Feature

Time, Tide and the America's Cup

Judging the flow of the tide in yacht racing has come a long way from throwing a line over the side...

Ross Vennell

In the high tech world of the America's Cup, hundredths of a knot in boat speed can mean the difference between winning and losing. Skippers can use knowledge of how tidal current patterns vary over the race course to make that difference.

Research at the University of Otago is providing Team New Zealand with detailed tidal current information to help them defend the America's Cup.

It's not just an aid to yacht racing. Tidal current research is also critical in understanding the marine environment, providing information on how sediments and sand are transported around our harbours and coastline, or about the movements of juvenile marine organisms. The technology developed for tidal research at Otago is not only benefiting Team New Zealand, but also the pilots of large vessels who have to safely berth vessels in confined ports.

The tidal research is based around an Acoustic Doppler Current Profiler. This instrument uses sound waves to measure water currents from a moving boat. In effect, the instrument is a very precise echo-sounder, capable of detecting echoes off both the sea floor and material in the water. From these echoes it can measure water currents at one-metre depth intervals.

Work at Otago University has been pioneering the use of this off-the-shelf instrument to give extremely high-resolution tidal current patterns.

One research application, undertaken in conjunction with NIWA (the National Institute for Water and Atmospheric Research), has been to better understand the physical processes in eddies which form behind headlands and their role in the movement of sand along our coastline. This, in turn, has an impact on coastal erosion. These eddies, up to 4km in diameter, can form in the wake of a headland as tidal currents speed past its tip.

Another research application has focused on the physics of tidal jets, which form outside harbours during the ebb tide, and how the jet's structure affects sediment movement around the entrance and into the harbour. This knowledge can assist ports companies with dredging strategies.

Careful measurements of the tidal currents on the America's Cup course may help give Team New Zealand an edge, while providing a useful understanding of the physics and dynamics involved.

Tidal Currents for Team New Zealand

The research for Team New Zealand has involved Acoustic Doppler Current Profiler measurements in the important areas of the Hauraki Gulf. Time didn't allow measurements to be made over the entire Gulf, so a computer model has been used to fill in the gaps.

The model has been developed by Paul Corney, a PhD student at Auckland University working at Montgomery Watson on a Graduate Research in Industry Fellowship.

Tidal current patterns in Bluff Harbour during flood tide show up to 4 knot flows in a narrow jet past the Aluminium Smelter Wharf at Tiwai. An associated counter eddy forms to the west of the jet.

Together the measurements and model give Team New Zealand a very detailed picture of tidal currents at every stage in the tidal cycle.

Spring tides during the first few races of the America's Cup, in February 2000, will produce strong tidal flows, so these pictures are vital, particularly if there are light winds at the time. Tidal current patterns will be used by the boat's crew to help them decide which side of the race course is more favourable, so they can build this into their tactical decisions.

Starts are critical in America's Cup match races. Knowledge of tidal currents helps the crew time their start to ensure they don't arrive at the line too early or too late.

Tidal knowledge also helps in picking "laylines", when sailing upwind. Laylines are the imaginary lines which the boat must sail to arrive exactly at a course mark when it is sailing at its optimal heading. Sailing above a layline means a boat travels a greater distance than it needs to, while sailing below a layline may mean the boat will have to do additional tacks to reach the mark, losing time. Tidal currents cause laylines to both shift and curve, compared to a layline when there is no current. Detailed knowledge of tidal currents will help the Team's navigator, Tom Schnackenberg, pick laylines.

The predominately north-easterly and south-westerly winds also drive surface currents in the inner Hauraki Gulf. Spot measurements with the Acoustic Doppler Current Profiler show that in high winds, around 20 knots, these currents can be significant. Paul Corney is modelling these wind-driven currents in the Gulf both for his thesis and to assist Team New Zealand.

Though big, Team New Zealand's boats are tiny compared to the 200m or longer container vessels which berth at New Zealand's ports. Another group benefiting from tidal research are pilots of these huge vessels at Bluff, Otago and Tauranga.

The use of Acoustic Doppler Current Profilers is providing the detailed current patterns pilots need. This knowledge is becoming increasingly important because while ships have been getting bigger over the years, harbours have not, largely because of the high costs of dredging larger shipping channels. This has meant captains and harbour pilots having to manoeuvre huge vessels inside restricted spaces -- an often delicate task.

The Team New Zealand tender boat on the America's Cup course, with Rangitoto Island in the background. The top of the Acoustic Doppler Current Profiler instrument can be seen attached to the side of the boat nearest to the camera. The instrument uses sound waves to give precise measurements of tidal currents.

Knowledge of currents is vital for harbour pilots to bring vessels in and out of wharves safely and efficiently. For example, if the current pushing sideways on the bow of a ship is not the same as that on its stern, the ends of the ship will try to move in different directions. The pilots and the tugs need to work closely to manage these twisting effects of currents in confined spaces.

Through experience, harbour pilots have learned a tremendous amount about the current patterns in their ports. Tidal research has mainly confirmed their knowledge, as well as providing details about the current patterns which they were not previously aware of.

Current measurements at Bluff show strong tidal currents at the harbour entrance of up to 6 knots. The measurements have been fed into the computer memory banks of Australasia's only vessel simulator, in Tasmania.

Bluff pilots are using the simulator, which runs on similar principles to computerised flight simulators, to train new harbour pilots and try out new ideas. The simulator provides a "virtual" representation of a port, allowing pilots to test options for moving vessels during different tidal states. Port Otago pilots are also working towards using the simulator to train pilots.

Other tidal research has developed algorithms to accurately predict tidal currents affecting access to the Port of Tauranga. One of the big benefits for Tauranga is that tidal currents are not as restrictive as traditionally thought, allowing for significantly increased growth in shipping.

The algorithm is part of a PC-based Harbour Management System, developed by New Zealand company Jade. This software allows shipping clients, such as cruise line operators, to book their time of arrival a year or more in advance and know the currents will be weak enough for them to safely enter the port at that time.

More information can be seen at http://www.otago.ac.nz/oceanphysics/

Ross Vennell is a lecturer in Otago University's Department of Marine Science.