance just in case the swell finds its way around a corner to you. Call the local harbormaster, if there is one, and ask where you'll be safest given the forecast. Most harbormasters know how swell from different directions and of different heights will affect the anchorage, and they are all too happy to assist. 

— Published: October 2012

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