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As your downwind performance improves and we look for more ways of
going faster, the attention shifts to the quality of the information
being used. How reliable are the target speeds, is the builders polar
diagram accurate for your particular boat and sails, is the speedo, wind
angle and wind speed, calibrated accurately ?
Many of the latest instrument systems can provide a bewildering array
of useful computed functions such as true wind angle, VMG, COG (course
over ground), SOG (speed over ground), calculated from data only sampled
from sensors once or so a second. You may think that a constant
instrument display of VMG would be the ideal answer to finding the
fastest angles downwind . Used intelligently, it can provide a useful aid
as long as the user is aware of its limitations and the errors which can
creep in from the way in which the function is computed. The fastest
instrument systems sample data from the various sensors; boat
speed,compass, wind angle, every four seconds and others can be less
frequent.The effects of heel angle on a fluxgate compass can cause error
of up to 4 degrees for every 10 degrees of heel. The wind cups at the top
of the mast require around 0.2 knots of breeze before they begin to
rotate which should be allowed for in the calibration as should an
allowance for up-wash of air flow up the rig, artificially accelerating
the flow experienced at the masthead.
If you are using a VMG read-out in a sprit boat to find the fastest
reaching angle, it is vital that the instruments operate fast, otherwise
the information you are trying to steer to will be from the previous gust
or wind shift. In the same way that the speed of computer operation is
measured by its Baud rate, the fastest instrument system operates at
28,800, others at 9,600 and the NMEA system at just 4,800. So if you are
looking to invest in a VMG instrument for a sprit boat check out
carefully the specifications and how fast and often it will update before
you start to trust the information it provides. In the reality of the
race course, the average VMG instrument is unlikely to be fast enough to
supply you with the quality of data you require. Conventionally rigged
boats are better off in my opinion using target speeds and have an
eagle-eyed tactician calling the shifts and choosing when to gybe. Never
forget that windshifts do not stop downwind. If you gybe the boat on each
major windshift you actually sail a shorter, straighter course to the
mark.
For sprit boat sailors racing in a handicap fleet, without the benefit
of similar asymmetric boats around them to judge the fastest angles, I
guess a VMG display would be a useful starting point to learn the best
angles to sail, but after a while you will learn to feel what angle the
boat likes for each wind strength. It seems to me that the best helms in
the Melges 24 class are mainly ex-dinghy sailors who retain a
'seat-of-the-pants' feel for keeping the boat on the plane and for
playing the gusts and waves offwind. There is a further point to be aware
of when using VMG displays for offshore or round the cans racing; that
VMG displayed gives you a velocity made good downwind value and may not
necessarily take you directly towards the leeward mark. Zig-zagging
downwind with the asymmetric up it can be all too easy to fly past the
mark half a mile away if you are just chasing the optimum gybe angles in
a shifting breeze. More useful is a VMG display linked into a GPS (Global
Positioning System), which can give you an actual VMG to the mark and
helps to guide you into the mark. This is where on board tactical
software systems or just an on-screen GPS plot can be a great help,
providing a visual picture of your track into the mark and even giving
such information as time and distance to go to the layline. We found our
way to a mark in thick fog in the middle of the English Channel in the
Rolex Commodores' Cup offshore race just using a tiny Magellen hand held
GPS , which could be easily used on the smallest of sprit boats.
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Target speed is the speed the boat needs to to be going
to achieve the maximum velocity made good, either upwind or down. The
simplest way to use target speed information is to have a chart of
target speeds for every 5 knots of wind stuck up in the back of the
cockpit in easy view of the helmsperson and tactician. Once you know the sort of speed the boat should be going in 15 knots it
is then just a matter of finding the right groove or wind angle that
will provide that target speed. Write the wind angle next to the target
speed on the chart to give the helm an idea of where he/she should be
headed. Offshore navigators often have this data provided by computer,
on rugged marinised PC's so that they can be used on the rail.
