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HSH Yacht Articles

How high are the seas?


Jeremy R. Hood

The other day I went out with a friend to measure the clearance under the Kemah / Seabrook Highway 146 bridge. An unnecessary task you may say, as all I had to do was to look on the chart to see it has a clearance at mean high water of 73 feet. True, that is what the chart states but my experience of these bridges is that there is usually more clearance than the stated height during most sea conditions. And this would be helpful to my friend who was considering a new boat with a mast height above the water of between 72 and 73 feet. Our calculations show he would normally have a foot or so to spare and so it was worth pursuing the boat.

How many times have you looked up as you go under the bridge? Maybe never, but I assure you that if you do, even those of you with 30 foot sailboats, you will tense as the mast approaches the bridge as it always look as if it will hit. And when you are passing under a bridge that you have never been under before, or worse, power lines stretched across the channel, you will hold your breath as you reach the point of no return and as the boat glides beneath you will let out a long sigh of relief. The fact is, that looking up from deck level to mast top, it is almost impossible to make any realistic assessment of mast height.

The same difficulty applies to assessing wave heights once they are much higher than your own height and it explains why many sailors describe seas as high as the top of the mast even though it is impossible to tell from deck level how much higher the mast is or, for that matter the wave! Trying to decide on wave heights is a matter of part observation, part estimation. When the seas are only 6 high, most sailors will give a close approximation to the real conditions but once they are more than 10 guessimates are likely to vary widely and, like the fish that got away, increase in the telling of the ordeal.

In practice, wave heights in open water are related closely to wind speed and, like windspeed, wave heights are given for the average (mean) conditions and so in a typical situation the tallest seas could be considerably larger than the average. In his book Meteorology At Sea (Standford Maritime, London 1982) Ray Sanderson gives the Beaufort Scale with considerable information on wave height and the following table is an extract from the table given in his book (with heights adjusted from meters to feet).

Beaufor t Force Wind Speed Range (knots) Mean Wind Speed (knots) Average Wave Height (feet) Max. Wave Height in 10-minut e periods Max. Wave Height in any 6 hr period Max. Wave Height in any 48 hr period
1 1-3 2        
2 4-6 5        
3 7-10 9 1.6 2.6    
4 11-16 13 3.6 5.9 7.9 8.9
5 17-21 19 6.6 10.5 14.5 16.4
6 22-27 24 10.2 16.4 22.3 24.6
7 28-33 30 14.8 23.6 32.5 36
8 34-40 37 22 35 48 54
9 41-47 44 30.5 49 67 74
10 48-55 52 40.5 65 89 98

From this table it is easy to see that in good sailing weather (Beaufort Force 4;p winds 11-16 knots) that the average wave height will be between 3 and 4 feet. But occasionally a sea as high as 9 feet will be encountered. Similarly in a Force 7 blow seas will around 15 feet with occasional ones at 24 feet and very, very occasional ones as high as 36 feet.

If you know the mean wind speed then the table will probably be more accurate in estimated wave heights than your own guesstimate made from the cockpit of your boat. But remember you need to use the mean (average) windspeed and not the highest number seen on your digital apparent wind speed indicator. Like wave heights, if the mean wind speed is around 10 knots then you are likely to have occasional gusts as high as 15 knots. And if you are beating to weather the apparent wind indicator may show 20 knots on these occasions. It may be more impressive to tell the story of 20 knot winds and 25 foot seas (see table for highest seas at 20 knots) but the reality is that you were experiencing a mean wind speed of 10 knots and the highest seas were no more than 9 feet!


The discussion above relates to open water conditions well away from land. Closer to shore and in some other circumstances, the height of the seas can vary a lot from the predictions given in the table above.

Shallow water

Where the water is shallow as in bays, estuaries and close to shore, waves will not be able to reach the heights predicted for open water. Instead, the waves will become steeper, closer together and begin to break sooner which explains the typical situation seen at the beach where gentle swells offshore produce short breaking seas at the beach. Shallow water makes for more difficult boat handling because of this even though the seas are not as large.

Strong currents

Where a set of waves is travelling in an area where there are also strong currents, the height, period (closeness of the seas to each other) and the steepness of the waves will be affected. When the current is pushing the body of water in the same direction as the wind is blowing (and thus the direction from which the seas are approaching) then the period of the seas will be lengthened and the height of them diminished which makes for easier sailing and boat handling. Such is the case when making a downwind passage in the trade winds. But when the current opposes the wind, seas become steep, closer together and the tops of the seas break sooner. This is a situation to very definitely avoid in anything more than light wind conditions and it explains to some extent the difficult conditions experienced in the English Channel where the tidal currents change every 6 hours almost ensuring some worsening of conditions in all winds. Closer to home it explains the notoriety of the Gulf Stream off the East Coast of the U.S. With a strong current heading northward and wind from the north (as in typical winter northers) will create this condition of wind against current. And in these conditions even big ships have foundered.

Cross seas

When a steady set of waves travelling across the ocean meets another set of seas the two will mix with each other in such a way that sometimes the peak of one sea will coincide with the trough of another creating an area of flat water but other peaks will coincide with peaks and troughs with troughs creating seas that are as high as the two individual heights combined. This situation will typically occur in storm conditions when a weather front passes through the area with its consequent wind shift. The new wind direction will create a new set of seas which will then interact with the existing seas creating confused seas with much higher peaks and troughs.

The same thing occurs when the wake of a ship passing close by interacts with the existing seas creating a short lived period of confusing seas. Another situation which creates unusually high peaks and troughs is where the seas are reflected back from a harbor breakwater or jetty. Here the reflected waves are travelling in exactly the opposite direction to the initial seas and so no confused seas arise but the heights of the peaks and troughs can be awesome and such conditions have accounted for the loss of more than one vessel in what were fairly normal conditions away from land.

When you are out in your boat, whether it is under sail or power, being aware of the wind, the seas, the depth of the water and the current is all part of seamanship. Because unless you are aware of these you will not be able to anticipate the conditions you may encounter or avoid. Close to shore you may want to consider putting in to port before a weather front arrives while even offshore you may want to alter course to get out of a strong current or at least prepare for turbulence. Being prepared is part of good seamanship but it has to begin with an awareness of what is happening around you.

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