When can deep water waves become dangerous?

Despite being frightened by stories about exceptionally dangerous storms, such the 1979 Fastnet or 1998 Sydney to Hobart races, not all storms create dangerous breaking waves. But some storms do and its been on my list for a while to understand what conditions will result in waves that may be dangerous in the open ocean.

We all know that inshore or in coastal sailing, there can be a number of parameters that impact the production of breaking waves including shallows and shoals, bar crossings,  etc. But in the open ocean, and in deep water how do you know if the forecast storm you are facing will generate the kind of breaking wave conditions that could potentially mean you are  caught in a situation where your boat may be knocked down or rolled by a wave?

The University of Southampton’s Department of Ship Science and a similar study by the Society of Naval Architects and Marine Engineers (SNAME) along with the United States Yacht Racing Union (now US Sailing) have shown that three conditions usually have to exist for waves to be dangerous to a yacht:

1. Wave height must exceed 30% of the yacht length before it contains enough energy to overcome a boat’s righting moment.

2. The boat needs to be broadside or oblique to the wave. The more a boat moves from being perpendicular to a wave, the greater the chance of a knock down. If the boat is bow or stern onto the wave, the chances of being rolled are greatly reduced. Yes, a boat can be pitch-poled (tossed end-over-end), but the size of the wave needed to do this greatly exceeds the size of the smaller wave needed to knock a boat down when broadsided.

3. The boat needs to be struck by a breaking wave. Unless the wave is breaking, the boat should ride up and over the wave regardless of the wave height in comparison to the boat length and the boat’s orientation to the wave. That is not to say that it will be comfortable or possibly cause the boat to go out of control. But without the wave breaking on the boat, the boat itself should not be knocked down.

A bit of Wave theory
Local wind causes waves and as these waves get separated out of a localized wind, it is called swell. Swell from outside an area and local wind waves within an area often interact. When a crest meets another crest, the result is the combined height of both.

For example, the crest of a 1.5m swell and the crest of 1m wave would stack, resulting in a 2.5m wave (for a wave to double like this, it requires that both waves have very long and regular swells). However, if the crest meets a trough, then they cancel. In this example, the would result would be a 0.5m wave. All ranges in between will also be encountered.

Because of these interactions, waves will appear different from one another and may seem confused. Waves will appear across a large spectrum of sizes, although to a local observer, there will appear to be a general height. This is why wave forecasts, such as from the NOAA or BOM or UK Met office, give a range, such as 1 to 1.5m, covering a broad area called a wave field.

This range, though, is often misunderstood. First, the forecast wave height given is the significant wave height, which is the average of the highest one-third of the waves that may be encountered. It is not the average of all waves, nor will all waves be in this range. Some waves will be smaller and some waves will be higher. In fact, you can expect to see a wave almost twice as high (!) as the significant wave height. This type of wave is rare, however, only showing up once every 3,000 waves.

Predicting Breaking Waves
Predicting dangerous breaking waves is complex. The best information that most sailors will have to rely on is the weather report. Therefore, when you are sailing, the wave forecast portion of the weather report is just as important as the rest of the report. The wave height (from bottom of the trough to the top of the crest) and the wave length (the distance from one crest to the next crest or one trough to the next trough) are the two pieces of critical information. Once you know the wave height and length, you can determine whether the waves will have the potential to be breaking or not in deep water.

A wave will collapse, or break, in deep water depending on the wave’s steepness.  The base of the wave (trough to trough, i.e., the wave’s length) can only support a wave of a certain height. If it exceeds that height, the wave becomes too steep and it collapses and breaks forward.

This is called the wave’s breaking point. If the wave length is 7 times or less than the wave height, then the wave may break. This is also known as the “Rule of 7 or Less,”  This is true for any measurement type, feet or meters.

For example, if the wave height is 15 feet, multiplying that by 7 will give a minimum wave length of 105 feet. If the wave length is 105 feet or less, then the wave has the potential to be breaking.

Is there danger?
To know if you have entered a wave-height danger zone, you must first know what wave height is dangerous to your boat. Remember, a wave will have enough potential energy to knock your boat down starting at 30 percent of its length and certainly will be large enough if it is 60 percent of the boat’s length. For Vega that is a wave of 4m. So anything approaching 4m and potentially breaking is our danger Zone.

Using the weather forecast you need to know the wave heights and lengths you will be expecting to encounter.  Will the wave heights exceed your danger zone? Could another condition, such as more wind or a contrary current, suddenly make safe waves dangerous?

If yes, then you need to know if the waves will be breaking. If the wave length is 7 times or less than the wave’s height, then you should take precautions. To determine dangerous wave heights and lengths for your boat, you can use these formulas:

Danger Wave Height
Wave Height = Boat Length x 30%

Danger Wave Length
Wave Length < = Wave Height x 7

The Rule of 7 or less is a pretty simple relationship, but a useful one. I have graphed it below for ease of reference.


Knowing the wave conditions that may be a danger to rolling your boat will allow you to take action to avoid or prevent it, next time you look at the forecast check the wave height and wave length and see if you are in your danger zone.

Extracts of this post have been taken form Steve Tredup’s article on the Ocean Navigator Web site.



2 thoughts on “When can deep water waves become dangerous?

Add yours

  1. Hello Richard,

    Excellent explanation and very interesting. I’d not paid attention to, or even noticed, the wave length in forecasts. When crossing the Great Australian Bight the swell was huge, built up over thousands of sea miles, but the wave length was proportionately long, so we lifted, lifted until the horizon was visible the dropped, dropped into the valley with little disturbance. We did have a steadying 15k SW breeze and were running before the swell – oh, and a steel panel on the rudder skeg went “boing.g..g” as the pressure bore hard at the bottom of each descent. With great depth, limitless fetch and steady wind is the wave height/length ratio fixed, and always so benign? What would cause the length to shorten and become dangerous, in the absence of summating cross swell? A most absorbing subject.





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