Waves and swell – assessing conditions for your vessel

Understand waves and swell to help you determine the performance capabilities and limitations of your vessel in different conditions.

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 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:

Short0-100 metres
Average100-200 metres
Longover 200 metres
Low0-2 metres
Moderate2-4 metres
Heavyover 4 metres

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.

Boat lengthWave heightBoating conditions
3.5 metres1.0 metresheavy going, limit of safety for this sized vessel
3.5 metres0.8 metresspeed reduced, water comes aboard, care required
3.5 metres0.5 metresspeed reduced, some pounding at speed
3.5 metres0.5 metresspeed reduced, some pounding at speed
3.5 metres0.2 metresgentle patter of water on hull, no spray, comfortable
4.3 metres1.5 metresfor the experienced only, lots of heavy water thrown aboard
4.3 metres1.2 metresthrottle backed right off, lots of spray, getting scary
4.3 metres1.0 metresspeed reduced further, green seas taken on the decks
4.3 metres0.8 metresspeed reduced, some pounding, spray taken aboard
4.3 metres0.5 metresmaximum cruising throttle, slight bumps, spray thrown clear
4.3 metres0.2 metresno spray, comfortable ride, gentle patter of water on hull
5 metres1.8 metreslimit of safe operation family comfort in this sized vessel
5 metres1.5 metressteering through seas individually and using throttle at times
5 metres1.2 metresdead slow, waves break on deck, lots of spray
5 metres1.0 metreshalf speed, spray and green seas come aboard
5 metres0.8 metresslight speed reduction desirable, moderate pounding, some spray
5 metres0.5 metresset cruising throttle, slight bumps, spray thrown clear
5 metres0.2 metresno spray, armchair ride
6 metres2.0 metresdead slow, occasional big seas break aboard, for the experienced skipper only
6 metres1.8 metresdead slow, steering individually for the seas
6 metres1.5 metreswindscreen sprayed, green seas on foredeck
6 metres1.2 metreshalf speed, lots of spray, water on foredeck
6 metres1.0 metresslight speed reduction, spray coming aboard aft end of boat
6 metres0.8 metresreduced speed, pounding increases
6 metres0.5 metresmaximum cruising throttle, some light pounding
6 metres0.2 metresgreat ride, like an automobile on bitumen
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