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 AMSA pamphlet "Wind and Waves".

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/hr, 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/hr, 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.

Length
Short	0-100 metres
Average	100-200 metres
Long	over 200 metres
Height
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 condition you expect to come across.

A Cautionary Note

The tables below are indicative only and do not reflect the performance of any particular design or model of craft. It should also be remembered that a combination of factors such as currents, tidal streams and geographical features can create dangerously unpredictable conditions. An example of this is when a fast running tide or current is flowing over a rock shelf or sand bar against a contrary wind.

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