Heavy Weather Operations

The ability to recognize wave patterns and characteristics is essential to safe operation in heavy weather and surf. A coxswain operating in these conditions must be able to determine the timing of lulls, series, and estimate wave heights accurately.

NOTE

The terms “Knockdown” and “Rollover” apply specifically to self-righting boats. A knockdown is when a boat has rolled in one direction 90º or greater but does not completely roll over (360º) to right itself. (Example: Boat rolls to port 120º and rights itself by rolling back to starboard.) A rollover occurs when a boat rolls in one direction and rights itself by completing a 360º revolution.

Wind velocity, fetch, and duration is the speed of the wind, the amount of ocean surface area affected by wind blowing in the same direction, and the amount of time the wind blows over the same part of the ocean. Ideally, to make a huge swell, one would want strong, steady winds blowing at maximum velocity over thousands of miles in the same direction for days on end. But, our atmosphere is highly dynamic, and rarely do such conditions exist or persist for long. During a typical open ocean winter storm, one could expect to see winds of 50-60 kts blowing for 600-1000 NM for 36 hours. In such a storm, the highest average wind waves (or seas) can commonly reach 30 ft towards the center of the fetch area. As the seas build under a storm, the speed of individual wind waves start accelerating as they combine. The higher the wind velocity, the larger the area and the longer the wind blows, the greater the opportunity wind waves have to combine and grow. Within the storm, waves of many different energy levels are created. Eventually either the storm dies or the wave speed exceeds the forward speed of the storm, and these seas escape into relatively calm waters. However, they are rough, ragged and cover a wide energy spectra. Now the waves have inertia and they're moving forward. Chop has little inertia or energy, so it dissipates when traveling long distances. This works against swell production when a wind wave has little energy, because it dissipates. However, when a wave has lots of time to accumulate energy, inertia works in its favor. In short, as a wind wave moves away from the storm, the choppy components dissipate, leaving only the pure swell energy to travel.

Heavy weather waves and seas are generated by weather systems, either local or distant. There are many factors that determine what conditions will be generated by a weather system or series of weather systems. Some factors that will effect wave height include: (01) State of the tide: Ebb currents often cause wave speed to decrease and wave height to increase. Conversely, flood currents often cause waves to gain speed and loose height, (02) Rainfall: Heavy rainfall can reduce the size of waves, but large runnoffs from rivers may stop the flood current or drastically change the conditions at inlets or bars, (03) The width of the body of water: The greater distance the body of water is allows for larger waves to be generated, (04) Depth of water: Deeper water allows for larger swells to be generated. As these swells approach shallow water on the coast, they will loose speed and gain height, (05) Air temperature: Cold air is denser, causing greater impact on the water and building larger swells than warm air.

Breaking Seas, also known as “Sea Breaks,” are wind driven waves that form crests, which tend to become unstable and topple forward, or “break,” creating a ridge of turbulence, white water, or foam (Figure 5-1).

CAUTION !

Depending on conditions, breaking seas can carry sufficient energy to affect the stability of a boat causing it to broach or be knocked down.

NOTE

Breaking seas are not to be confused with surf, which is a type of breaker, whose formation is largely dependent on the topography of the seabed over which it passes.

After the deepwater waves are generated far out at sea, they move outward, away from their wind source, in ever-increasing curves, and become what are called swells. The farther the swell moves from its source, the more uniform its characteristics become, as it travels in a series of waves, relatively equidistant, and moving at a more or less constant speed. Because of this, swells generated from storms far out at sea can be distinguished by their smoothness and uniformity from those that are coarser (peaked and irregular) which have recently originated nearby. The usual period of these swells is from 6 to 10 seconds. This corresponds with wave lengths of 184 to 1310 feet and velocities of 18 to 49 knots. Interference between different swell systems, which are traveling in nearly the same direction, causes groups of waves to travel outward in patches. As these groups of several waves (normally 7 to 12) progress outward, those waves in the forefront disappear and new waves, of the same characteristics, appear at the rear of the patch. This process continues until the waves dissipate their energy at sea, or transfer it to the shore as surf. The ability to recognize wave patterns and characteristics is essential to safe operation in surf and heavy weather. A coxswain operating in these conditions must be able to determine the timing of lulls and series, and estimate wave heights accurately. Some factors that affect wave patterns are: (01) Refraction, (02) Reflection, (03) Interference, (04) Shoaling water.

Refraction means bending. Wave refraction occurs when the wave moves into shoaling water, interacts with the bottom and slows down. As the waves encounter the shallows, they slow down, causing the crests of the waves to bend forward toward the shallower water (Figure 5-2). The key to the amount to refraction that occurs is the bottom terrain. This can also occur when a wave passes around a point of land, jetty, or an island (Figure 5-3).

