Saturday, April 2, 2016

Winds and Coastlines | Issuma in Antarctica

Winds and Coastlines | Issuma

Nautical chart Maxwell Bay, South Shetland Islands, Antarctica
annotated for wind data by Richard Hudson
Winds and Coastlines | Issuma 

Richard, George and Sarah are the crew aboard Issuma. Richard, with his usual matter of fact style, here recounts a rough spot while anchored in one of the most protected bays around. BTW the centerboard is not like the one on your dinghy.  It carries several hundred liters of diesel fuel - and swings up the better to minimize damage from hitting ice.  Issuma was built for these waters. - gwc
Weather forecasts can be tricky sometimes. We were anchored peacefully where the black arrow is on the picture, with a moderate (20-25 knot) SSW wind forecast for the general area (I get these forecasts from Gridded Binary (GRIB) data from a US National Weather Service computer via email). I figured we would be well protected by the long island south of us (Ardley Island).

What I didn't think about was the possible effect on the wind that the shape of the bay we were in could have. As the wind steadily built, it moved to the ESE, from which we had no protection. While the anchor was holding, it might drag if the wind increased a lot (one always needs to think about what might happen if the wind is stronger than expected), and floating ice would then be blown towards where we were anchored. It was time to leave.

We raised anchor and motored out into a 25 knot headwind and choppy seas. Still surprised by the wind direction and increasing strength, I called up the nearby Chilean base on the VHF radio and asked if they had a forecast for Maxwell Bay.

They said yes, the wind forecast was for ESE to SE 40 knots. That would be a major problem to have stayed anchored where we were. It took several hours of motoring into the wind and seas to reach a bay on the eastern side, where we found a place to anchor near a glacier. At one point, we hit either a growler or an uncharted rock with the centerboard--the centerboard pivoted up and then broke the centerboard cable when it fell back down after passing over the obstruction. We replaced the centerboard cable later at anchor (this is not a big job, but it involves climbing into the narrow centerboard case to thread the new cable, which Sarah--who is thinner than I am--did).

Looking at the chart later, I began to understand why the SSW wind that I expected was actually a SE wind inside Maxwell Bay. The way I've drawn the arrows on the chart picture, it looks obvious, but it wasn't until I looked closely at the chart, and considered the height of the mountains (which deflected the wind, instead of letting it pass over), that I concluded the outside wind probably was a SSW and that it was deflected by the shape of the bay.

The Chilean forecast is probably available to vessels with Inmarsat C, but is not on NAVTEX (which Issuma can receive). I could receive the Chilean Antarctic forecasts via email from the internet, but only after a delay of 24 or more hours (for a 24-hour forecast). Later, I learned of a great Norwegian website ( that collects weather forecasts from various sources and makes them available (in a way that can be retrieved via email). I was then able to get China's forecasts for Maxwell Bay via email.
(if you read the Norwegian Maxwell Bay forecast above remember that is Beaufort Scale, so a `strong breeze' is 21-27 kts gusting to 31 kts with wave height 91/2 to 13 feet.)