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THE PRINCIPLES
Calibration of a speed log involves nding the
mounting angle of the sensor (heading offset)
and the speed error at various speeds.
If the vessel has a laminar water ow near
the sensor, a single calibration point will be
sufcient. If the ow changes with speed,
(due to the friction of the hull, or nearby
constructions) then extra calibration points will
be required. The sensor is mounted on a pole
that can be turned to adjust the angle. This may result in a small angular error.
This can be calibrated away in the 2 axis version, but not in a single axis version.
This should be done before the main calibration procedure is performed.
Full calibration is designed to ignore water current effects. The procedure
requires the vessel to sail a xed length track at a constant speed. To remove
the current and wind effects, the same track should then be sailed in the
opposite direction at the same speed. The average of these tracks will be used
to calculate the speed difference between the real speed (measured using the
actual distance and the time it took), and the measured speed (using distance
from the sensor and the time it took). This procedure must be performed at least
once and then other speeds checked with the GPS.
Figure 3.1
If necessary, the procedure should be repeated at different speeds. SKIPPER
recommend that calibration is performed at a low speed and a high speed. If the
user is not able to turn the vessel and is sure there is no current, the calibration
procedure can be stopped after the rst leg and saved as a calibration point.
This will give values that are correct for that particular condition. (If current is
present, the speed through water will contain an error).
Boat
Speed of water
Forward
Mounting offset
Callibration parameters
5.0
0.2
1st leg
2nd leg
4.0
0.2
Callibration procedure
A
B
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