Almost any obstacle can reflect part of a wave, including underwater barriers such as submerged reefs or bars, even though the main waves may seem to pass over them without change. These reflected waves move back towards the incoming waves (Figure 5-4). When the obstacles are vertical or nearly so, the waves may be reflected in their entirety.

Waves which have been refracted or reflected can interact with each other. Waves can also interact with incoming waves resulting in higher wave heights. Interference may even result in standing wave patterns (waves that consistently appear to peak in the same spot). Interference can be of particular concern because it may result in a boat being subjected to waves from unexpected directions and of unexpected size (Figure 5-5).

Knowledge of the characteristic grouping of waves is useful when operating in shoaling waters such as over bars, in inlets, or working in surf. The wave groups can be observed and their group periods determined. The boat or boats can be best maneuvered during that time when the wave motion is at a minimum, during the space between groups. When deepwater waves move into shallow waters, the waves are influenced by the bottom, becoming shallow-water waves. In the approach to shore, the interaction with the bottom causes the wave speed to decrease. This decrease causes refractions, and one effect is to shorten the wavelength. As the wavelength decreases, the wave steepness increases and the wave becomes less stable. Also, as the wave moves into water whose depth is about twice the wave’s height, the crest peaks up; that is, the rounded crest of a swell becomes a higher more pointed mass of water with steeper sides. This change of waveform becomes more pronounced as the wave moves farther into shallow water. These changes in wavelength and steepness occur before breaking. Finally, at a depth of water roughly equal to 1.3 times the wave height (the actual formula used to determine when the wave will break is when the height is equal to 80% of the depth ratio, H=.8d), the wave becomes unstable. This happens when not enough water is available in the shallow area ahead to complete the crest and the wave’s symmetrical form. The top of the onrushing crest is left unsupported and collapses. The wave breaks, resulting in surf.

The lull period in a wave system is the safest time to transit a bar, inlet, or shoal area in heavy weather/surf. By timing the duration of the lull, a coxswain can be prepared to make a transit while the waves are smaller. They will also have some idea of how much time is available before the next big set comes through. The basic technique is to use a stopwatch. After the last big wave of a series has passed, the time is started. When the first big waves of the next set arrive, the time is stopped. This is the duration of the lull, which may range from less than a minute to several minutes. This pattern should be observed for as long as possible until arriving at a useful consistent time. It may also be useful to time the duration of the series and number of waves in the set.

NOTE

The lull is the time between a series of swells.

An accurate estimate of wave height is subjective and sometimes difficult to accomplish, but there are a number of methods that, with practice, will give good results.

With the boat in the trough and on a level and even keel, any wave that obscures the horizon is greater than the height of a person’s eye. One can compare a wave to the deck edge or a structure such as the handrail. The wave face is observed while bowing into it, with the boat on an even keel in the trough.

This technique is most useful when observing from land, but may be applied while underway. If the freeboard of a buoy is known to be 13 feet, that information can be used to determine the height of the waves passing it. A buoy can also be used to determine the wave period. One can observe a vessel underway and by estimating the freeboard of the vessel and observing its motions on the water, he or she can gain a fair estimate of the seas in which it is operating.

Observation of waves as they pass a fixed structure, such as a break- wall, jetty, or pier, can be very accurate and can also provide wave period.

Using a digital depth sounder with a fast update speed can be very accurate for determining wave height. By comparing the depth in the trough on even keel with the depth at the crest on even keel, an accurate measurement can be obtained. All of these methods can be useful and reasonably accurate, but they require practice and experience. By comparing a local Weather Service buoy report with the crew’s observations, they can fine tune their sense of wave height. With enough practice, they should be able to judge wave heights simply by looking at the waves themselves.

A wave or swell of the sea breaking on the shore, shoal, reef, bar, or inlet. Breakers are a result of wave interaction with the bottom contour of the sea, shoal, reef, bar, or inlet. With each of these waves/swells the bottom of the wave slows on the ocean floor, shoal, reef, bar, or inlets while the top of the wave moves ahead of it causing it to break.

There are three basic types of breaking waves: (01) Plunging (Figure 5-6), (02) Spilling (Figure 5-7), (03) Surging (Figure 5-8). Each type of breaking waves brings its own hazards, such as suction currents, dropping huge quantities of water, and exerting a great deal of force. It is important to remember that when operating in heavy weather, these hazards are magnified beyond those found during calm water operations.

NOTE

A 20 foot breaker will drop 1,500 tons of water on a boat.

Plunging breakers are created when a wave encounters a sudden decrease in depth, such as a reef or a steep rise of the ocean floor. The momentum caused by the breaking top of the wave will cause the water to curl.

Spilling breakers are created when wave energy encounters a gentle sloping ocean floor. The spilling breakers normally have a crest of white water spreading down the wave face.

Surging breakers are created on very steep beaches. The wave builds very quickly and expends its energy on the beach.

NOTE

It is unlikely you will encounter surging breakers while aboard a boat unless you are beaching it on a very steep beach.

