Catching the 'magic carpet'

DEVELOPMENT OF NEW SOFTWARE TOOLS TO HELP OFFSHORE RACERS FIND THE MOST FAVOURABLE OCEAN CURRENTS, BY PENNY HAIRE.

ANY COMPETITOR in the Rolex Sydney to Hobart race will be aware of the East Australian Current. It is the largest ocean current close to Australia; moving as much as 30 million cubic metres of water per second in a broad ribbon up to 100 kilometers wide and 500 metres deep. At times it can flow at up to 7kts with large eddies featuring at various points.

Positioning the yacht to avoid northgoing current and take advantage of the strongest south-going flow is a crucial factor in the race and has contributed to the success or failure of many a well campaigned boat over the years.

So how does the navigator put the boat in the right place?

In races past, samples of seawater were routinely taken by bucket and temperature measured by thermometer (warmer water theoretically indicating a stronger south going flow). More recently the boats'
instruments have been able to do this.

More recently, navigators have accessed ocean current information from CSIRO at www.marine.csiro.au/remotesensing/ oceancurrents, where the best available observational data about geostrophic current and sea temperature can be found. The information is presented as images, but their usefulness is limited because software such as Expedition, Maxsea and Deckman, now used by many navigators, requires data to be in GRIB format to perform optimal routeing functions.

We set ourselves the task of converting the CSIRO data to GRIB and ten days before the most recent Rolex Sydney Hobart race and after lots of nasty maths and midnight oil burned, we achieved this. It then became possible to view the current data as strength (knots) and direction (degrees) and Sea Surface Height deviation (metres) on an electronic chart and to perform optimal routeing functions using wind and current GRIBS.

The science of ocean currents

So how accurate and valid is the data? To evaluate this, it's worth touching briefly on the science of how ocean currents are formed and how CSIRO uses data from satellites and other sources to make an observational model of the ocean.

Surface ocean currents are analogous to atmospheric winds:
• Differences in sea level exist between adjacent areas of the ocean. This gives rise to pressure gradients.
• Water needs to flow from an area of higher pressure to an area of lower pressure, but the Coriolis effect forces a geostrophic flow parallel to isobaric contours (lines of equal pressure).
• If you can ascertain the pressure (height) distribution over an ocean, you can then determine the geostrophic current.

CSIRO produces a map of the ocean surface using, principally, sea surface height anomalies (height deviations from a mean surface) measured by satellite altimeters (accurate to within 5cm). The geostrophic current at any point can be calculated from the differences in surface height.

The satellites can't sample the whole world every day because they can only measure directly beneath them. To make a complete ‘quasi-synoptic' map, data up to ten days old must be used with the most recent data points given more weight than the older ones. Fortunately, the ocean changes more slowly than the atmosphere, so the map doesn't change very quickly.

The ocean surface is affected by the density of water below it. Warm water tends to cause a rise in the surface and cold water a dip in the sea surface. CSIRO Sea Surface Temperature (SST) data is derived from a number of satellite passes a day and the data is therefore more ‘recent' than the surface current data.

When overlaid with the current it can help to confirm the presence of cold eddies (which rotate clockwise in the southern hemisphere) and warm eddies (which rotate anti-clockwise). The SST map is best for precisely locating where the ocean currents are, while the sea level map is good for resolving ambiguities of which way the currents are generally flowing, and whether they are weak or strong.

The 2008 Hobart race
Strong downwind conditions for the first part of the race meant that the current was likely to play less of a factor for the faster boats, which were seeking more pressure over favourable current and the optimal routeing outcomes suggested this, with only a two-four hour difference being predicted for a TP52, when pre-race routeing with and without the current GRIB.

It was a different story for the smaller boats, where differences of up to ten hours were predicted. Ironically though, few of these boats use routeing software on board.

Tidetech supplied surface current and SST GRIBs to 17 boats, including line honours winner Wild Oats XI, IRC overall and Div 1 winner Quest, IRC Div 0 winner Quantum Racing and IRC Div 2 winner Ragtime. We were very pleased with the results and feedback.

Navigator on Quest, Bruce Baker, said, “We tested Tidetech's GRIB files with data from the CSIRO for the month before the race and found it to be pretty accurate. This meant that on this Sydney to Hobart race we could use them in our tactical routeing software with confidence and we believe that the information made a difference to our overall result.”

Navigator on Quantum Racing, Bryan Northcote, said the current GRIB file was the most accurate he had used. “I was able to run the optimum route in Expedition software for Quantum Racing using the current GRIB in combination with four different wind GRIB files. In reality the SST and current position and direction enabled us to confidently make tactical decisions regarding wind speed and direction that are more susceptible to change.

“At sea, the information contained in the current GRIB was very accurate. Any navigator or tactician using routeing software such as Expedition could confidently use the optimum route program with a current GRIB file produced by Tidetech when comparing effects of wind and current to produce an optimum route.”

Tidetech is developing a subscription service where GRIB files of current, sea temperature and tidal stream data will be available for download for selected locations worldwide.

For more information, visit www.tidetech.org.

Tidetech was founded by Penny Haire and Dr Roger Proctor. Penny Haire is the chief examiner for the RYA/Yachting Australia National Yacht Training program and has 20 years' experience as
a skipper and navigator with specialist knowledge of electronic navigation and racing systems.

Dr Proctor is an internationally recognised coastal oceanographer responsible for the integration and coordination of the Australian marine observing system, based at the University of Tasmania. He
has also been a tidal consultant to the UK Olympic Sailing team since 1988.

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