The Rottnest Channel Swim is a staple of the WA sporting calendar. Every year on a Saturday late in February hundreds of enthusiastic swimmers take to the ocean to race 19.7 km from Cottesloe to Rottnest Island. The winner normally completes the swim in just over 4 hours, whilst the last solo swimmer will emerge at Thomson Bay after around 10 hours.
Part of the appeal of the race is that the conditions encountered by swimmers can vary from year to year. On a good day Gage Roads becomes a swimming pool, whilst on a typical day swimmers will enjoy calm conditions close to Cottesloe, and then choppy seas with a Northerly current as they get close to the island (see Figure 1).
Figure 1.
Strong currents can present a significant challenge for swimmers to stay on course. 2021 was a good (or bad!) example of this – northerly currents of up to 0.8 m/s close to Rottnest caused about 50 swimmers (equal to an eighth of the total field) to abandon the race because they had drifted too far North and could not make headway against the currents.
How can engineering help?
In the wake of 2021, the Rottnest Channel Swim Association teamed up with The University of Western Australia and Aurora Offshore Engineering to improve swim safety by developing a route optimisation tool. This tool accounts for forecast surface currents and the swimmers’ ability to calculate the optimum route to swim between Cottesloe and Rottnest in the shortest time.
Figure 2. The results given by the app.
The optimisation algorithm was developed by a multi-disciplinary team, including UWA engineers Associate Prof. Scott Draper and Adjunct Associate Prof. Terry Griffiths, UWA physical oceanographer Associate Prof. Ivica Janekovic, and UWA sports scientists Dr Grant Landers and Dr Nat Benjanuvatra. The group have been developing the tool for several years to improve their own Rottnest swim times.
Figure 1 shows an example of an optimum route in 2021. The curved shape is optimum because it allows swimmers to head South of the rhumb line at the start of the race when there is little current opposing them, before drifting North back onto the rhumb line close to Rottnest.
The benefits of taking an optimum route can be significant. In 2021, for example, a swimmer who would take 10 hours to travel along the rhumb line could cut 90 minutes off their time with the optimum route. This time reduction could be the difference between a successful and an unsuccessful crossing.
Since its introduction, the route optimisation tool has been very well-received. In 2022 and 2023 more than 80% of swimmers used the tool, with almost all finding it useful and more than 70% declaring it extremely or very useful. Owing to its success it was also adopted for the 2023 Port to Pub Swim event.
So how did 2023 go?
The route optimisation tool made waves in the days leading up to the 2023 swim, predicting some very fast crossing times for swimmers, due mostly to the forecast of a strong Easterly current.
And this prediction didn’t disappoint.
2023 turned out to be the fastest day in the 32-year history of the event. Every race record was broken, with the quickest solo time - 3 hours and 48 minutes – clocking in at around 12 minutes faster than the previous best.
For further details on the optimisation tool visit:
https://rottnestchannelswim.com.au/routeoptimisation/