Waves and swell – assessing conditions for your vessel

Coastal wave heights

Modern well-designed small craft are highly seaworthy. However wind and waves have a major bearing on performance and, in extreme conditions, can present a considerable threat.

This information is designed to assist drivers of small vessels to assess the effect of waves at sea.

Manly Hydraulics Laboratory provides information on the latest coastal wave heights for:

• Sydney
• Batemans Bay
• Byron Bay
• Coffs Harbour
• Crowdy Head
• Eden
• Port Kembla.

Visit Manly Hydraulics Laboratory for the most up-to-date wave data for these locations.

Sea waves

Waves created by direct local action of wind on sea are termed 'sea waves' and are measured by length and height.

When wind reaches 2.5 km/h, gravity takes over from surface tension as the dominant force on wave form. The crests become more pointed, the troughs rounded. The wind reinforces the wave shape by pressing down on the windward side and eddying over the crest to reduce pressure on the leeward side.

As wind increases further to around 13 km/h, the crests of the waves steepen until they become unstable and break, producing whitecaps.

Swell

Swell is generally regular wave motion caused by large meteorological disturbances operating at a distance. Swell persists after the disturbance has disappeared and maintains a constant direction as long as it keeps in deep water.

Swell can travel for considerable distances. Since swell may have been instigated by a storm, the arrival of onshore swell can indicate that a storm is approaching.

Sea waves and swell

Sea waves caused by the local wind, are often superimposed on swell moving in from a distance. Interaction between the two can cause unpredictably high waves and dangers for mariners.

The following terminology is generally used to describe the length and height of swell:

Wave size and boat size

It is vital to know the performance capabilities and limitations of a boat before buying. Take great care to select a boat that will be able to handle the types of conditions you expect to come across.