UPWIND LEG IN VENUE WITH INCREASING ROTARY CURRENTS |
| Sailing with Currents: currents
affect upwind sailing in two ways. (1) current pushes the boat off the line in which it is
headed; and (2) current changes the direction of the "true" wind. The first
effect is obvious, but the second is more subtle. The current contribution to the
"true" wind is equal in magnitude but opposite in direction to the current flow
(this effect is different from "apparent" wind, which differs from
"true" wind as the result of the boat's motion relative to the water). If the
(ground) wind during the upwind leg is at 10 knots, and the current flows directly
downwind at 1 knot, the boat sees a "true" wind of only 9 knots. If the current
flow is shifting from right to left (clockwise rotation), the "true" wind will
appear to shift counterclockwise. This is not something you have to calculate, since it
will be automatically included in the "wind" measured by your boat instruments.
If current is constant during the leg, any combination of tacks will reach the mark in the
same amount of time. If, however, the current is shifting during the leg, due to the
passage of time or because there are point-to-point variations, one sequence of tacks may
be significantly faster than another. To calculate the optimal route, you need to separate
the current contribution to find the ground wind, and then recompute "true" wind
at intermediate points. Computing the Layline: for a given wind speed and a given
boat, there is an optimal tacking angle at which the boat makes the maximum speed upwind.
At any intermediate point, the tactician adjusts the heading to maintain the optimal
angle. Even if wind is constant, the effect of current is such that the true wind
direction, tacking angle, and heading will change point to point if the current is
different. The layline is the path that the boat will sail to the mark, assuming that it
sails at its optimal tacking angle at each point. The layline is dependent on both effects
of current, and typically differs from the heading of the boat. In the most general case,
where current varies in strength and direction point-to-point, the layline is not a
straight line, but rather curved (see example below and at www.goflow.com/advfacts.htm).
Computing the Optimum Upwind Route: Advantage has a special windward-leeward
(W/L) routing function that allow you to set the maximum number of tacks, after which it
computes and compares all combinations of tacks to the mark. Each possible route consists
of a number of tacks until the boat tacks along the layline, at which point the boat is
assumed to continue along the layline to the mark. The W/L router indicates the quickest
route (red route below), and a number of comparison routes (other colors), and
displays the time for each (see below). You can apply the router with realistic
"Local Knowledge" currents, or create a current scenario of your own in a matter
of seconds and check out the tactical alternatives. |
ANSWER TO MONTHLY QUIZ (UPWIND IN INCREASING ROTATING CURRENT) |
| We applied the Advantage W/L Router,
assuming a wind of 10 knots and 1D35 polars. The difference in time for different routes
will be greater for a lighter wind or slower boat. At first glance, one might assume that
it would be better to sail to the right in the early part of the leg, when the opposing
sideways current is zero, or small, and then to approach the mark from the right, towards
the end of the leg, with the aid of the increasingly favorable current. This is wrong
because the right (starboard) layline will rotate upwards (counterclockwise) as the
current shifts, so the boat will have to sail farther to reach it. The exact calculation
is shown in the graphic below, and shows the left side favored by about 2 minutes (out of
30). The optimal route for the quiz scenario is a starboard tack towards the left (port)
layline, which is almost two minutes (1.8 min) faster than a port tack to the starboard
layline (blue route). The green route is almost as fast (additional tacks on left side of
course), and in fact any sequence of tacks on the left side of the course is faster than
tacks on the right.
Note: the approximate time the boat will take to reach the mark is used to compute
where the layline will be at the time the boat reaches it, and this is reflected in the
graphic. Note that the routes are slightly curved, to reflect the changing true wind,
tacking angle and heading. |
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| The example below shows actual flood
currents in San Francisco Bay, with wind from due west. Note the difference in stength and
direction of currents from one part of the Bay to another, and the resultant curvature of
the laylines. In this case the quickest (red) route takes just under 36 min, and the
slowest (green) route takes 41.6 min, a difference of about 5.7 minutes for this one
upwind leg. This illustrates how route selection in venues with variable currents is
critical to racing success. Only Advantage, the latest in our line of tactical software,
is designed for variable current and is able to compute these effects. |
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