Marblehead - Halifax Race.
If you have haven't done the Marblehead to Halifax Ocean Race (MHOR) it is well
worth considering. Begun in 1905 and formalized as a biennial race in 1939, the
MHOR has become a very popular North-East USA event. It is a challenging race
and also offers easy access to Nova Scotia, Cape Breton and Maine - some of the
best cruising around. Check out the official
MHOR site for all
SailFast uses an
"isochron" methodology to find the fastest route between any two points. An
isochron is simply a locus of points showing where a boat can be at a certain
time. Wind predictions and boat polars are the major inputs. When current is
significant the optimum route can be meaningfully affected. A good knowledge of
all three factors can give you a nice "unfair" advantage in offshore racing. For
Gulf of Maine races SailFast uses built-in tidal data. Grib files with current
data are also supported, which is appropriate for other venues, such as Bermuda
races through the Gulf Stream.
So does current
really matter? Below are a few interesting examples.
Using Sailfast I set
a starting position about 75 miles due West of Brazil Rock. This is above the
rhumb line from the Marblehead start. Perhaps half the races in recent years
have had winds that tend to put the fleet above the rhumb line. Using the
What-If dialog in SailFast the wind was set to be a fixed 10 knots coming from
75 degrees true. A Grib wind forecast could have been used but this keeps the
analysis simple and lets us just look at current effects.
The screen shot
below shows an isochron solution with a predicted ETA to the finish of 40.5
hours. This optimum course takes a long first tack below the rhumb line, then
way back up before the third tack comes back down close to Brazil Rock.
Click any picture for a larger image.
This next screen shot is the sailing solution for exactly the same conditions
except that the Gulf of Maine current has been disabled in the Tools/What
If/Current dialog. Normally this is done when you aren't sailing in the Gulf of
Maine, but we can use it here to see the current effects.
Without current the
optimum track's ETA to the finish is 42.8 hours, which is 2.3 hours longer than
with current. Perhaps more interesting than the time difference is the course
difference, with a long first tack above the rhumb line followed by a long tack
down near Brazil Rock.
Let's go back to the real world case with current. What happens if we sail above
the rhumb line for a while instead of sailing down on the first tack? It's not
obvious from the isochrones how that will change the ETA. To answer this
question in SailFast the sailing boundaries can be adjusted to restrict the
sailing to certain areas.
In the case below
the lower sailing boundary has been set much higher, preventing an isochron
solution below the rhumb line for half the distance to Brazil Rock. In this case
the finish ETA is 41.2 hours, or 0.7 hours slower.
Now let's say we are a little nervous about going well above the rhumb line and
relatively close in shore before getting to Brazil Rock. We are concerned that
the wind may become light and we don't like the risk. What happens to the ETA if
we stay farther offshore before passing Brazil Rock?
Here the top
boundary line has been moved down. The finish ETA is now 40.9 hours, or 0.4
hours slower than the optimum track with no boundary constraints.
With Grib wind
forecasts available the analysis gets a lot more complex. Fortunately Sailfast
handles the calculations easily. Of course the data SailFast uses is never
perfect, and wind and sea have a habit of doing the unexpected. But how the
navigator combines sound judgement and experience with the tools available can
make all the difference. And let's not forget the importance of the sailing
happening on deck. Hey - this is what makes sailing fun!