Calibration

As the northern hemisphere sailing season is approaching, pre season shakedowns to check sails and systems are starting to happen. At the top of everyone's list should be to check calibrations. Rigs have been out, speedos have been cleaned and new parts installed and gear moved.

1. Speed is naturally at the top of the list and is pretty straight forward to get the baseline cal checked without disturbing any linearity and heel offset calibrations.

2. MHU offset to zero out the MWA should be the next on the list, and again pretty straight forward to identify any errors and apply a correction without disturbing previous seasons TWA calibrations.

3. It may seem that the next sensible calibration to check is TWA - to remove tack to tack, reach to reach or gybe to gybe TWD shifts. Before any adjustments are made to your TWA table however, it is important to remember that a major contributor to TWD 'wobble', is your compass.
   

Magnetic sensing compass should be put through their respective manufacturers calibration process at regular intervals, especially when sailing in a new location. It is a simple process and takes very little time.

How good is my compass?

Even post calibration is it important to know your compasses limitations - its error margin. Magnetic field sensing compasses are very good, but in the real world only offer +/-2 deg of accuracy in ideal conditions. This can equate to a 4 deg error from tack to tack. Couple that with any error in your leeway calibration and you may see a TWD tacking through up to 10 degrees before TWA offsets can even be considered.

Further, measuring current reliably and accurately very much depends on your compass. One degree of compass error will manifest itself in a large variation in current calculation.

High end instrument systems such as A+T and FaRo recognise this and have tables that can be edited to offset these errors. This one of the primary reasons that high quality GPS and/or FOG (Fibre Optic Gyro) compass solutions are chosen by high end race teams.

Unfortunately these high end AHRS solutions come with a hefty price tag - FOG solutions typically cost many tens of thousands of pounds/euros/dollars and as such are out of reach of most teams.

However, GPS compasses are available at a more sensible price point, but be careful, not all GPS compasses are equal. Off the shelf GPS compass solutions from many of the main marine electronics suppliers are not suitable for most sailing yachts due to the high rates of turn in manoeuvres such as pre start and mark roundings. The heading solution can degrade to a point where they are rendered useless, or their accuracy is worse than a regular mag field sensing compass.

Specialist GPS compass solutions are available, but are not necessarily 'off the shelf' and require setup and interfacing from a specialist marine instrumentation installer.

One stand out product is the SBG system's Ellipse D product. They are not the only solution available in their target market space (survey, aeronautics, vehicle, defence etc), but are being adopted by teams that are looking for a very reliable AHRS (attitude and heading reference system), and have a flexible toolkit to be fitted to most yachts and instrument systems. Still not for the price conscious, but at sub £10k are within reach of most.

An example of the real world difference between two well respected magnetic field sensing compasses and the Ellipse D is shown here. 24 hours of logged data at two headings (gybes) shows the performance difference between the systems:

The results speak for themselves.