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Calibration procedure

Calibration of instrument systems is necessary to make displayed information correct and useful..If liftsThus far you have set the majority of the calibrations in the system, which provide the "coarse" tuning of the system. The last two, Cal Windspeed and Cal Upwash, are very powerful calibra-tions which have substantial effect on the Wind Direction solution. This makes these two calibra-tions very effective in fine tuning the Wind Direction solution and allows further tweaking for differ-ent wind strengths.

Automatic calibration

For those without the time or inclination to manually calibrate, Ockam offers an automatic calibration service called DeWiggler™. For more information, go to the DeWiggler page.

Manual calibration

This is the traditional way to calibrate an Ockam system. All you need is a good day, some crew, basic math skills and a screwdriver.

In this page, we will concentrate on correcting sensor inputs to adjust for things like boundary layer (paddlewheels), upwash (masthead units), and installation variables such as compass deviation or sensor misalignment.

The actual readouts covering calibration are listed here.
Manual calibration procedure
Preliminary checks
  • If this is a new installation, check that the calibrations are set to reasonable values.
    Cal boatspeed master=1.06 Cal boatspeed offset=0.000 Cal leeway=7.0 Cal Windangle offset=0.0
    Cal Windspeed=1.09 Cal Upwash=0.0 Cal Upwash Slope=0.000
  • Check that heel angle is zero with the boat upright. Confirm you are getting a reasonable reading of apparent wind speed.
  • Go in a slow circle and confirm that the compass readout agrees with the binnacle, and the apparent wind angle (not true wind angle) readout agrees with the masthead vane. Do this all the way around the clock. If you turn up any problems, you should fix them before proceeding.
  • Check the errors readout. You may get errors 33 thru 35, but if the vane passes the above check, these errors can be ignored.
  • Heel the boat to port. The heel angle readout should be positive. If it isn't, flip the heel sign switch on the masthead interface.
A word about your compass

Wind direction is built upon the instrment's compass. Any error in it reflects 1 to 1 in wind direction solution. There is no manual way to confirm its accuracy. Traditionally, compass adjusters would be called in for this. But nowadays, they're way to few of them. One thing you can do is to sniff around the area where the compass is mounted with a hand-bearing compass to see if there is any iron in the vicinity.

Most people assume that modern compass auto-calibration guarantees a good heading. Nothing could be further from the truth. If the compass is mounted in the wrong place (e.g. near iron), no autocal will correct for it. DeWiggler provides a compass deviation table, so it actually provides a calibration for which there is no normal manual equivalent.
Calibrate boatspeed and windangle offsets
Do these calibrations only if no signifigant wind shear is present. Here's how to tell.

For all the sailing calibration time, you should look for a developed breeze of around 12-14 knots, generally later in the day. Choosing this type of condition helps to reduce the possibility of wind shear (changes in wind speed with altitude) and wind gradient (changes in Wind Direction with altitude) which are often present early in the day, when the breeze is light or the sea breeze is developing. Cold water and warm air (especially in the spring) can also cause wind shear and gradient.

For offsets, you will want to sail upwind to gather sufficient data to calculate the necessary changes in the calibrations. This normally will require between 30 and 45 minutes of concentrated upwind sailing. The purpose of these two calibrations is to develop symmetry in boatspeed and apparent wind angle readings from one tack to the other. Thus, even with a boatspeed paddlewheel that is off centerline, and a masthead unit which is slightly angled to one side, you will be able to achieve the same boatspeed and apparent wind angle readings on each tack.
  1. Sail the boat close hauled with careful attention to the details of trim on one tack. You will want to duplicate the same trim settings on the other tack. You should sail by the telltales or the angle of heel or a method in which you can easily and consistently keep the boat "in the groove". The idea here is to maintain symmetry in the way you are sailing the boat, and to keep the boatspeed and apparent wind angle as stable as possible.
  2. Record the boatspeed and apparent wind angle on each tack as often as you can while the boat is in good stable trim. Allow the boatspeed to accelerate and then level off after coming out of a tack. It is important to collect meaningful data from each tack.
Print out the Offsets Worksheet

Calibrate boatspeed
Boatspeed transducers measure water flow close to the hull, but have to be adjusted to read the boatspeed thru the water. The reason that flow near the hull does not equal boatspeed, is that the hull distorts the flow near itself. Calibrations for boatspeed are therefore required to compensate for hull shape and the position of the transducer.

Boatspeed can be calibrated in many ways: timed runs over a measured distance, comparison with a good standard (i.e. another boat known to be well calibrated, or a towing calibrator), dead- reckoning, or a combination of these. You should use the best standards available and should continue to further improve the calibration as you gain more experience.

If you use calibration by time between marks, make timed runs over an ACCURATELY MEASURED distance of at least 1/2 mile, going over the course in both directions to negate current effects. Remember to keep as straight a course as possible, because sinuous courses always make the actual distance traveled longer than measured. Also, if you are powering or being towed over the course, prop wash will make the indicated boatspeed higher than actual. Both of these effects tend to make your calculated boatspeed lower than it actually is. Take the log readings over the course in each direction, trying to interpolate to 1/1000 mile.
Repeat the procedure several times until the applied corrections are less then 2%, which is about as good as running a measured course can do.

Print out the Boatspeed Worksheet

Calibrate Apparent wind speed and Upwash
Thus far you have set the majority of the calibrations in the system, which provide the "coarse" tuning of the system. The last two, Cal Windspeed and Cal Upwash, are very powerful calibrations which have substantial effect on the Wind Direction solution. This makes these two calibrations very effective in fine tuning the Wind Direction solution and allows further tweaking for different wind strengths.
  • Check that QuikCal is set to zero.
  • Get warmed up by sailing upwind and rechecking your work on the boatspeed and apparent wind angle offsets. Besides confirming your previous efforts, this exercise will hone your senses for the real excitement.
  • Then tack or jibe back and forth at the appropriate apparent wind angle. The important idea here is to steady the course of the boat down once you are close to the required apparent wind angle. The wind angle is not as important as good data achieved by a steady compass heading.
  • On you feel that the Wind Direction has settled in, record the data. Then tack or jibe over to the other board, and reestablish a steady course.
  • With plenty of Wind Direction data, at least 6 to 8 sets, you can calculate the change in the calibration.
  • Finally, sail back upwind using the same care to develop symmetry in sail settings and steer-ing technique. Concentrate on "groove" sailing, not "scalloping", to enhance your data col-lection.
Print out the Windspeed & Upwash Worksheet