Measuring Angle of Attack of a Self-Trimming Wing Sail

An important performance characteristic of a self-trimming wing-sail assembly is the relationship between trim-tab deflection and the resulting angle of attack (AOA).
Within the normal operating range, this relationship is approximately linear.

A practical method for quantifying this relationship uses the existing control and sensing capabilities of the sailing drone.
The onboard system can both set the trim tab to a defined angle and measure the corresponding equilibrium orientation of the wing sail relative to the wind.

Calibration Method

A dedicated calibration procedure was implemented within the control software.
During calibration:

1. The vessel is held stationary in a steady, uniform breeze.
2. The software sets the trim tab to a known deflection and measures the resulting equilibrium AOA, averaged over several seconds to reduce noise.
3. The trim tab is then set to the same angle in the opposite direction, and the new equilibrium AOA is recorded.

The effective change in AOA produced by the trim tab is calculated as half the difference between these two measurements.
This process is repeated across a series of trim-tab deflections to construct a calibration curve of trim-tab angle vs. resulting AOA.

Illustation of the trim tab on opposite sides with the change in wing angle

The illustration below shows the measured response of the wing sail to a reversal of trim-tab deflection:

• On the left, the wing has rotated to an equilibrium position 52.5° from the reference line.
• On the right, with the trim tab reversed, the wing equilibrates at 22.6°.
• The total change in wing angle is 29.9°, giving an effective AOA of 15° (half the difference).

 

Wing Sail Foil Testing - Downwind Settings

The current algorithm used within Voyager to set the Wing Sail Trim Tab is simple:

• The trim tab is rotated clock-wise on starboard tack.
• The trim tab is rotated counter-clock-wise on port tack.
• The angle may be changed, but has typically been set to 15 degrees or majority of sailing.

The following three illustrations (figures 1,2 & 3) show the typical operation of self-trimming wing sails.

In each case, the wing sail effectively doesn't change while the vessel changes heading from beating, reaching to running, on starboard tack.

Figure 1. Starboard Tack Beating, Trim Tab rotated CW 15°

Figure 2. Starboard Tack Reaching, Trim Tab rotated CW 15°

Figure 3. Starboard Tack Running, Trim Tab rotated CW 15°

One issue with self-trimming wing sail operation that has not previously been clear is:

What is the best position of a trim tab whilst running.

Should the position of the trim tab be reversed when deep running ?

The following figure 4, illustrates the vessel running on starboard tack, with the trim tab reversed, and swapped to CCW position.

Figure 4. Starboard Tack Running, Trim Tab rotated CCW 15°

The Test rig was set  up for different headings, including deep running, and the load cells were used to provide a measure of relative forward thrust and force to leeward.

The aim was to measure thrust generated by the two strategies for trim tab position while deep running.

Once tests were performed to assess the forward thrust it became clear that reversing the trim tab is not a good idea when deep running.

Figure 5. Test Rig set for Starboard Tack Running- CCW Trim Tab

Figure 6. Test Rig set for Starboard Tack Running- CW Trim Tab

Experimental test results are presented in the following table.

In all cases the thrust was reduced when the trim table angle was reversed.

The last results with 157.5° off the wind and 20° trim tab showed a reduced difference.

At 135° off the wind the thrust appeared to be reversed.

Of course, at reduced angles to the wind such as 90°, the effect of reversing the trim tab will definitely cause the vessel to travel in reverse.

Figure 7. Test Results

Conclusion

The is no benefit in reversing the trim-tab position when deep running.

It is best to retain the behaviour where trim-tab is rotated CW on starboard tack, and CCW on port tack.

Wing Sail Foil Testing - Trim Tab Size and Control Authority

One objective of testing was to clarify the relationship between trim-tab size and control authority. This was investigated by splitting the trim tab into two equal halves and comparing the aerodynamic response when operating one half versus both halves together.

Test foil with split trim tab allowing testing of a full size and half size trim tab

As expected, the relationship between trim-tab area and effectiveness proved to be approximately linear. Tests conducted with the wing pivot at 18% chord showed that halving the trim-tab area produced roughly half the change in angle of attack. The results confirm that, within this range of operation, trim-tab authority scales proportionally with its surface area.

Conclusion

Trim-tab effectiveness varies linearly with area.

