What is a good swell for boating?

Wave height and wave length are two pieces of critical information to help determine whether it is safe to go boating in big swells. The power generated by a dangerous swell will have enough energy to put your boat at risk.

Some boaters use the 30 percent wave height to boat length ratio. If a wave’s height is 30 percent of the boat’s length, then it is pretty good idea to turn back.

However, this is only a rule of thumb. Understanding waves and swell can help you know the limitations of your vessel and keep you safe out on the water.

What is swell?

Swell is basically wave motion generated out on the water, and originates from distant weather systems that have far-reaching effects. Swell can travel for considerable distances and will maintain its energy in a constant direction.

Swell is affected by waves caused by the local wind. Waves are formed when the wind is powerful enough to break the surface tension of the water, forming crests and troughs. The wind presses down on the windward side and eddying over the crest to reduce pressure on the leeward side, steepening the crests of the waves until they become unstable and break, producing whitecaps.

When these two elements interact conditions can become unpredictable and dangerous for boaters out on the water.

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How swell affects your boat

The size of your boat and the height of the waves will impact your vessel's ability to navigate the swell. For example, if your boat is around 3 metres in length a wave height of 1 metre will be quite challenging, while a vessel 6 metres in length will be able to handle waves up to 2 metres in height before reaching the limit of safety.

While most modern boats are designed to handle wind and waves, sudden and extreme changes in conditions can present operational challenges for your vessel.

It is important that you factor in the performance capabilities and limitations of a boat before attempting to go out boating. While out on the water you will experience a wide range of wave heights depending on a range of factors like wind, swell and tide.

Waves generally arrange themselves into sets, and reading these patterns will help you decide whether your vessel can withstand the impact of the conditions.

One way to determine this is by measuring the ‘significant wave height’ to estimate the size of swell and wind waves.

Significant wave height is the average wave height, from trough to crest, of the highest one-third of the waves. It is estimated that about one in every 2000 to 3000 waves (three to four times a day) will be approximately twice the height of the total wave height.

This means you need to be prepared for the wave with the maximum height before heading out on the water.

Planning for the conditions

There are a number of other factors to consider when navigating swell. Ocean currents, the depth of the water, natural formations such as reefs and rocks, and even the temperature of the ocean can all affect the formation of swell.

Swell can travel for thousands of kilometres, influenced by wind speed and direction. Knowledge of the area will help determine the impact of big swells on your next trip. Checking the marine forecast will give you the tools to make an informed decision to head out on the water. The Deckee app provides information real-time updates on wind, swell and tides.

Whitecaps on the water are generally a good indication that conditions will be challenging, and depending on the type of vessel and your experience level, it would be a good idea to stay home if there is any hesitation about going out.

Knowing the swell conditions that may be a danger to yourself and your boat will allow you to prevent it.