|Day 28||Position||Daily run||Av speed|
The boat's position at 15:00 GMT today: 44°29S - 139°38E
Distance travelled in 12 hours : 239,37 nautical miles
Average speed over the last 12 hours: 19,95 knots
Geronimo, the Cap Gemini and Schneider Electric-sponsored trimaran continues to close on Tasmania through difficult sea conditions: a south-westerly swell, crossed by wind-generated waves from the west north-west. This wind, now blowing at a sustained 30 knots, is very close in direction to the boat's current track and has meant a lot of gibing for the crew during the last 72 hours (multihulls do not perform well with the wind right behind them).
So, for Geronimo to manage an average point-to-point speed in excess of 19 knots over 24 hours, she has to sail much faster than 20 knots actual speed. The trimaran is currently having to cope with a breaking sea, whose waves are shown by satellite imaging to be over 7 metres high, making progress tricky and obliging her crew to be very careful indeed. There are many factors that affect the amplitude, period and wavelength of ocean swell. The effect of the wind on sea conditions depends on the distance that wind blows before meeting any obstacle. This distance is called 'fetch', and in the open sea, is generally determined by the size of the depression generating the wind. The other determining factor is the length of time for which the wind blows continuously at a given strength. So, for any given wind speed, there will be a certain period of time before the waves created reach a state of equilibrium. Beyond that point, even if the wind continues to blow, the swell and wave frequency will not increase any further.
Studies show that waves 10 metres high, with a period of 11 seconds, can be produced locally by a 45-knot (50 mph) wind blowing for 20 hours over a distance (fetch) of 135 nautical miles, and that a 2-metre swell, with a period of 14 seconds, can be produced by a 36-knot (40 mph) wind blowing over 1080 nautical miles, 62 hours earlier.
These are the reasons why Geronimo is experiencing such difficult sea conditions, which have been aggravated by at least two weather patterns in an area where there is nothing at all to stop the waves. Forecasts for tonight show even higher waves south of the boat's current position, forcing her to stay much further north than the skipper would prefer.
In the next few hours, the Cap Gemini and Schneider Electric crew should pass south of Tasmania, the island first discovered by Abel Tasman in 1642 and separated from Australia by the Bass Straight. Tasmania is Australia's smallest state.