For all the books on weather forecasting I have yet to see one which helps you forecast sea state which is just, if not more, important. Can it be done? For years now I have been collecting books and articles on the subject, and while it may be no more reliable than weather forecasting, it is quite possible to cast your eye over a chart and make some predictions about the conditions you are likely to experience.

We are concerned with the waves formed by wind. Waves are also formed where two streams meet or where a stream meets an obstacle or a sudden change of depth, either on or below the surface. Without wind these waves are not usually much of a problem, though there is many a harbour bar that will produce breaking waves with no wind at all. Wind waves, as we all know, increase in size with the strength of wind, the fetch and the length of time the wind has been blowing. You can easily find graphs that will enable you to work it all out, but there is a snag — all the formulae are for deep water waves, depths, say, of over 150 feet.

Waves coming into shallow water deform: the wavelength shortens, the waves get steeper and may also be higher. Just how shallow the water has to be before the waves begin to change shape is not clear. It has been claimed, for instance, that the Labadie Bank, which rises out of the Continental Shelf, of 60 fathoms and is itself 21 to 50 fathoms was partly responsible for the confused sea during the Fastnet storm. The change is of course progressive, but in most open conditions around our coast the following figures from Tony Marchaj’s “Seaworthiness” are probably about right:

Wind Speed (knots) Significant Wave Height (feet) Period (secs) Average Wave Length (feet) Fetch (nautical miles) 10 1.4 2.9 27 10 14 3.3 4.0 59 28 18 6.1 5.1 90 55 22 10.0 6.3 134 100 26 15.0 7.4 188 180

These figures are for waves that are as high as they can go and tell us what we can expect on open water. But, of course, not all waves are the same height and the term ‘significant wave height’ is the average height of the highest 2/3rds waves or Hs. The highest wave in ten minutes will be 1.6 times Hs and 2 times Hs every three hours. More rarely waves will be 4 times higher than Hs. These are not freak waves: they will happen, and over time will work out to this frequency. This means that in a day’s sailing, you must expect to encounter some unusually high waves and these do not arrive singly, there is always a build up.

If the fetch is less than in the table above, the wave height will be less. The effect of fetch is shown in these figures:

Sea Area Proportion of waves exceeding 5 feet (%) winter summer Southern North Sea 30 4 Dover Strait 25 12 Morecambe Bay 21 8 Sevenstones 80 40

The North Sea is more landlocked and gets more offshore winds, consequently the waves are not as high. Here are some figures which I have extrapolated from a graph:

EFFECT OF FETCH ON MAXIMUM HEIGHT OF WAVE

Fetch (miles) Wind speed (knots) 10 15 20 30

10 1.5 2.5 3.5 5 25 2 3.5 6 10 50 2.25 4 7 13 100 2.3 4.7 9 16 200 2.5 5 9.5 18

Wave maximum height (feet)

Waves will build up rapidly to near their maximum height in the first five hours, taking longer, of course, at the higher wind speeds. Waves build up in height faster than in length and although the waves earlier in the blow are smaller, they are also steeper. A wind shift will also complicate the wave pattern as the new is imposed on the old. Here are some figures showing average steepness taken from a book published by the U S Navy:

AVERAGE STEEPNESS OF WAVE (wind duration in hours)

Wind Speed (mph) Wind Duration (hours) 5 10 15 20

10 12:1 23:1 31:1 42:1 15 14:1 20:1 23:1 26:1 20 14:1 16:1 18:1 23:1 30 13:1 15:1 16:1

Average Steepness of Wave

Time, more than wind strength, determines steepness. Once we have some idea of what the waves will be in open water, we can start trying to predict the effect of topography. I have not tried it, but since the wave period does not change, it might be worth timing the waves that hit the shore as a guide to what to expect in open water — see first table.

Several things can happen to waves. Waves refract when they come in at an angle to shallow ground and are bent towards the shallows. This means that, in the middle of a bay, waves spread outwards and dissipate some energy, while, at the headlands, they concentrate and consequently are higher and more confused. Waves reflect when they bounce back from an obstacle and combine with the waves coming in, so that some are larger and a bit like pyramids breaking at the tops while others are smaller. You have only to stand on any wall against which waves are beating to see this effect. Waves refract when they spread outwards having just pushed through a restriction. For instance, after passing the breakwater at the entrance to a harbour, waves will spread out as space opens up in front of them. When this happens, the wave height at the sides is liable to be bigger than that of the original wave. Then waves break up completely. The following is from Wind Waves at Sea — Breakers and Surf, U S Navy 1947:

“A wave first begins to show measurable deformation when it reaches a point of a shoaling bottom where the depth of the water (measured below undisturbed sea level) is about one half the wave length. The length of waves are progressively reduced in their future progress from this point onward, so that they are telescoped together as it were, as they near the shoal… the nearest approach to a definite rule that we dare offer for the relationship between heights of breakers and depth of water is as follows: if the bottom slope is gentle — less than 1:40 — waves may break where the depth is twice as great as their own heights if the wind is strong on shore or if a strong current is running about 1.3 times their own heights if the weather is moderate to calm with little or no current: but they may not do so until the depth is only about ¾ as great as their heights if there is a strong offshore wind.

