Salt Water and Electronics - 5th Bass Strait Voyage

After quickly recovering from the prior launch and recovery on Friday 22/3/2024, the vessel was ready for another launch 11 days later on Thursday 2/4/2024, the 5th voyage on Bass Strait.

Following the prior launch and recovery on the Friday, the compass had been replaced and recalibrated yielding much better accuracy. The aim was specifically to improve the determination of the True wind Direction, which contributed to the failure as discussed in the last post.

Om that Friday night, the vessel had been recovered high and dry on the beach after it had been washed ashore, through the small surf.

There was little damage, but the sail and its electronics would have been submerged in the breaking waves on beach.

It was considered that there were no electronic problems with the Sail, and so the vessel was launched again on Thursday 2/4/2024.

Launch from Torquay 2/4/2024 at 8:15pm, soon after sunset

It sailed well, in the light to moderate conditions for the next 16 hours, and almost 15 miles down the course, at which point it failed to proceed down the course, and to started to drift back toward the launch place.

The conditions at that point were very calm.

The vessel drifted back to the west for 2 days and returned coincidentally almost back to the launch point. 

As the boat approached the shore it was possible to interrogate it using the LORA telemetry, when it was within around 2km of the headland.

It was possible to confirm that most of the components were operational.

However the wing sail had not responded to regular polling, since the time of failure, 2 days earlier.

This implied that the Wing Sail controller had failed.

The vessel was safely brought to shore by swimming out to retrieve it, to avoid it entering the breakers. 

Safely retrieved, and brought to shore before entering the breakers.

It was noted that wing sail servo was not operational, and was in the neutral position.

This suggests that it is likely that the last action performed by the controller was to partially reboot, and center the servo. If it had failed without involving a reboot, the servo would most likely have been set to one side or the other.

The electronics were switched off, and then switched back on again, and the Wing Sail returned to operation.

Later inspection of the Wing Sail controller revealed evidence of corrosion, despite the epoxy coating.

It appears that the epoxy coating was incomplete, and allowed salt water to come in contact with the electronic components.

It is believed that this was the reason for the failure of the Wing Sail controller.

It seems likely that the voyage 11 days earlier, where the Wing Sail would have been submerged in the sea water may have allowed salt to remain in voids or fissures in the epoxy coating.

Whilst dry, the salt would not have had any effect.

 

It seems likely that on the next voyage, the moist atmosphere would allow the salt to affect the circuitry.

Wing Sail Controller showing corrosion - top view

Wing Sail Controller showing corrosion - side view

To address this problem, the Wing Sail controller will be fully potted in epoxy.

A new PCB mount, incorporating a potting box has been 3D printed.

New Wing Sail Controller, located in 3D printed potting box

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.