Wave series are irregular because of constant shifting of wind direction and speed. Storms at sea create masses of waves that build up in groups higher than other waves. Breakers vary in size and there is no regular pattern or sequence to their height. But while the space or interval between series of breakers may vary, it is fairly regular. Despite the interval, breakers tend to stay the same for hours at a time. The height and period of a wave depends on: (01) The speed of the wind, (02) The amount of time the wind has been blowing, (03) The distance over water which the wind travels unobstructed, known as fetch. Nearness to land will limit fetch, if the wind is blowing offshore. The lifecycle of a wave consists of its: (04) Generation by wind, (05) Gradual growth to maximum size, (06) Distance traveled across the sea, (07) Dissipation as wind decreases or when the wave impacts against the shore or an object.

NOTE

Tidal currents going against the waves will make the waves steeper.

Irregular waves of deepwater become organized by the effects of the contact with the bottom. They move in the same direction at similar speeds. As the depth of water decreases to very shallow, the waves break and the crests tumble forward. They fall into the trough ahead usually as a mass of foaming white water. This forward momentum carries the broken water forward until the wave’s last remaining energy becomes a wash rushing up the beach. The zone where the wave gives up this energy and the systematic water motions is the surf (see Figure 5-9).

Sometimes there are two breaks of surf between the beach and the outer surf line. These breaks result from an outer sand bar or reef working against the wave causing the seas to pile up. The movement of water over such outer bars forms the inner surf belt as the water rolls toward the shore. The surf that forms around an inlet depends on the size of approaching swells and the bottom contours. The waves’ speed and shape change as they approach shallow coastal waters. They become closer together (as their speed slows) and steeper as they contact the bottom. This change typically happens at a point where the water is approximately one half as deep as the wave’s length. As a wave steepens, its momentum will cause it to fall forward or curl. It is this momentum that gives a curl of breakers its tremendous force.

WARNING

Stay out of the wave’s curl. Boats not authorized to operate in breaking surf or bar conditions should remain well clear of these hazards.

Following are descriptions and definitions relating to breakers encountered during surf conditions.

Several waves or swells of the sea breaking on the shore, shoal, reef, bar, or inlet.

A wave on the point of breaking. A comber has a thin line of white water upon its crest, called feathering.

The top of a wave, breaker, or swell.

The outermost line of waves that break near shore, over a reef, or shoal. Generally refers to the outermost line of consistent surf.

The area where surf exists, between the outermost and innermost breaking waves.

In normal operations and especially in heavy weather, there are a number of conditions created in the surf zone and in individual waves of which the coxswain must be aware. These include: (01) Windows, (02) High/low side of a wave, (03) Wave saddles, (04) Closeouts, (05) Wave shoulder, (06) Rip currents.

A window is an area where the waves have momentarily stopped breaking, opening up a safer area of operation for your boat. Windows often form in the area of aerated water where a large set of waves has just finished breaking. The window may remain for a long time or may begin breaking again almost immediately. It is preferable to operate the boat in the windows whenever possible.

Window Shoulder

The “high side” is defined as the section of a wave which carries the most potential energy. The “low side” is where the least potential energy exists and represents the safest direction to turn when facing the wave/swell (Figure 5-11).These high and low sides often change rapidly, and the ability to quickly navigate the high and low sides is a critical skill for surf operations.

High Side Low Side

The “saddle” is the lowest part of a wave, bordered on both sides by higher ones. Often it is a small, unbroken section of a wave that is breaking. It is preferable to drive a boat in the saddles if possible, thus avoiding the white water. While saddles are very useful, they must be watched carefully, because they easily turn into “close-outs.”

“Closeouts” occur when a wave breaks from the ends toward the middle, or two waves break towards each other. The middle may look like a good saddle, but can quickly turn into whitewater. Closeouts should be avoided because they can create more energy than a single break.

The “shoulder” is the edge of a wave. It may be the very edge of the whitewater on a breaker, or the edge of a high peaking wave that is about to break. The shoulder is usually lower in height than the middle of the wave. Driving on the shoulders can be particularly useful in a narrow surf zone because it allows driving very close to a break in relative safety.

Rips are created along a long beach or reef surf zone. The water from waves hitting the beach travels out to the sides and parallel to the shoreline, creating a “long-shore current” that eventually returns to sea. This seaward flow creates deep channels in the sand offshore that can shift from day to day. In the case of a reef, the channels are permanent parts of the reef, but otherwise behave the same. In these channels, the waves or surf are usually smaller because of refraction over the deeper water. Because of this, a rip channel often represents a safer route into or out of a surf zone. A rip current may also carry a person-in-the-water or a disabled vessel clear of the surf zone (Figure 5-15). If using a rip current, great care should be taken to stay in the channel by watching the depth sounder. Boat crews should always be alert for debris, which tends to concentrate in these areas.