Passage hazard
Sources: J.K. Mallory, Abnormal waves on the south east coast of South Africa (International Hydrographic Review); Yachting World (secondary). Builds on: L2.2.
The current
The Agulhas Current is one of the strongest western boundary currents in the world. It carries warm Indian Ocean water south-west along the South African coast, close to the edge of the continental shelf. Yachting World gives speeds of up to about 5 knots, in a stream usually less than 50 miles wide.
For a yacht heading south-west from Richards Bay or Durban to Cape Town, the current is a gift in fair weather. In the wrong weather it is the most dangerous water on the route.
Why abnormal waves form
J.K. Mallory, a master mariner, analysed reports of ships damaged by abnormal waves off the south-east coast. His findings:
| Factor | Finding |
|---|---|
| Wind | On every occasion, south-westerly winds with a fetch over 1,000 miles had blown for some time |
| Position | 8 of 10 ships were within 2-3 miles outside the continental shelf edge |
| Inside the shelf | None were inside the 100 fathom (200 m) line |
| Height | Superimposed waves up to about 18 m (60 ft) |
The mechanism: a long south-westerly swell and a local wind sea run against a strong opposing current, which steepens and shortens them. Where two wave trains coincide, a single wave far higher than the rest, preceded by a deep trough, can form.
Avoidance
Mallory's advice comes down to two choices:
- Stand in towards the coast before the wind changes, into about 50 fathoms (100 m), and stay there until the wind and sea moderate, then edge back out beyond the 200 m line.
- Stay well out, at least 20 miles seaward of the shelf edge, outside the core of the current.
The one place not to be in a south-westerly is just outside the shelf edge, in the core of the current.
WARNING - The front comes fast Cold fronts sweep up the coast from the south-west, and the wind can go from a fair north-easterly to a gale from the south-west within hours. Make the decision early.
A worked example
Illustrative figures: a 45 m yacht is heading south-west, and on this stretch of chart the shelf edge lies about 15 miles offshore.
| Option | Position | Distance offshore |
|---|---|---|
| Ride the current | About 3 miles outside the shelf edge | 15 + 3 = 18 miles; exactly where abnormal waves were reported |
| Stand in | 100 m depth contour | Inside the shelf edge; here perhaps 8-10 miles off (read from the chart) |
| Stay out | 20 miles seaward of the shelf edge | 15 + 20 = 35 miles or more |
With a south-westerly forecast in 10 hours, the shelf-edge track is the wrong answer. Standing in, or making port if one can be reached safely before the change, is the conservative choice.
Using the current in good weather
In a north-easterly, the current and wind run together and the sea is kinder. Many yachts ride the current's edge on those days, then move inshore or into port ahead of the next front. Check the current's actual position: it moves, and satellite-derived current charts show where the core lies.
TIP - Know the refuges Before each leg, list the harbours and anchorages you could reach, and how long each would take, if the forecast changes. L3.2 covers the planning.
CAUTION - Old data, stable lesson Mallory's study dates from the 1970s. The physics has not changed, but check the SA Weather Service and current Sailing Directions for up-to-date guidance.