| EFFECTS OF DIFFERENTIAL CURRENTS ON LAYLINES AND BOAT MOTION |
| When currents vary point to point, there are
significant effects on laylines and courses. Current not only pushes the boat offline, it
also changes the "true wind" felt by the boat. This is distinct from the
"apparent" wind, which differs from "true" wind as a result of the
boat's motion relative to the water. Current causes the boat to move through the
surrounding air, adding a component to the true wind, even at zero boatspeed. This changes
the "true wind angle," or TWA, experienced by the boat, and TWA is what
determines boatspeed. This is reflected in the boat's polars, which predict boatspeed as a
function of TWA. In tacking (or jibing),
there is an optimal TWA which maximizes velocity-made-good (VMG) towards the mark. If TWA
is affected point to point by current while tacking, the boat has to change direction to
the extent necessary to maintain the optimal TWA. In zero current, the layline is a
straight line along which the boat can sail to reach the mark. With varying current,
laylines are curved to reflect the actual path the boat sails at optimal TWA.
In the graphic below, the boat is tacking upwind towards
mark 46. The straight blue lines ending at 46 show the laylines as they would be in zero
current. The magenta dots show the laylines as they really are, corrected for differential
current. Current is shown at various points in the blue boxes, in knots, with a streamer
to show direction. To the left (west) of 46, current flows at about 2.5 knots in a NE
direction. Along the upper layline, the current flow is more northerly. Wind is assumed at
12 knots, and with current running to 2.6, there is a significant shift in TWA. Combined
with the direct effect of current on the boat, there is a dramatic shift in the laylines. |
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| Likewise, the boat while sailing to the
layline follows a curved path, partly to compensate for current and partly to keep TWA
optimal. The dashed-dot lines emanating from the boat show the track the boat will follow
on (optimal) port and starboard tacks. The dashed line from the boat shows where the boat
will intercept the layline if it maintains its present heading; Force 4 computes the
intercept and the time to reach it (the boat will not sail the straight dashed course, but
will arrive at the intercept at the time computed). |
WHAT-IF COMPUTATIONS IN THE PRESENCE OF DIFFERENTIAL CURRENTS |
| "What-if" computations compare
alternative ways to reach a mark. With constant current, laylines and boat motion are
straight lines, and any combination of port and starboard tacks will reach the mark at the
same time. With differential currents, there can be a big difference for different routes.
The simplest comparison is a two-tack solution, with either a port or starboard tack to
the layline, and from there along the layline to the mark. These calculations are just a
matter of geometry when there is no current, or constant current. For this reason ALL
other programs use an average current to simplify the computation. This cannot produce
accurate results when currents vary over the venue as they do in the example below. In
contrast, Force 4 cuts no corners, and accurately computes the time and intercept in the
general case, when both layline and boat motion are curved. In the example below, with the boat approximately 1 nm from the laylines,
there is more than a 2 minute difference between the favored starboard tack (to the lower
layline) and the port tack (to upper layline). This is a huge difference in racing (this
example used 1D35 polars, a fast boat). The red line connects boat position to the
computed intercept point while on the port tack (the dashed-dot line shows the actual
path). Likewise the green line connects to the intercept while on the starboard tack. The
solution box gives the comparison times, the optimal TWA and the heading to achieve this
TWA at present boat location (heading will need to change to preserve TWA until the boat
reaches the layline). |
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| These computations are difficult, but are
necessary to obtain dependable results. ONLY Force 4 has made the effort to do them
correctly, and ONLY Force 4 provides the graphic representation of actual laylines and
boat motion shown above. These calculations are updated every few seconds and adjusted for
windshifts as they occur. The "time-to-layline" computation gives you the
precise moment when you should tack. |