Wing Sail Foil Testing - Optimum Axis Position

What is the optimum position of the axis of a self-trimming wing sail expressed as a percentage of the chord ?

A self-trimming wing sail must naturally weathervane into the wind. 

It is well documented that the aerodynamic centre of a simple thin foil is at 25% of the chord.

NASA - Aerodynamic Center

This means that 25% is the absolute maximum position along the chord, because beyond that the wing sail won't weathervane into the wind.

As the position of the pivot is moved toward the 25% point, the wing sail becomes more balanced, requiring less effort to increase the angle of attack.

This also means that manufacturing tolerances will have an increasing effect as the pivot point moves toward 25%.

 

As the pivot point moves closer to the leading edge, the effort to increase the angle of attack of the foil increases.

Also manufacturing and assembly issues encourage the pivot point to be a reasonable distance back from the leading edge.

The image below shows the sail from Voyager 2.9. A practical issue is the need to have pivot far enough away from the leading edge to allow for mounting the magnetic encoder ring used for measuring the wing sail angle.

Voyager 2.9 wing sail with mast pivot at 16% of chord

The wing sail with 16% pivot point has been used successfully in lake trials and on the recent ocean passages.

Testing Pivot Position

A series of tests were performed with the test rig by setting the position of pivot at the following percentages of the chord:

15%, 16%, 18%, 20%, 25% and 27%.

Then the trim tab moved in 5° steps from -40° to +40°.

The resultant angle of attack was recorded at each step.

The results are plotted below with the trim tab angle on the horizontal access, and the resulting angle of attack (AOA) on the vertical access.

The Eppler 169 foil has an optimum AOA for lift vs drag, of around 5° to 6°.

The overall trends observed in testing were consistent with expectations.
The sensitivity of the wing’s angle of attack (AOA) to trim-tab deflection increases as the pivot axis moves aft toward the 25% chord point.
At 25% chord the system became too sensitive for practical use: we aim to operate the wing at an AOA below about 10°, yet a trim-tab change of only 1–2° was enough to drive the wing into stall.

It is important that the wing’s response to trim-tab input is not excessively sensitive.
The trim tab must move by a practical amount during normal operation—large enough to be repeatable and to overcome any mechanical backlash or stiction in the linkage—while still giving fine control over the AOA.

The tests revealed an apparent offset of approximately –3° in the trim-tab calibration.
This could be due to small manufacturing or alignment errors, but is more likely caused by flow-field asymmetries in the test setup.

A useful benchmark is the trim-tab deflection needed to achieve a 10° AOA:

• about 7° of tab deflection when the pivot was at 15% chord,

• about 5° when the pivot was at 18% chord.

For completeness, we also explored operating with deliberately large trim-tab deflections that drove the wing well past the stall angle, even though such conditions are outside the intended flight envelope.

Conclusion

Overall, a pivot-axis position in the range of about 15–20% chord appears to be well-suited.
Within this range the wing shows stable weathervaning, and the tab response is strong enough to overcome backlash yet not so strong that small tab motions cause abrupt stalling.

I expect to continue using the 16% chord position for the pivot of self-trimming wing sails.

 Wing Sail Foil Testing - The Setup

The designs of the wing sails used with the Voyager sailing drones have been established by studying other designs, by using a lot of intuition and by judging whether it looks right. 

It is time to perform some more rigorous testing to determine the optimum values of some key parameters of a self-trimming wing sail.

Design Questions

These are some questions to be answered to assist in designing and operating the next wing sails.

1. What is the optimum pivot point for a self-trimming wing sail ?
2. What is the relationship between Trim Tab angle and Angle of Attack, and hence what size should a trim tab be ?
3. What is the optimum trim tab angle when running ? Should the trim tab be reversed when running ?

Wing Sail Test Rig

 I developed a test rig to allow a series of relative measurements to be performed indoors.

The airflow is provided by a large domestic fan.

But tests quickly showed that the airflow from a fan is too turbulent to be useful for performing measurements. So a columnator or flow straightener was developed to improve the quality of the airflow.

This was constructed primarily from rolled up sheets of A4 paper, contained within a wooden frame. It wasn't great, but it was good enough to get useful results.

 

Fan, flow straightener and the test article.

