The Importance of True Wind Direction for a Sailing Drone

 The Importance of True Wind Direction for a Sailing Drone - 4th Bass Strait Voyage

An opportunity arose on the evening of 22/3/2024 to commence a passage in Bass Strait from Torquay to Western Port. An 80km (50 mile) passage that would be expected to take around 48 hours.

The mission failed within a couple of hours, with the vessel running ashore before midnight of the same evening.

Torquay Fishing Beach 7pm launch

Conditions were calm as the vessel ran downwind to the lee shore.

The vessel was found with minor damage on the beach.

The first question was whether the damage occurred at sea and caused the failure of the mission, or whether the damage occurred when the vessel was beached.

The equipment housing was completely intact and the equipment was fully operational.

Later analysis of the log files contained on the SD Card showed that the failure was due to the sailing algorithms making bad decisions.

Midnight on the beach

Spot GPS Satellite position reports

The sailing algorithm aims to get the vessel to the next waypoint, while remaining within a maximum permissible cross track error (CTE) from the rhumb line. 

It does this by trying to sail the course, using the favoured tack.

If it can't make the waypoint directly, it will sailing until the CTE reaches the Maximum CTE (boundary) permitted for the current waypoint and then change tack.

If it is unable to directly sail for the next waypoint, it is assessing at all times whether it is able to reach the next waypoint directly on the other tack.

The determination of whether it is possible to sail directly to the waypoint on the other tack is a key issue.

Analysis of the logs showed the following:

• The vessel could not quite sail directly to waypoint.
• It gradually reached CTE Max on the leeward side of the course (port side of course) on starboard tack.
• Having reached the boundary, it gybed on to port tack.
• Then it reassessed the course to the waypoint  (if it were to change starboard tack) and decided it was directly sailable.
• So, it gybed back on to starboard tack again, but it wasn't directly sailable, and it was still beyond Max CTE.
• The process repeated.
• The effect was to gybe and gybe again. effectively running downwind, until it beached on the lee shore.

Position Log from SD Card

The key issue was the wrong assessment of whether the waypoint could be directly reached on the other tack.

When racing sailing a full size yacht it can be tricky to assess when to tack to make the next mark without overlaying or underlaying.

The algorithm for assessing whether the waypoint can be reached directly on the other tack is as follows:

• Compare the Bearing to Waypoint (BTW) with the True Wind Direction (TWD) less the minimum sailing angle from the wind, less an additional sailing margin.
• The sailing margin is currently set to 25 degrees. This is effectively a measure of how much to overlay the mark before considering it is sailable.

Correctly estimating the TWD is critical for making correct decisions, and this was the source of the problem.

The determination of TWD is performed approximately as follows:

• True heading of vessel (HDG_T),
• less Apparent Wind Angle (AWA) 
• less correction adjustments to include an approximation for boat speed..

So the main requirements for determining TWD is to measure an accurate vessel heading, and apparent wind angle.

The main problem with Voyager 2.7 was the accuracy of the magnetic heading.

The following plot illustrates the relationship between the vessel heading and the calculated AWD, as it was during the failed mission.

The plot was prepared from data recorded on the SD Card, while the vessel was rotated on a turntable to assist gathering measurements.

It shows that the AWD varied by up to 47 degrees with different headings.

AWD versus Heading as it was for the mission

The compass was upgraded by removing the UFSF Max IMU , and replacing it with the LSM303.

The improvements brought by the new compass, and careful calibration, and minor adjustments to the calculations greatly improved the results.

The resulting plot of AWD versus Heading after the upgrades performed, yielded significant improvements, as seen by the near flat orange line.

The variation of AWD was less the 6 degrees with heading completing three full turns.

AWD versus Heading after upgrades

Then end result should yield a much more accurate AWD and hence estimated TWD.

The next ocean voyage should be telling.