A yacht’s instrument system can only be as good as the care with which it was set-up; ‘garbage in, garbage out’ is as true of a boat’s computer as any other, writes Mark Chisnell

These tweaks will help make any integrated instrument system accurate and effective, whether the goal is to win races, or cruise efficiently, comfortably and safely.

The real key to setting up any instrument is careful calibration. I hate to say it, but this is one of the times when it’s worth reading the manual. It’s particularly important to approach sailing instrument calibration in a systematic order.

A lot of the data is interdependent, with much of the most useful information derived from the measurement of only five values: boat speed; compass heading; heel angle; apparent wind speed; and apparent wind angle.

It’s vital to calibrate these sensors first, and only when they are done accurately move onto whatever calibrations are provided for other functions like true wind speed and direction.

Compass set up and calibration

Electronic compasses will have the exact details of the best calibration routine described in their manuals, but it usually involves turning the boat in a circle or two. This allows the compass to calculate its own deviation curve and compensate for the local magnetic environment.

For example, to set up a Simrad Precision-9 compass requires the yacht to complete a 390° turn with a steady turn rate of 2°-3° per second. Once the turns are completed the compass applies the new deviation adjustment and sends a message that it’s finished.

There are a few things to watch out for in this process. It’s a lot easier and more accurate in flat, open water with no wind and no current because the turns need to be steady and even – so it won’t work if the yacht has to adjust course to avoid hitting something during the calibration.

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Check beforehand that there is no magnetic material anywhere near the compass sensor, with everything stowed in its normal sailing position. Beware of mobile phones in pockets of people sitting on deck, often right above the compass fitted below.

Once compass calibration is done, check the physical alignment of the instrument relative to the centreline of the boat by sailing down a known transit line. I also always check the boat compass against any hand bearing compasses on board – it’s useful to know if they disagree, because then a deviation card can be created for the difference.

Boat speed

Boat speed calibration should be one of the first jobs completed on any new boat, or at the beginning of a new season – particularly on a race boat. The helmsman and trimmers will get used to the setting, and it will be disruptive if the sensor has to subsequently be calibrated after a lot of sailing time.

Again, each individual system has its own calibration routines. The B&G H5000 has three variations. The first allows the navigator to compare directly with the GPS speed over the ground. This is only useful when there is absolutely no current.

Secondly, boat speed calibration can be manually adjusted as a percentage of a previous value. Or, for old school obsessives like myself, there’s a routine for correction against a measured mile.

Whatever system is used, some general rules can be followed to get a more accurate result. Set up when the water is flat; the logs measure the water flowing past it and are not too choosy whether the flow is created by the boat moving forward or up and down. If the boat is pitching it will record more distance than has actually been travelled.

Do the runs at fixed engine revs to keep the speed consistent. If using a measured mile, make sure the speed is the same at the beginning and end of every run – otherwise the acceleration will affect the results – so don’t slow the boat during the turn between runs.

Always steer as straight a line as possible between the chosen distance marks. If it is a proper measured mile then the chart will provide the bearings.

Any wavering from a straight line means the log will measure extra distance that will not be accounted for in the calibration calculation.

And finally, if the measured mile is in a tidal area then three runs are required to eliminate the tidal error and get an accurate calibration.

Apparent wind angle

The only real calibration possible for the apparent wind angle is symmetry on both tacks. The critical thing is to set this up on a day when there is very little or no wind sheer – usually a day with a well-mixed, gradient (not thermal or sea breeze) wind.

Apparent wind speed

It’s impossible to do much with this one, unless you’ve got access to a wind tunnel! If you have concerns ask the dealer or manufacturer, as they should be able to check the sensor.

Heel angle

On a calm day set the boat up with slack warps in the dock and put all the gear in its normal sailing position – including boom and spinnaker pole on the centre line. Whoever stays on board should also stand on the centreline while they read the heel meter.

Under these conditions the heel angle should read zero; if it does not then adjust it till it does, either with a software calibration or by physically moving the unit.

First published in the July 2019 edition of Yachting World.