Fan and flow straightener

The airflow in the vicinity of the test article was around 2.8m/s.
This was measured using an Air Velocity Sensor Module, the Renesas FS3000-1015.

Test Article

The test foil is an Eppler 169 (E169) with a 400mm chord. This chord size represents the approximate size to suit a wing sail for Voyager 3.

The 400mm chord yields a Reynold's number of around 76,000 for the 2.8m/s airflow.

The test foil has been designed to support testing with the following characteristics and adjustments:

• The position of the axis may be varied from 15% of the chord to well over 35%.
• The trim tab is adjustable with a scale to easily set a desired angle.
• The trim tab is split in two, to allow for measuring authority versus size.
• The test rig includes a scale to measure angle of attack.
• The wing sail mast bearings are supported by load cells to provide a relative measure of the load in 2 dimensions. The load cells are rated at 1kg max, and include digital readouts in tenths of grams as a relative measure of force.

Test wing section, Eppler 169 with 400mm chord

Adjustable pivot point shown at 18%

View of Trim Tab and scale showing +10 degrees.

Angle of Attack scale showing 

View of mast mount and load cells providing independent support in the X and Y axes.

Digital readouts of relative force expressed in grams

Tenth time lucky - Finally !! - a Successful Ocean Passage

Sunday 7th September 2025 around 6pm Voyager 2.9 was launched from Torquay.

She was bound for Flinders in Western Port, around 50 miles or 80kms to the east.

Winds were generally 10 to 15knots, peaking at 20knots from the northwest and backing around to southwest over the next two days.

Launch from Torquay Fisherman's Beach 6pm Sunday.

Course from Torquay, 80km eastward to Western Port

The intended destination was Flinders beach, just inside Western Port. This beach is calm, and surrounded by hills. The hills tend to block the southwesterly wind. As the boat cleared the last waypoints to head into Flinders, there was not enough wind power to overcome the incoming tide.

This meant that Voyager was swept out of its corridor and deeper in Western Port.

I could see from the satellite tracking that it was not going to be able to reach Flinders. I was able to move to high ground and gain line-of-site to establish a telemetry link and redirect Voyager further north to Point Leo, where a landing would be easier, with more favourable winds.  

The telemetry uses 433MHz LoRa. It is reasonably reliable over 2 to 3km provided there is line-of-site. This requires being elevated above the beach, on the hills behind.

Approaching the beach at Point Leo, Tuesday midday, 100m to go.

Happy skipper.

I swam out 50m to bring Voyager to shore.

Safely ashore at Point Leo after 44 hours

Battery Life

The main battery consists of 12 x 18650 cells arranged as 2S x 6P.

When the nominated minimum voltage is set at 6.5Vdc, the estimated battery life is around 10 days.

The Wing Sail battery consists of a pair of 18650 cells arranged as 2P.

With a nominal minimum voltage of 3.5Vdc, the estimated battery life is around 60 days.

Wrap up

The boat was in good order on arrival, responding well to manual override via the LoRa telemetry channel. It appeared that it could have continued at sea for several more days.

Summary of failures on previous ocean voyages:

1. Fatigue failure of aluminium mast, and loss of equipment housing due to inadequate strength and fastenings. Changed to carbon fibre mast, and greatly improved the strength of hull fastenings.
2. Water ingress into the Wing Angle Sensor, which is 3D-printed part. Filled the sensor housing with epoxy resin so that we don't rely on the 3D-print being waterproof.
3. Software errors related to weather changes and wind direction transitions in particular cases that were not correctly handled. Some software errors are not revealed with lake testing on short courses. Some are only revealed on multi-day multi-mile courses with wind direction transition that were not anticipated.
4. Poor compass calibration, combined with software errors lead to failure. This was addressed by focusing on improving compass performance and calibration, and also software improvements to make better decisions and be more resilient in the cases where compass accuracy is critical.
5. Failure of standard servo with a brushed motor. Changed to brushless servo motors for steering. There may still be an issue with the life-span of the mechanical potentiometer used for positional feedback. On low-cost servos, the feedback potentiometer may wear out. So this is still an issue of concern.
6. Failure of the Wing Sail controller due to water damage. This is now potted in epoxy and has performed well over multiple missions at sea. This includes the failed 6th voyage, lying on a beach for 100 days. It worked perfectly after that, once powered up, so the one controller has been reused on all voyages since.
7. Likely failure of the Wing Sail structure. It is difficult to prove, but it is believed that one or two failed missions may have been caused by the Wing Sail suffering a structural failure with the tail.
   A new design has now been established for the Wing Sail that eliminates the separate tail with trim tab and integrates the trim tab into the main foil. This design appears to be inherently stronger because of the elimination of the separate tail.

