Computing Optimal
and Comparison Solutions with the Force 4 Router |
| Variations in wind and current, from point to point and over time, can lead to large
differences in the time to sail different routes to the mark. It just isn't possible to
sort out and weigh the various possibilities in your head. Routing software does the hard
work, applying your boat polars (to estimate boatspeed for different wind angles) and wind
and current predictions to find the quickest route. All programs by other vendors employ a
method called "isochrones," which results in a single predicted
"optimal" route, and a complicated pattern resembling a spiderweb. In principle
this pattern tells you more information, but in practice it is next to impossible to
interpret. Force 4 takes a different approach. Our router builds a fine web of points
between the start and final destination, and computes every possible route connecting
them. This takes a few seconds longer, but produces more accurate results and more
information. In the example below, the router finds the quickest way from the boat, upwind
to mark "48." As a result of current variations over the venue, there are
significant differences between possible routes. The router computes an optimal route,
shown in red, and up to 5 comparison routes, shown in different colors. The corresponding
times are shown in the "Best Course Comparisons" box (here in minutes). For
example, there is over a 3.5-minute difference between the red and yellow routes.
It can be just as important to know the time-difference between alternate routes as to
know the optimal route. Sometimes the time-differences between several routes are small,
and there may be reasons to prefer a slightly slower course (such as to avoid shallows, a
hazard or to cover the competition). Often there is a "band" around the optimal
route within which you can sail without significant time difference. Force 4 is the ONLY
program to compute and display quantitative comparison solutions.
Note: the route does not show the boat heading, except in a reach, but rather the
location in which you want to sail. Upwind or downwind, this will involve tacking back and
forth across the route direction. |
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| In the above example, Alcatraz Island lies beween the boat and the mark. Force 4
supplies data such that the router "knows" the shorelines, islands and other
obstacles. Note how the central "black" route skirts around the north end of
Alcatraz. Third-party routers either ignore land, or identify it by the corresponding
color of "water" on the chart. The latter method is practical only for
relatively simple charts. Many charts, such as BSB, Maptech and Softcharts, have many
shades of color for individual pixels, which make them unsuitable for this technique. ONLY
Force 4 routes independently of the underlying chart. Once computed, the solution is
redrawn if you change charts. |
Stepping Along
Intermediate Timepoints of the Force 4 Solution |
| When you click the "A" (for animation) box at the lower right of the
Comparisons box, the stepping control appears at the bottom of the box, as shown below.
Click the forward and back arrows to move between the timepoints, or type in the time of
your choice on a given day. The black dot on each of the routes shows the predicted
position of the boat at the timepoint. The right-hand column in the Comparisons box shows
distance to the destination from each black dot. If you click the yellow "T"
option box, the display will reset to the date and time of the timepoint, so you can see
the winds and currents at that time. In the example below, winds are shown at 2213 hours
on June 14, 2002, during the Newport-Bermuda Race. |
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| A new feature of the 2006 edition of Force 4 is the "Winds" button on the
Comparisons box (above). When you click this, the display changes to the form below,
showing the minimum and maximum winds encountered during each of the routes. If a route
skirts the vicinity of a "hole" in the wind, the solution is sensitive to the
exact position and motion of the hole over time. A solution may be optimal because the
predicted wind allows the boat to traverse the edge of a hole, or pass through a region
before the hole arrives, but it may not work out that way if the wind prediction is off. A
low value of WMin (say 5 knots) signals possible sensitivity to the forecast. If the
predicted time for another route is not too much greater, but it has a larger WMin, it
might be a safer choice. Likewise, a boat might want to avoid a route where WMax is large
(say 40 or 50 knots). Polars predict that a boat will go at least as fast at 50 knots as
at 30, but, in the real world, wave action, and stress and strain, make this unrealistic
and risky. Correspondingly, the latest edition of the router allows you to specify a
miniumum and/or maximum acceptable wind in your optimal solution. ONLY Force 4 provides
this option and max/min wind information. |
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| Routers can be extremely effective in predicting the optimal route, and can put the
amateur navigator on a level playing field with the professionals (all of whom are using
routing software themselves!). The only caveat is the accuracy of the underlying wind and
current predictions, and that of the boat polars. You can check this as you go along by
comparing the measured (GPS) position of the boat (shown onscreen) with the corresponding
black dot of the routing solution at that time. If actual position and predicted position
track pretty well, it is likely that your weather and boat performance data is accurate.
If not, you will want to check alternate predictions, or fix polars. |
Force 4 Routing
Control Provides Many Options |
| Force 4 provides a UNIQUE set of options and features that control routing. The Router
control is shown below. You select a mark to route to or from by clicking on "Start
from Mark" or "End at Mark," which brings up the list of marks, organized
by region. You can route or from any mark you have defined, on any chart. You can also
constrain the solution to pass near one or more intermediate points. These are not marks
that you must round (in which case you route to them), but are used to focus the router on
a route that you prefer for other reasons (such as passing to one side of an island). You
can elect whether or not compute alternative routes, and can modify the resolution and
adjust the solution grid in several different ways. You can route with a wind model (grib
file), fixed wind or fixed boat speed, and modify a wind model by scaling and/or rotating
the wind vectors. You can change the parameters which determine the comparison solutions,
and eliminate solutions which include windspeeds below or above preset values (Wind Min
and Wind Max). The router always includes current drawn from our detailed tidal-current
models (see Local Knowledge currents), or from our custom ocean-current modules, available
for major races worldwide. |
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