If the slope of the bottom is steep or if it changes slope abruptly, it is safer to reckon on waves breaking where the depth is twice as great as their own heights, especially if the wind is blowing strong onshore or if there is a strong current against the waves, for while it is uncertain if the slope of the bottom per se has any effect on the depth at which rollers break, it is always wise to be on the safe side when dealing with waves. Waves may even break where the depth is as much as three to six times in extreme cases.”

Since waves are not all the same height, it seems reasonable to take a factor of three, i.e. when Hs is 3 ft some waves will begin to collapse in 9 ft of water. Before that happens from a depth of about half the wave length (say 30 to 40 ft of water) waves will begin to steepen appreciably. In windless conditions, or a weather shore, the break up will be delayed. Shallow water is not the only cause of waves breaking: once their steepness is 1:7 they will break up anyway and since they are steeper at the crests than the troughs, crests will break before the whole wave. Waves can also be broken by other waves interacting with them, the wash of a boat for instance and also by a current pushing against them — the familiar wind against tide conditions.

RATIO BETWEEN CURRENT VELOCITY AND WAVE VELOCITY IN STILL WATER

Contrary current as a proportion of wave speed -0.25 -0.20 -0.15 -0.10 -0.05

Height 2.35 1.75 1.39 1.21 1.08 Length .43 .52 .67 .79 .90 Steepness 5.49 3.40 2.07 1.53 1.21

This means that a wave will more than double in height if it is opposed by a current a quarter its own speed. Our benign 3.3 ft wave travels 59 feet in four seconds, which gives it a speed of 11 mph. If it meets a 3 knot current head on, its height will be more that doubled, its length more than halved and its steepness increased more than five times. It becomes a vicious monster curling up and breaking in all directions. Even if it only meets a one knot current, its height will be increased by nearly 25%. If the current is with the wind then the waves will be flatter than normal, but not a lot.

Following current as a proportion of wind speed +0.05 +0.10 +0.15 +0.20 +0.25

Height 9.93 0.87 0.82 0.79 0.76 Length 1.08 1.19 1.26 1.36 1.43 Steepness 0.86 0.73 0.65 0.58 0.53

In this case, a three knot following stream will reduce our 3 ft wave by 25%. I am not suggesting that any of these tables should be taken as absolute truth, but they do shown tendencies and you can make your own rules according to how much rough water you can stand. For me the amber light comes on when:

Waves are more than 3 ft high The current is more than 2 knots The water is less than 30 ft deep The wind is more than 15 knots The fetch is more than 25 miles

None of these are bothersome singly, but when they combine in the wrong way, they can signal an uncomfortable sail if not a risky one.

With all this behind us, what can we tell just by looking at the chart about the western entrance to the Solent? On the flood with a 14 knot south westerly condition should be reasonable in the main channel, except where waves bounce off the end of the Needles. The Shingles may have about 20 ft or less over it and the waves will be unpleasantly steep, as the current pushes into shallow water and breaking in many places. This will filter out the larger waves and, in the lee of the bank, the sea will be more even. We can expect a branch of the stream to follow the bay round and the appearance of the coast suggests a strong stream through the North Channel which, meeting the main stream at right angles just before Hurst Point, will give irregular waves breaking at the crests at least.

On the ebb, which can run up to 4 knots, the situation changes dramatically. The waves will now double in height, which means breaking waves all over the Shingles. The Needles channel will not be much better with steep frequently breaking waves. A northerly wind will change conditions again. Presuming any SW swell had died, a 14 mph wind would produce waves of something like 2 ft high, getting smaller as we approach the coast. But against the flood, that would still give 4 ft waves which would break on some parts of the Shingles. To take another example, Chichester Harbour entrance is surrounded by sandbanks and shallow water, often not much more than 12 ft deep. Being a shallow slope, waves can break all over, but the larger waves will, of course break first in the deeper water. In the westerly sector, the fetch is short and it would need about 20 knots of wind to put up a 3 ft wave, which would probably only break if the ebb were against it. The situation changes with the longer fetch of southerlies, especially on the ebb when 67 ft waves would be possible, where the ebb runs strongest and these would certainly break, but if approaching it, you will know how high the waves are in the open and can make an estimate of the conditions you are likely to meet at the entrance.

I do not suggest that any of this is scientific, despite the tables to two places of decimals, but it is one step up on having no idea at all what conditions you will have to face or, worse, thinking that, because it was all right last time, it will be OK the next.