Tuesday, 17 November 2015

Mozambique To South Africa - Racing South With The Agulhas Current

Google Track Image By Thomas Herter
After five days sheltering from strong southerly winds,  we departed Basaruto in Mozambique with a decent weather window, in company with six other vessels.

Basaruto is a very large sand island, with a decent pass to the ocean at the southern end, though the pass has a bar at both the seaward and inshore end.

You can see our track on the Google Earth image at right - it is a very beautiful part of the world.  What that image doesn't show is how residual south easterly swells, even with a rising tide, can setup a steep breaking sea on the ocean side of the bar.  I'm fairly sure the south setting ocean currents played a role in this.

As we approached the bar, we were not quite sure that there was adequate water depth to handle the swell size, however we stuck our neck out and nosed gingerly into the shallow area, having studied the breaks and picked the best line.  Watching the fish finder screen very closely, we tracked the depth at the bottom of each swell and crossed the bar with 1.5 meters under the keel.

We relayed the minimum depths to the other vessels by VHF radio and they then started to commit to the sea in one's and two's.

It was a vigorous bar, much like the river bars we dealt with so often in Australia, and we were glad to have it behind us.

Some other boats in our group had never been through surf of this type, and they learned another aspect of boat handling that morning.

The 46 foot catamaran Papillon looked impressive coming through the sets, a first for the owners and crew.  Of course what goes up must surely come down and the crew had quite a ride over several wave sets.

Papillon Images By Diane Selkirk
From the southern pass at Basaruto Island to the port entrance at Richards Bay in South Africa is a distance of around 490 nautical miles.  We covered that in 2 days and 21 hours, using the favourable currents for almost the entire voyage.

The winds were almost dead astern for most of the passage, a sailing angle that we don't really enjoy.  So we angled off and tacked downwind, keeping the wind at around 150 degrees apparent with consistent pressure on the sails and the vessel - a much more comfortable ride, and easier all round on both the vessel and the people.

Despite this conservative approach we still managed to chalk up some breakages.  On the first night out, under staysail and reefed main,  I shone the torch forward for a quick inspection of the deck and noticed the staysail had started to come down of its own accord.  Not very interesting.

The shackle under the swivel had given way, so we had no choice but to drop the sail to the deck, and stow it away until we could go up the mast (in calmer conditions) to retrieve the halyard.   The next morning, as we were enjoying our breakfast coffee, we watched our fibreglass dome television antenna float rapidly past the cockpit, having broken free from the mast and fallen (thankfully) into the sea, without touching the boat.

What else could go wrong ?


Fortunately, that was the last failure for the voyage, and we were able to focus on tracking the ocean currents and working to synchronise our course to take best advantage of them.

In the Mozambique Channel the currents can be enormous.  While the popular notion has them as a clearly defined "river" running south, the reality is more complex.   Current streams loop and whirl, often creating reverse eddies and spiral whirlpools that can be several hundred miles across.

If you can stay on the right side of the spirals and find the main south setting stream it can halve the voyage time.

We were able to download RTOFS (Real Time Ocean Forecast System) charts for the area, which gave us surprisingly accurate predictions of where we would find the currents we wanted.  The RTOFS ocean current forecasts come from the National Weather Service in the United States, so we owe a debt of gratitude to Uncle Sam for this information.  Those little green arrows in the image at right denote a current of over three knots, so we worked hard to find them.  For this voyage, the Agulhas current reduced our passage time by at least a day - we arrived safely in Richards Bay earlier than expected and well clear of the next dangerous southerly weather shift.

2 comments:

J Harrison said...

I just wanted to say that I'm enjoying reading your travel blog. I've followed it for a few years now. Safe travels.

J. Harrison (Fort Worth, Texas)

SV Crystal Blues said...

I have been remiss in not responding earlier to your comment - my apologies. Thank you for the comment, Ley and I are both very pleased to know that you can enjoy the ride along with us. Kind regards, Neil Langford