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IMS is offering different methods of calculating corrected times.
Each method is suitable for different level of competition, wind conditions,
type of races or size of the fleet. Scoring method should
be defined in the Sailing Instructions and list of all ratings
shall be available to all competitors before the start.
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Performance
curve scoring |
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Performance curve scoring is the most powerfull engine of the
IMS. The unique feature of IMS, making it
fundamentally different from any other handicap system and much
more precise, is its capacity to give and rate different
handicaps for different race conditions because yachts do not
have the same performance in different conditions.
This means that yachts will have a different time allowance in
each race depending on the weather conditions and the course
configuration for that particular race. This gives consideration
to the fact that yachts can behave very differently according to
their characteristics. For example, heavy under canvassed boats
are slow in light airs but fast in strong winds. Boats with deep
keels go well to windward and light boats with small keels go
fast downwind. The use of system is not at all
complicated as it may look.
For any type of course for a given race a set of 7 values is
derived by the computer program constructing each of them as a
function of length, bearing and wind direction for each leg of
the course. The points so obtained are linked by a curve which
represents the predicted optimum performance along a scale of
wind speeds. This curve is called the Performance Curve, and for
each yacht this curve is different for any different course
sailed.
Figure
The vertical
axis represents the speed achieved in the race, expressed in
seconds per mile. The horizontal axis represents the wind speed
in knots. When the finishing time of Yacht A is known, its
elapsed time is divided by the distance of the course to
determine the average speed in seconds per mile. This number is
represented by point A on the vertical axis. The computer then
finds the point on the horizontal axis that corresponds for that
course to the average speed obtained. This results in point Aw,
the so called “Implied Wind”. This means that the yacht has
completed the course “as if” it has encountered that wind speed.
The faster the boat has sailed, the higher the Implied Wind,
which is the primary index for scoring. The yacht with the
highest Implied Wind wins the race. The Implied Wind is then
transformed into a corrected time.
Figure
The Implied
Wind is intended as an interpolation between time allowances,
not an extrapolation. This means that when the Implied Wind
drops below 6 knots or raises above 20 knots, the time
allowances used for calculating the corrected times will be
those of 6 knots and 20 knots respectively. This does not mean
that IMS races need to be stopped (or not started) with wind
below 6 knots or above 20. When the “implied wind” results to be
lass than 6 knots or more than 20, the respective values are
used, as using a “fixed wind” option.
Figure
In order to
present the result of the race in a comprehensive format we use
a “Scratch Boat”. In most cases this is the potentially fastest
boat of the fleet, yacht B. Her Performance Curve is the lowest
in the drawing. From the point where the vertical line yacht A
intersects with the curve of the Scratch Boat, a horizontal line
is drawn to the left towards the vertical axis. This point, Ac,
produces the corrected time when the seconds per mile are
multiplied by the distance of the course in miles. The
corrected time of the Scratch Boat is, by definition, same as
its elapsed time. This exercise produces corrected times,
expressed in hours, minutes and seconds, a format most sailors
are familiar with.
A more detailed explanation of the Performance curve scoring
can be downloaded as
PDF file or
Power Point presetnation.
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Performance line |
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Performance Line Scoring is a simplified
variation of Performance Curve Scoring where performance curve
is approximated by the straight line intercepting the
performance points of 8 and 16 knots of wind for a given course.
