Agulhas Current - Note One

The sources of the Agulhas Current are the East Madagascar Current, the Mozambique Current and a recirculated part of the south-west Indian subgyre (a smaller circular current within a gyre)south of Madagascar. The flow of the Agulhas Current is directed by the topography. The current follows the continental shelf from Maputo to the tip of the Agulhas Bank (250 km south of Cape Agulhas). Here the momentum of the current overcomes the vorticity balance holding the current to the topography and the current leaves the shelf.

The core of the current is defined as where the surface velocities reaches 100 cm/s (39 in/s), which gives the core an average width of 34 km (21 mi). The mean peak speed is 136 cm/s (54 in/s), but the current can reach 245 cm/s (96 in/s.

As the Agulhas Current flows south along the African east coast, it tends to bulge inshore frequently, a deviation from the current's normal path known as Agulhas Current meanders (ACM). These bulges are occasionally (1-7 times per year) followed by a much larger offshore bulge, known as Natal pulses (NP). Natal pulses move along the coast at 20 km (12 mi) per day. An ACM can bulge up to 20 km (12 mi) and a NP up to 120 km (75 mi) from the current's mean position. The AC passes 34 km (21 mi) offshore and an ACM can reach 123 km (76 mi) offshore. When the AC meanders, its width broadens from 88 km (55 mi) to 125 km (78 mi) and its velocity weakens from 208 cm/s (82 in/s) to 136 cm/s (54 in/s). An ACM induces a strong inshore counter-current.

Large-scale cyclonic meanders known as Natal pulses are formed as the Agulhas Current reaches the continental shelf on the South African east-coast (i.e. the eastern Agulhas Bank off Natal). As these pulses moves along the coast on the Agulhas Bank, they tend to pinch off Agulhas rings from the Agulhas Current. Such a ring shedding can be triggered by a Natal pulse alone, but sometimes meanders on the Agulhas Return Current merge to contribute to the shedding of an Agulhas ring.

One Very Unplesant Incident

Ship’s Radar Observer’s course was a part of our study program. We did this on board our training ship "Sir John Cass". The training ship was berthed in Rotherhithe. Another Sri Lankan student joined our batch only to follow the Radar Observer’s course. It was a two-week course, and every morning by 9.00 a.m. we were all on the ship. Soon after that, the ship left the berth and proceeded down the river Thames and returned only by 5.00 p.m.

On Friday of the first week of the Radar Observer course, the Sri Lankan student, Ram, and I boarded the train at Rotherhithe to come to central London. There were a few dock workers in the train and they were drunk. One of them came close to us and said "You black bastards you are smelling!" I understood the situation and I just kept quiet. Ram was new in London and retaliated with some harsh words. I tried my best to advise Ram to be quiet but he would not listen. At this time, two others joined and started assaulting both of us. I pulled Ram and jumped out of the train when it stopped at the next station. Those men also jumped out and continued to hit us. Ram was bleeding from his nose. So many people passed by and nobody, repeat nobody, even turned to look at what was going on. A Chinese gentleman came to our rescue and then those three men moved out. We thanked him and thanked God for sending him. Ram was trying to maintain a very brave face but I knew that he was in pain. There was blood all over his shirt. He was staying in Wembley park and he took the train in that direction while I boarded the train that brought me to Finchley Central. That was one very unpleasant incident that took place during the time I was studying in London.

                                   Section of the Rotherhithe Underground Station in London

The First Phase of Academicals

Highest sea-going qualification for the deck officers is the Master mariner. The Extra Masters which is like a Ph.D. is more of an academic qualification which was highly recognized, worldwide.

For Merchant Navy Officers there are phases of academicals for which we must enter a Nautical College. During our time, the first phase of studies was to prepare us to sit the exam to obtain the Second Mate Foreign Going Certificate. From the time I joined my first ship as an Officer cadet, I had accumulated sea time of about three and a half years which enabled me to enter college for the academicals. The required sea time was three years and three months. After looking at many options, I, finally opted to do my studies at Sir John Cass College in London and also at the Irish Nautical College in Cork, Ireland. During the time I worked on board I saved some money for my studies. But, later I found that was not enough and my parents and some friends helped me with the finances from time to time while I was studying in London and in Ireland. 

I joined Sir John Cass College in February 1978. During the time of my studies in London, I lived with some of my cousins in Finchley Central. We shared an apartment, cooked our meals and managed our lives.  During the summer holidays, I worked in a bakery in the Wembley area. I could not save much from what I earned because with that money I had a good summer with my cousins and friends.

It took around ten months for me to complete the first phase of my studies and sit the exam.

I passed my exam on December 16^th that year. It was great because it was two days before my birthday. Soon after I passed the exam, I was offered a job with a British Shipping Company but since my Visa was only for the purpose of studies in the UK I was unable to join. After spending Christmas and the New Year with my relations and friends in London I flew to Venice to join a ship owned by the company that I had my training.  

                                                  Sir John Cass College in London

                                               Photo of Sir John Cass Training ship.

Marine Sextant

With the sextant for determining latitude and the chronometer for longitude, sailors by the 1800s were able to navigate the high seas with great precision.


For millennia, as sailors from the Phoenicians to the Polynesians knew, the heavens remained the best way to find one's north-south position. Increasingly sophisticated devices were designed over the centuries to measure the height of the sun and stars over the horizon. The gnomon or sun-shadow disk operated like a sundial, enabling the user to determine his latitude by the length of the sun's shadow cast on a disk floating level in water. The Arabian Kamal was a rectangular plate that one moved closer or farther from one's face until the distance between the North star and the horizon exactly corresponded to the plate's upper and lower edges. The distance the plate lay away from the face—measured by a string tied to the center of the plate and held at the other end to the tip of the nose—determined the latitude.

In the Middle Ages, sailors relied on the astrolabe, a disc of metal that one held suspended by a small ring. The disc had a scale with degrees and a ruler for measuring the height of an astronomical body. Other medieval mariners preferred the cross-staff, a T-shaped device whose base was held up to the eye. One measured the sun's height by pulling the slid able top of the T toward one's eye until the sun lay at the top and the horizon at the bottom. Since blindness resulted from frequent use, the explorer John Davis invented the back-staff in 1595, which enabled one to get the same measurement with one's back to the sun. The sextant was the most advanced of these devices, allowing users to determine their latitude to within a sea mile or two, even from a swaying deck.

In the years after the sextant was invented in 1731, many held out hope that it would aid in east-west navigation as well—that is, in finding longitude. Sailors could employ the sextant to figure longitude using the lunar-distance method, but with the astronomical tables of the 18th century, the process could take several hours to work out one's position—not remotely good enough for sea travel. In the end, it was the dogged clockmaker, John Harrison, who solved the longitude problem with his chronometers. And today, the precocious step-child of these highly accurate clocks, the Global Positioning System, has finally proved the Roman dramatist Seneca right, when he wrote in the first century:

 There will come an age in the far-off years

When Ocean shall unloose the bonds of things,

When the whole broad earth shall be revealed....

Dead Reckoning

This ancient form of navigation known as dead reckoning begins with a known position, or fix, which is then advanced, mathematically or directly on the chart, by means of recorded heading, speed, and time.

The navigator finds a position by measuring the course and distance they have sailed from some known point. Starting from a port, the navigator measures the distance from that point on a chart. Each day’s end position would be the starting point for the next day’s course and distance-measurement.

The navigator plots his 9am position, indicated by the triangle, and, using his course and speed, estimates his position at 9:30am and 10am.

Without an accurate way to tell time, dead reckoning was notoriously inaccurate in the open ocean, making Columbus’s feats all the more remarkable.