Americas Cup 2000

Local Knowledge Current Models and Tactical Software

We specialize in the development of detailed, point-by-point current models in racing venues worldwide. When the Americas Cup came to New Zealand, there were no local predictive models for currents in the Hauraki Gulf. We contracted with the Kiwi Defense to build a PC-based current model that could be integrated with the team's tactical software. Since then we have supplied many of the Americas Cup syndicates (and individual tacticians and navigators) with current models and unique software that exploits subtle current variations for tactical advantage.

FOR SOME EXAMPLES OF SHORT-RANGE TACTICAL COMPUTATIONS, CLICK HERE

        The discussion below relates to the 2000 Americas Cup in New Zealand. For information about our
        latest tactical software, ADVANTAGE, click HERE.

About "Local Knowledge" Currents: We build detailed, current-prediction models for racing venues worldwide, from the English Channel, to the east and west coasts of the US, to Auckland, site of the 2000 Americas Cup. We incorporate these predictive models in a number of navigational and tactical software programs for the PC. For details on our current models and racing programs, See our Website.

Currents and Racing Tactics

Effect of Current on Racing Tactics: The net motion of any boat is determined by the (vector) sum of its velocity over the water, and the velocity of the water itself (the current) over the bottom. So, to reach a given mark, the helmsman must steer to the right or left to compensate for cross currents, and the time to reach the mark will be longer (or shorter) than it would be without an opposing (or following) current. For a sailboat, however, there are additional effects. The speed of the boat depends on the speed and angle at which air flows over the sails. If there is current, moving with the current adds to the air flow over the boat, so there is an effective "wind," even on a windless day. The net wind experienced by the boat is the vector sum of the ground wind and a component equal but opposite to the current velocity. (In addition, the boat's own motion over the water contributes to the "apparent" wind measured by its instruments.)

Even a relatively small current can alter conditions on the course. If the wind is blowing 10 knots and there is a 0.5 knot cross-current, there is an effective wind "shift" of about 5%, or a little less than 3 degrees. This doesn't sound like much, but over an upwind leg of 3 miles or so, it can significantly affect the position of laylines (where racers tack to avoid sailing extra distance in tacking to an upwind mark). The example below is from the Hauraki Gulf in a modest, 0.4 knot cross-current.

In the graphic, the solid blue lines represent the laylines in the absence of current. A sailboat tacks back and forth between the laylines to work its way upwind to the windware mark ("WM"). Going outside the laylines involves extra distance and hence extra time, but any combination of tacks within the laylines takes the same total time to reach the mark (ignoring the time lost in each tacking maneuver). The dotted magenta lines show the laylines when corrected for the current (in this case an almost uniform cross-current of 0.4 knots, at about 250 deg). There is a sizable shift of the left (almost vertical) layline away from the boat, and a shift of the right (almost horizontal) layline towards the boat. This example is very typical of the AC courses, with a distance of approximately 3.1 nautical miles from leeward mark ("LM") upwind to WM.

Effects of Differential Currents on Strategy (Wind 8 knots at 240 deg)

In this example, the ebb current is aligned more along the wind direction and changes in strength and direction across the course (the circles represent the "tail" of the current arrow, which streams away in the direction of flow). We have used our "Force 4" tactical program to compute and compare the time it would take the boat to tack to either layline and then sail along the layline to the mark. This shows that sailing along the starboard tack (green line) to the lower layline is some 45 seconds faster than the alternative port tack (red line) to the upper layline. This difference is 2 to 4 times the usual margin between the AC boats on such a leg, and so it becomes really important for the racers to know how currents vary across the course.

Current Effects are Magnified when Winds are Light: Compare the graphic below, using the Force 3 "best course" routing function for moderate winds of 12 knots, indicating very little to no advantage from one upwind route to another, to the following graphic, for the same time and location with a 5 knot wind. The latter predicts a TWO MINUTE advantage in the right-hand side of the course, a huge difference in this kind of racing. Such light winds are not at all uncommon in Auckland, particularly as the season advances and we approach the semis and finals.

What Determines Currents: currents along a land mass, like New Zealand, are almost entirely due to tidal "forcing," and reflect the motion of water towards (or away) from land as tidal heights rise (or fall). Tidal effects are primarily driven by the relative position of earth, sun and moon, and follow a daily variation which is well understood and can be predicted quite accurately. Currents, however, also vary from point to point due to the presence and shape of nearby shorelines, varying depths of water, and other factors, such as inflow from rivers and other sources of water into the system. Unlike wind, currents are inherently predictable, given enough detailed knowledge of these factors, but in practice one faces only partial knowledge and difficult computations. It is next to impossible to accurately determine currents from boat instruments, while on the water, and in any case all the instruments can measure are the currents behind the boat. The trick is to predict currents ahead of the boat, allowing for the time it will take to reach each point as well as current differences from one point to another.

We at Local Knowledge combine what is known about the underlying factors, with whatever actual measurements are available, to produce a computer model of currents which is both accurate and amenable to rapid calculations. Our models enable a great variety of detailed and complex computations of interest to the racer or cruising sailor, incorporated in our PC software.