Multiple Tack Function

Advantage's "Super-If" function compares two alternative 2-tack sequences to an upwind or downwind mark (a starboard tack to the port layline, or a port tack to the starboard layline). A typical result is shown below, indicating a 53 second advantage for the starboard tack sequence. Advantage Level 3 allows you to perform the same calculation with an arbitrary number of tacks, a more general result. Just edit the 'Max Number Tacks' parameter on the Super-If form by clicking the up or down arrow, and click 'Calculate.' The result for a maximum of 5 tacks is shown in the second image below.

USING MULTIPLE-TACK FUNCTION

This function computes a precise result for the specified number of tacks of equal length (in time). One possibility at the end of each tack segment is to continue on the same heading (to extend the prior tack), so the function also considers solutions with a smaller number of tacks. When a tack segment ends close to a layline, the solution assumes continuing to the layline and along it to the mark. This function produces realistic alternatives, with a display that shows the actual boat path. The example below is for the same situation as the two-tack result above, and (in this case) finds that the two-tack solution is the quickest. For comparison, the function computes a number of comparison sequences, color-coded to correspond to the times in the 'Best Course Comparisons' box. The optimal (red) route is predicted to take 22:47 min:sec, in agreement with the 2-tack solution above. The comparison routes take up to 53 seconds longer (blue route). [These routes may coincide in some portions; for example, the green route is identical to the red route to the point where the green route first appears, corresponding to a port tack away from the red route.]

The examples above are computed in a venue with relatively modest currents and minor variations point to point. In venues where currents vary more sharply, the time difference between different tack sequences can be much larger. The example below is taken on a day of fairly modest currents for San Francisco Bay (the maximum current over the portion shown does not exceed 2 knots), but which has rapid current variations point to point. In this case a port tack to the starboard layline is some 6 minutes faster than a starboard tack to the port layline (green line), and 1:22 faster than tacking up the middle (black line).

The Multi-Tack function has three advantages compared to the 'Best Tack' (2-tack) function: (1) it can find optimal routes which lie in the middle of the course, not just along the laylines; (2) it shows how much faster the "optimal" route is compared to alternatives; and (3) it shows a more detailed, step by step approach. The example below finds a red optimal route that is (more or less) tacking up the middle/right, but which takes the same time as a port tack to the starboard layline (blue route) and thereby to the mark. This gives the racer more flexibility, perhaps to stay closer to competitors in the knowledge that it won't cost additional time.

In the same location, but at another time in the current cycle, the route to right of center is preferred to a route to the far right (blue), or left of center (black), but only by about 15 seconds. In contrast, routes to the far left take 2 to 5 minutes more (yellow and green routes).

Advantage enables you to create your own current models, quickly and easily, with the 'Quick Model' facility. You can use this to create your own scenarios to explore tactical options. Below a Quick current model has been defined that features less adverse current up the middle of the course than to the sides. As one would expect, the Multi-Tack optimal route consists of tacking back and forth to the middle, rather than going early to a layline. Because this function sets a maximum on the number of tacks, each covers a fairly large distance. To see a solution staying closer to the middle, you would need to increase the Max number (note, however, that this will increase the time required for the computation).

RACING IN A VENUE WITHOUT A PREPARED CURRENT MODEL: If you will be racing in a venue without prepared 'Local Knowledge' current, you can use the Quick Model facility to create simple current models based on your own observations. These models can then be used with the Super-If and Multi-Tack functions to explore the most effective tactics, for different assumptions regarding wind, and different times of the tidal cycle.