Speed calibration unfortunately requires a lot more effort than just utilising an instrument systems auto-cal procedure.
Auto-cal routines are a good way to get your speed reading accurate for the exact conditions that you have calibrated in, however the following needs to be considered:
Mechanical paddle wheel type sensors are inherently non-linear and typically under-read below the speed you calibrated at. This is due to the change in water flow near the hull (boundary layer) at different speeds.
Interference from any transducers or hull deformations up stream of the speed transducer, or a poor hull finish, will introduce errors and/or amplify the boundary layer issues.
Leeway and heel cause an off axis flow of water over the speed sensor , and heel moves the sensor closer to the surface for another potential boundary layer change.
Initial calibration:It is important to do a calibration run as close to a typical sailing speed as possible to set a speed datum. If your target speeds upwind are in the region of 7 to 8 kt through the TWS range, then ~7.5kt would be a good speed to calibrate at.
It is prudent to record a manual calibration alongside the auto-cal procedure. With a manual record you can cross check the new calibration numbers and if one run of many has an error or is interrupted you can still complete the calibration manually.
Recording a trip log distance on each run between two fixed points of a know distance will give you an idea of how much your speed sensor is over/under reading. From this you can calculate a delta % and apply this to the existing calibration.
Remember:
The more runs the greater the accuracy of your calculations
A longer run will reduce the effect of errors identifying the start and stop points of the run
More than two runs will give you a picture of any effect of changing current effects between runs 1 & 3 (& 5 etc) and 2 & 4 (&6 etc)
Linearity & heel errors:Different high end systems have varying methods of correcting errors induced by varying speeds or Heel/Leeway. Most use a lookup table of some form and adjust speed by either altering the calibration number itself or the speed in kt, with a value or % change. I personally prefer the value offset as it reduces the mental maths when adjusting errors on the fly.
A good rule of thumb for linearity correction is to increase speed reading in the order of 5% under your datum speed and reduce by 5% over, and refine from these values, remembering that the linearity correction is in itself not necessarily linear.
Heel error offsets are different from boat to boat as they are dependent on hull form and leeway angle, and again there is a variety of methods for addressing this in high end systems.
Quantifying errors:A powerful tool to verify the quality of your speed calibration is to monitor calculated current set and drift, noting any unreasonable changes in calculated current at different speeds or heel angles. An added benefit of this is to refine your leeway calculation.
If you are sailing in an area of very low or no current (rare!) then this is easy to compute.
However is a little harder in areas with significant current.
In no current:
If you always have current from behind, your speed is under reading at the specific speed that you are sailing at, and vice versa.
If your current is always tacking with you and coming from the windward side, your leeway is under-reading, and vice versa.
In areas with current, this process is a bit more complex but not impossible. If your navigation software has a logging or graphing tool this can make the process easier.
Even better, software such as Expedition can draw current vectors live on your track and you can graphically monitor changes in current set/drift live.
A tack from Stbd to Port showing a leeway error in current arrows
Bottom mark rounding (to Stbd) showing both a speed and leeway calibration error
Remember to always have a reasonable amount of damping on your current calculations to eliminate changes due to 'noise' from your COG/SOG and other inputs to that equation. Typically 20 to 30 seconds of damping is effective.
The overall speed calibration process is laborious and ongoing, but vital as speed (and leeway) are key primary inputs to your wind solution so will go a long way to making calibration of your TWA and TWS much easier.
Speed calibration unfortunately requires a lot more effort than just utilising an instrument systems auto-cal procedure.
Auto-cal routines are a good way to get your speed reading accurate for the exact conditions that you have calibrated in, however the following needs to be considered:
Mechanical paddle wheel type sensors are inherently non-linear and typically under-read below the speed you calibrated at. This is due to the change in water flow near the hull (boundary layer) at different speeds.
Interference from any transducers or hull deformations up stream of the speed transducer, or a poor hull finish, will introduce errors and/or amplify the boundary layer issues.
Leeway and heel cause an off axis flow of water over the speed sensor , and heel moves the sensor closer to the surface for another potential boundary layer change.
Initial calibration: It is important to do a calibration run as close to a typical sailing speed as possible to set a speed datum. If your target speeds upwind are in the region of 7 to 8 kt through the TWS range, then ~7.5kt would be a good speed to calibrate at.
It is prudent to record a manual calibration alongside the auto-cal procedure. With a manual record you can cross check the new calibration numbers and if one run of many has an error or is interrupted you can still complete the calibration manually.
Recording a trip log distance on each run between two fixed points of a know distance will give you an idea of how much your speed sensor is over/under reading. From this you can calculate a delta % and apply this to the existing calibration.
Remember:
The more runs the greater the accuracy of your calculations
A longer run will reduce the effect of errors identifying the start and stop points of the run
More than two runs will give you a picture of any effect of changing current effects between runs 1 & 3 (& 5 etc) and 2 & 4 (&6 etc)
Linearity & heel errors: Different high end systems have varying methods of correcting errors induced by varying speeds or Heel/Leeway. Most use a lookup table of some form and adjust speed by either altering the calibration number itself or the speed in kt, with a value or % change. I personally prefer the value offset as it reduces the mental maths when adjusting errors on the fly.
A good rule of thumb for linearity correction is to increase speed reading in the order of 5% under your datum speed and reduce by 5% over, and refine from these values, remembering that the linearity correction is in itself not necessarily linear.
Heel error offsets are different from boat to boat as they are dependent on hull form and leeway angle, and again there is a variety of methods for addressing this in high end systems.
Quantifying errors: A powerful tool to verify the quality of your speed calibration is to monitor calculated current set and drift, noting any unreasonable changes in calculated current at different speeds or heel angles. An added benefit of this is to refine your leeway calculation.
If you are sailing in an area of very low or no current (rare!) then this is easy to compute.
However is a little harder in areas with significant current.
In no current:
If you always have current from behind, your speed is under reading at the specific speed that you are sailing at, and vice versa.
If your current is always tacking with you and coming from the windward side, your leeway is under-reading, and vice versa.
In areas with current, this process is a bit more complex but not impossible. If your navigation software has a logging or graphing tool this can make the process easier.
Even better, software such as Expedition can draw current vectors live on your track and you can graphically monitor changes in current set/drift live.
Remember to always have a reasonable amount of damping on your current calculations to eliminate changes due to 'noise' from your COG/SOG and other inputs to that equation. Typically 20 to 30 seconds of damping is effective.
The overall speed calibration process is laborious and ongoing, but vital as speed (and leeway) are key primary inputs to your wind solution so will go a long way to making calibration of your TWA and TWS much easier.