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The rhumb line is not necessarily the fast line — currents, tides, wind direction, even wave height, all playing a part. For some boaters the number of variables to consider are daunting, but fear not, there’s a wealth of cheap downloadable info out there


Remember the amusing scene in the movie Finding Nemo, when the hippy turtles are "surfing the EAC, man"? Well, unless you have their natural abilities, you’ll generally rely on some kind of software package to navigate the East Australian Current and other anomalies surrounding our lovely shores. This southerly flowing warm current is one of the great challenges for yacht-race navigators, who obviously want to avoid it when heading north during the Gold Coast Race or hook into it when heading south as in the Sydney Hobart race.

For the race guys time is of the essence but for general boaters time is money with a lot of diesel wasted butting against an adverse tide. Referring to the tidal diamonds on the paper charts is about as good as it gets for many boaters but taking advantage of the latest oceanic current data is what top navigators such as the Adrienne Cahalans’ and others do.

All very well if you’ve got a degree in oceanography or similar but most of us nowadays haven’t the time or inclination for statistical analysis so turning to packaged software is the easy way. For weather information a myriad of sources are available that can be downloaded onto your software chart in the non-proprietary GRIB format.

GRIB refers to ‘Grid In Binary’, a data file format used for generating gridded meteorological data and is maintained by the World Meteorological Organization. For basic wind direction information this data, technically known as GFS files, are small and sometimes less than say 100KB, so can be easily downloaded via a satellite phone or Sailmail SSB HF radio link while at sea and overlaid onto chart-plotting software.

GFS stands for Global Forecast System and is a worldwide weather prediction model run by NOAA/NWS and generates four updates per day. The data is available from various sources, such as which allows completely free global downloads and even include a software package to display this info. More typically, the GRIBs are loaded into navigation software — if you’re a racer it might be to complex packages like Deckman or Expedition — but for general boaters it would be your chartplotter software.

GRIBs for currents are very much a developing area so the success of the Australian company Tidetech is worth noting. Tidetech technology is based on hydrodynamic modelling, which uses various satellites to analyse ocean currents. It is a service used by winners of the Sydney Hobart, Gold Coast and Lord Howe races since December 2008.

Using what’s known as ‘satellite altimetry’ both current and wave-height data can be derived as well as interpretations of sea colour and density. This is where it gets interesting because this science purports to show phenomena such as plankton rising, so has fascinating possibilities for commercial fishing and even anglers. The other major area is ship routing — finding the most economical way for, say, a container ship from Botany Bay to go to Japan could reduce fuel costs significantly.

Tidetech director Dr Roger Proctor explained to Bright Sparks that their subscription service supplied processed data gleaned originally from the CSIRO, which was an analysis of data based on roughly a week. So, for instance, the Sydney Hobart racers would receive the week’s analysis prior to the race and then perhaps an update during the race.

What this all means for the general boater is greater accuracy in software charting data, so if you are at home using passage-planning software to prepare a summer cruise you might want to include wind and tidal averages based on a certain month and so on. For the bluewater sailor, who may use ocean-passage planning software, such as Digwave Visual Passage Planner 2, for example, the straight line may not always be the best route between two ocean waypoints based on this GRIB data. So, apart from the ubiquitous rhumb line you may choose a bumpy, fast route or a slower smooth route.

But remembering what Disraeli said about statistics and more pertinently what weather expert Dr Simon Keeling warns about reliance on gridded data — it is based on averaged data points, usually of 24nm intervals, so for example a headland in-between grids could have a much higher wind speed. So, like all things electronic the Mk1 Eyeball Method is always the best ultimate judge of what’s going on around you at sea.


Photos: Accurate weather-routing data can help minimise those rough rides, as seen here heading to Lord Howe across the East Australian Current; Strength and direction of the East Australian Current according to Tidetech GRIB data; Free weather GRIB data and software is available from; Ocean-passage planning can be done with software such as Visual Passage Planner from Digwave; Tidetech subscription service supplies ocean-current data in GRIB format.


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