Extract from "SOUTHERN AFRICAN CRUISING NOTES" by Tony Herrick (available from 'Cruising Connections') http://www.cruiser.co.za/tony.asp

The sailing route from Durban to Cape Town is governed by three major factors:

(1) THE AGULHAS CURRENT: The current is one of the great ocean currents running mainly from northeast to southwest following the two hundred meter contour of the continental shelf, and dissipating over the Agulhas Bank south of Mossel Bay. Main axis of the current is on or near the two hundred meter line and can run up to 6 knots at it's fastest point. Further details in the relevent sections.

(2) THE VARIABLE WEATHER PATTERNS: These patterns are governed by low and high pressure systems moving across from the Atlantic Coast of South America and travelling eastwards up the eastern seaboard of the South African coast.

(3) SHELTERED HARBOURS AND ANCHORAGES: The third factor is the lack of sheltered harbours and anchorages , particularly between Durban and Port Elizabeth.

ABNORMAL WAVES: It is a known fact that giant waves occur on the South African coast in the Agulhas current region, where southwesterly gales prevail against the southward flowing Agulhas current. Professor Mallory of Cape Town University analysed the recorded conditions that prevailed each time a number of ships were damaged by exceptional waves, and found that in all cases the dominant waves were always from the southwest. The weather patterns play a major part in that the most dangerous period occurs when cells of low pressure are moving along the coast in a northeasterly direction. These lows are a regular feature of the eastern seaboard and it often happens that during their passage the wind can change from a near northeasterly gale to a southwesterly gale, sometimes in a matter of minutes. The southwest wind then reinforces the existing waves generated by a short choppy sea, which acts directly against the Agulhas current.

It is the interaction between the strong southwesterly wind and the strong south flowing current which at times can reach 6 knots that creates monstrous freak waves, of which the charts warn: "abnormal waves of up to 20 meters in height, preceeded by deep troughs may be encountered in the area between the edge of the continental shelf and twenty miles to seaward thereof."

The warning also describes the necessary evasive action to be taken under unfavourable conditions, namely, to stay clear of the areas seaward of the continental shelf. In other words, move inshore inside the 200 meter line. This well established rule has given rise to the belief that the bottom topography plays a part in the generation of giant waves, but in fact this only plays an indirect role.

Please remember that the conditions along the southeast coast of South Africa are unique; the region can only be made safer through an understanding of the forces involved and by treating the seas with the respect they deserve, regardless of loss of time.

"Do not have a deadline to meet at the other end". (Reference is made to a research paper - "Giant Wave - Anomolous Seas of the Agulhas Current" - by Ecxart H Schuman.)

WEATHER RELATED SAILING CONSIDERATIONS:

(1) The major wind belts around Southern Africa are influenced primarily by two high pressure systems, namely the South Atlantic High (SAH) and the Indian Ocean High (IOH). These high pressure systems form part of what is known as the 'subtropical ridge'. Because of the effect of Coriolis force, the winds associated with such high pressure systems are deflected to the left in the southern hemisphere, and therefore blow around the highs in an anticlockwise direction. Such systems are also called 'anticyclones'. In contrast to the high pressure systems, to the south there is a belt of low pressure systems, or 'cyclones', with associated winds blowing around the lows in a clockwise direction.

(2) The closer the isobars are to one another around such highs or lows, the greater the pressure gradient, and consequently the stronger the associated winds. Latitude also affects wind strength, with the winds being weaker closer to the poles for a given pressure gradient. However, because of the balance of forces in the two systems, anticyclones tend to be large, with the strongest winds near the perimeter and light variable winds near the centre. In contrast, for cyclones the strongest winds are associated with small, deep systems.

(3) Frictional effects at ground or sea level cause an imbalance between the pressure gradient and Coriolis force, with the result that the winds do not blow exactly parallel to the isobars. (see diagrams in SOUTHERN AFRICAN CRUISING NOTES by Tony Herrick). There is a net flow towards the low pressures, causing convergence in cyclones (lows) and divergence in anticyclones (highs). The result of this is an updraft in the centre of a low together with the possibility of cloud formation and rain, while the centre of a high will be cloudfree, drier and hotter.

(4) The air above particular regions may acquire reasonable uniform properties while passing over these regions. Temperature is one such property that can be used to identify air masses. But what is of importance are the rapid changes that can occur when one air mass is replaced by another at a particular place.

(5) Weather, in terms of clouds, rainfall, winds, etc., results from the vertical upliftment of air. In particular, fronts occur when two air masses with substantially different temperatures meet. The denser air undercuts and forces the less dense air upwards. Cold fronts occur when, over a period of time, cold air replaces warm air at a particular place. Warm fronts occur when warm air replaces cold. Quite apart from temperature changes, abrupt changes in wind can also be expected with the passage of a front. There are also distinct cloud formations associated with such fronts. Thus cirrus, altocumulus and stratocumulus clouds typically preceed a cold front, with the possibility of cumulonimbus clouds bringing heavy rain. On the other hand, the clouds associated with warm fronts are cirrostratos, altostratos and nimbostratus.

(6) The duration of an individual frontal low is usually from three to five days. Cold fronts tend to travel faster than the associated warm fronts, and an occlusion occurs when they merge. In these conditions the warmer air rises above the surface, and the system decays rapidly.

For "Southern African Cruising Notes" (Tony Herrick) - passage planning from Richards Bay to Durban to Cape Town and beyond - ports and approaches, weather, lights list, diagrams, radio details, harbour photos, etc. http://www.cruiser.co.za/tony.asp