Rough seas off Cape Agulhas. Photograph : Grindrod

Meteorological effects on shipping

Study Sections 11.4.1 and 11.4.2 in the Grade 11 section.

Meteorological data that will affect a ship during her voyage

Strong wind and heavy seas are among the more serious dangers that face seafarers. The highest wave height recorded along the South African coast is around 23 metres, although other abnormal waves have perhaps been higher but not recorded. While ships are designed to withstand adverse weather conditions, exceptionally heavy seas can damage ships. Ships can also be delayed by heavy seas as they need to reduce speed; cargo can shift, causing a ship to list (a situation that can be dangerous to the ship) and there are a number of incidents where containers have gone overboard from ships during heavy seas.

It is important that when the route of a ship is planned, the weather is taken into account. For most voyages, ships need not divert form their planned course because of the weather. However, in the case of a tropical cyclone (hurricane or typhoon) or a severe depression, alterations to the planned course can be made to avoid the worst of the heavy seas associated with the weather system.

The following is an account, written by the Master of a ship heading from South Africa to Asia late in April.

On our way across the Indian Ocean, I was surprised to see on the synoptic chart a tropical storm developing on our course line, few days ahead. As this is late in the season, I expected it to fizzle and die. It was not to be. The tropical storm developed into a fully-grown Tropical Cyclone – and a big one with winds around the eye forecast at 120 knots. What was more interesting is that it was behaving what would be considered as impossible, as it was heading on a north-westerly course, towards the equator. In the southern hemisphere, a tropical cyclone is supposed to start south of the equator, then head WSW until it reaches around latitude 20S before curving and carrying on with a south-easterly course. Tropical Cyclone Fantala was doing her own thing! This meant that I had to alter course to pass on the dangerous side of her. But by the time we got to that side, she was well on her way and in the end passed north of Madagascar.

The map below shows the position and course of the tropical cyclone as described by the ship’s Master.

MD = Madagascar; M = Mauritius.

The following is a typical weather forecast for that area during the passage of a tropical cyclone, and describes these conditions. You will understand why a ship’s Master will decide to alter course to avoid the cyclone.

Although all ships keep a careful watch on the weather, heavylift ships or tugs undertaking ocean towage operations keep a particularly close watch for adverse weather systems such as depressions or tropical cyclones so that they can avoid any heavy weather or strong winds that so easily can cause problems for them during their voyage. The Chinese heavylift ship shown bunkering in Cape Town) is carrying a cargo of container gantry cranes from China to Europe, and, to ensure that she is not exposed to unnecessary dangerous sea conditions or very strong wind, her owner will provide a very specific route for her. Photograph : Brian Ingpen

The effects of weather on cargo operations

Cargo operations (especially container handling) cannot be done during times of strong wind. Containers will swing in the wind, making it difficult for container gantry operators to load containers into the container guides on a ship, or to fit them onto the twistlocks that will secure them to hatch covers or onto the container below. The South African ports of Cape Town, Port Elizabeth and Ngqura are particularly prone to being affected by wind. (See also Section 11.4.1 in the Grade 11 section of this website.)