Wave heights

This page outlines information relating to waves and swell and how to determine the performance capabilities and limitations of a vessel based on a vessels length.

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 web page aims to assist the small craft operator to assess the effect of waves at sea and is a summary of the Australian Maritime Safety Authority pamphlet Wind and Waves (PDF).

Information on the latest coastal wave heights at six locations along the NSW coast provided by the Department of Public Works and Services Manly Hydraulics Laboratory.

Please read the following and select the maximum wave height you are comfortable with for your boat.

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:

Length Description
Short 0-100 metres
Average 100-200 metres
Long over 200 metres
Height Description
Low 0-2 metres
Moderate 2-4 metres
Heavy over 4 metres

Wave size/boat size

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

Boat length Wave height Boating conditions
3.5 metres 1.0 metres heavy going, limit of safety for this sized vessel
3.5 metres 0.8 metres speed reduced, water comes aboard, care required
3.5 metres 0.5 metres speed reduced, some pounding at speed
3.5 metres 0.2 metres gentle patter of water on hull, no spray, comfortable
4.3 metres 1.5 metres for the experienced only, lots of heavy water thrown aboard
4.3 metres 1.2 metres throttle backed right off, lots of spray, getting scary
4.3 metres 1.0 metres speed reduced further, green seas taken on the decks
4.3 metres 0.8 metres speed reduced, some pounding, spray taken aboard
4.3 metres 0.5 metres maximum cruising throttle, slight bumps, spray thrown clear
4.3 metres 0.2 metres no spray, comfortable ride, gentle patter of water on hull
5 metres 1.8 metres limit of safe operation family comfort in this sized vessel
5 metres 1.5 metres steering through seas individually and using throttle at times
5 metres 1.2 metres dead slow, waves break on deck, lots of spray
5 metres 1.0 metres half speed, spray and green seas come aboard
5 metres 0.8 metres slight speed reduction desirable, moderate pounding, some spray
5 metres 0.5 metres set cruising throttle, slight bumps, spray thrown clear
5 metres 0.2 metres no spray, armchair ride
6 metres 2.0 metres dead slow, occasional big seas break aboard, for the experienced skipper only
6 metres 1.8 metres dead slow, steering individually for the seas
6 metres 1.5 metres windscreen sprayed, green seas on foredeck
6 metres 1.2 metres half speed, lots of spray, water on foredeck
6 metres 1.0 metres slight speed reduction, spray coming aboard aft end of boat
6 metres 0.8 metres reduced speed, pounding increases
6 metres 0.5 metres maximum cruising throttle, some light pounding
6 metres 0.2 metres great ride, like an automobile on bitumen
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