The base points can be based on different wind speeds, obtaining
different coefficients. Corrected time is calculated by two coefficients as follows:
Corrected time
= (PLT * Elapsed time) - (PLD * Distance)
With the time
coeficient PLT and distance coefficent PLD, two boats may be
rated differently in the light or heavy wind conditions. While
length of the course is fixed value, it is not the same for two
boats if the course is sailed in shorter or longer time and it
may happen that one boat is giving a handicap to another in
light wind conditions, while there will be opposite in heavy wind
conditions. |
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Time on distance |
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Corrected time is calculated by the distance coefficient as
follows:
Corrected time
= Elapsed time - (TOD * Distance)
This is very often
method of calculating corrected time. With TOD coeficent time
allowance of one boat will not change with wind velocity, but it
will change with length of the course. One boat will be giving
to another alwas the same handicap in s/NM, and it is easy to
calculate difference in elapsed time between two boats needed
for win in corrected times. Time on distance method is best in
use in the constant wind conditions. |
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Time on time |
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Corrected time is calculated by the time coefficient as follows:
Corrected time
= TMF * Elapsed time
With time-on-time
scoring, time allowance will increase progressively as the wind
velocity increase. Course distance has no effect to the results
neither needs to be measured. Corrected time will depend only on
the elapsed time and difference between to boats may be seen in
seconds depending of the duration of the races. The longer the
race, the bigger is handicap. Time on time method can be
problematic to use only when no wind situations can be expected
on the race course. |
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Triple-number
scoring |
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The Triple-Number system provides a set of three time
multiplying factors TMFs (as described above for Time on time)
for each of two course types, a) Windward/Leeward and b)
Circular Random. Within each of these two course types TMFs are
given for three wind ranges; 1) Low Range, 2) Medium Range and
3) High Range. In practice, the course selection for a race is
pre-specified and the race committee signals at the start the
wind range to be used for scoring.
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Course types |
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Different scoring methods can be caluculated for different course types:
Constructed course determines the correct wind angles
over the course constructed leg by leg and defining: wind
direction, length and direction of each leg and optionally, the
direction and rate of the current on each leg. Any leg can
be split in sub-legs in case there is a marked shift.
Once all data are entered, the computer calculates the mix of
wind angles and distance through the water for each yacht for 7
wind speeds, corrected for the tide in it is also entered. The
constructed course method is usually associated with the
performance curve system, but in particular cases or for special
reasons it can also be used with fixed wind speed
Constructed course with fixed wind speed
- The course constructed table of time
allowances provided by the scoring software can also be used
with a fixed wind speed. This can be a single wind speed for the
whole race, or split in a wind speed for each leg of the
constructed course.
Windward/Leeward
course (up and down) is conventional course around windward and
leeward marks where race course consist of 50% upwind and 50%
downwind legs.
Olympic 6-leg is an Olympic
triangle type of course, with approx. 55% beat and 45% reach and
downwind angles, so is best suited for inshore round the buoys
races.
Ciruclar random course is
calculated simply averaging the boat speeds around the circle,
from VMG downwind to VMG upwind in 2 degrees intervals. It can
be represented by a hypothetical course sailing on a perfectly
circular course (for instance around an island) in a steady
wind.
For Non-spinnaker course,
the same procedure used to calculate
Circular Random is performed but without spinnaker. In case of
mizzen mast rigs, the calculation will include a mizzen staysail
if the yacht was measured with one.
Ocean Course
type is best suited for long distance/offshore races
with a relatively high content of reach and downwind angles.
This course type can be used where the fleet will be widely
spread, a wide range of wind and sea conditions can be expected
and which cannot be accurately predicted. It is a composite
course, the content of which varies progressively from 30%
Windward/Leeward and 70% Circular Random at 6 knots of wind
speed to 100% Circular Random at 12 knots of wind speed and then
to 20% Circular Random and 80% reaching at 20 knots of wind
speed.
Time on distance or time on time coefficients in the single
number handicaps can be calculated either as GPH or ILC.
GPH means
General Purpose Handicap and it is on of the most important
numbers in IMS/ORC Club that is used for general comparison
between two or more different boats. It is calculated in s/NM as
average of of Circular Random time
allowances for 8 knots and 12 knots wind speed.
ILC is calculated as combination of time allowances
in s/NM for VMG beat, Reach of 110 degrees to wind direction and
VMG downwind for wind speeds of 6, 10 and 20 knots.
When calculating time-on-time single number handicaps
600/GPH is used for offshore TMF, and
675/ILC is used for inshore
TMF. |
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