The article series
- Encoders should therefore be adjusted
- Adjusting and calibrating the compass
- Setting the echo sounder
- Check log
- Adjusting the wind sensor
Perhaps you are familiar with this problem? According to the plotter, there are only a few minutes left before the layline on the display has crept so far that the system signals: Time to tack! According to the calculation, the course on the new bow is clear. So let's get round. But as soon as the sails are trimmed over the stay and on the new bow, the wind seems to have shifted. There is no longer any question of a layline to the waypoint; there are easily ten degrees of height missing. To confirm this, the wind indicator also shows a shift.
One tack later, the same game, again the wind seems to shift just during the manoeuvre. Is it just bad weather, or is there some kind of system behind it? A glance at the compass doesn't provide any real insight. According to the steering compass, the boat is travelling at around 200 degrees, the autopilot shows 190 degrees, and the GPS reports a course of 220 degrees over ground.
Measurement errors propagate
Such deviations can almost always be explained by incorrectly adjusted instruments, sometimes even by faulty installation. A single measurement error propagates itself across several values: with the wrong speed through the water, the system calculates the wrong height in relation to the true wind, and therefore the wrong tacking angle and the wrong geographical wind direction. The cruising sailor is suddenly unable to reach the waypoint he thought he had before tacking, and the regatta skipper sees wind shifts where there are none.
This widespread problem can hardly be completely avoided, but it is possible to trim the measured values from the compass, echo sounder, log and wind so that their display comes as close as possible to the actual values. However, this involves some time and computational effort. Whether reliable data is worth this effort is up to each skipper when it comes to logs, soundings and wind. Not the compass, however, because according to the Federal Maritime and Hydrographic Agency, it must be properly compensated.
Calibration or adjustment?
Colloquially and in the manuals, there is usually talk of calibrating the instruments, but strictly speaking it is merely a matter of adjusting the displays using data that is assumed to be reasonably known. This is still better than simply hoping that the factory settings of the instruments match the ship. Real calibration would require a calibrated reference, and any remaining errors would have to be accurately documented. There is virtually never a calibrated reference on board and documentation is only produced for the magnetic compass in the form of the deflection table, if at all.
So we leave it at adjustment. However, it is still useful to document the remaining errors, at least for the log. The paddle wheel can usually only be set precisely for one speed.
Observe the sequence
To ensure that the adjustment is successful, you should pay attention to the sequence. The log, plumb line and compass work independently of each other and should be adjusted first. The wind indicator, on the other hand, not only provides the apparent values measured by the transducer at the masthead, but also uses the boat speed and course to calculate the true wind direction and strength as well as the geographical wind direction. It can therefore only be set properly if the log and compass provide reliable data. Some systems even calculate the apparent wind angle, taking speed-dependent correction factors into account.
The situation is similar with the autopilot. Sophisticated systems take the boat speed into account. It is therefore important to adjust the log before starting to tune the autopilot.
The article series
- Encoders should therefore be adjusted
- Adjusting and calibrating the compass
- Setting the echo sounder
- Check log
- Adjusting the wind sensor