Ninth Voyage in Bass Strait - Argh! Software Bug!

Conditions were good for a 2-day passage commencing from Torquay Fisherman's Beach on the evening of Monday 28/7/2025.

The weather on the course was initially running, and was predicted to back toward a reach, and continue backing to a beat with moderate winds near the end of the main leg to Western Port.

But it didn't get there, and this time the reason was a software bug.

Ready for Launch at Torquay Fisherman's Beach, 28/7/2025.

The software bug was introduced in 2021, but had not shown itself in ocean conditions before.

When beating to windward the software limits the upwind course with a minimum upwind angle, forcing the vessel to tack to reach a windward waypoint.
In early 2021, a similar constraint was added for downwind sailing, forcing the vessel to tack downwind to reach a waypoint within the minimal downwind angle. This was to improve downwind performance by avoiding sailing dead-downwind.

This all appeared to be fine, but there was a latent error, that was not recognised until now.

The error was revealed when sailing a running course that requires tacking downwind, and then the wind changes so that the course requires tacking upwind, without a period in between where the course is directly sailable.

Given this specific scenario, the software had an error where it did not allow the vessel to leave the running course. I've named it the "Stuck Running" bug.

This lead the vessel off the course and it did not recover.

The software has now been updated to handle the direct transitions between upwind and downwind courses that are not directly sailable. 

On a positive note, everything else with the vessel appeared flawless.

 New Self Trimming Wing Sail Design - Tailless

A symmetrical foil generally has the Centre of Pressure (CoP) at a position about 25% of the chord from leading edge.

Hence a reliable configuration for a self-trimming wing sail is to place the axis of the wing sail at the 25% point, and then add a separate tail to ensure the CoP is well behind the pivot point.

Influential Sailing Drones

These are images of significant Sailing Drones equipped with self-trimming wing sails that have strongly influenced the design of the Voyager Drones.

Saildrone Explorer

Saildrone Surveyor

Maribot

Voyager Sails

The following images show a series of different Self-Trimming Wing Sails on Voyagers.

Voyager 3

Voyager 2.0 with first version of wing sail

Voyager 2.8 with evolved wing sail with tail

As at April 2025, Voyager has made eight voyages into Bass Strait with different problems occurring in each voyage. Before commencing each new voyage the issues encountered on the previous voyage would be addressed and the design evolved. Issues have been related to waterproofing, software design errors, robustness, servo failure during to wearing out.

One problem appeared to repeat on two different voyages.

This was a loss of sailing performance, probably due to the wing sail tail being damaged.

It was determined that the sail assembly might be more robust if the tail is removed from the design.

This can be done if the pivot point is moved forward of the 25% neutral point.

It is unclear what optimum point is, perhaps between 14% and 20%.

Tailless Self-Trimming Wing Sail

Voyager 2.9 has been rebuilt with a tailless self-trimming wing sail with a pivot point at 16% of the chord. 

Voyager 2.9 with Tailless Self-Trimming Wingsail

The Tailless Self-Trimming Wing Sail with an axis at 16% of the chord appears to perform as well as previous sail with tails. It appears to be just as stable, showing no signs of unexpected behaviour.

This sail should be more robust with the elimination of the tail.

A future refinement may be to construct the sail from fibre glass coved foam core for increased strength to resist to damage in breaking waves.

A further design change was the choice of foil. The recent Voyager Wing Sails have all been NACA 0018 (a symmetrical foil with 18% thickness),
The new sail employees an Eppler 169 (E169) foil (~15% thickness). The Eppler foils are intended for use with low Reynold's numbers, typical of those found on small vessels such as Voyager.

Voyager 2.9 with Tailless Self-Trimming Wingsail on the water

Saildrone's Hurricane Sail

I later realised that Saildrone had already developed a tailless version of the self-trimming wing sail.

It is designed with a smaller highly robust self-trimming wing sail for deployment during hurricane season.

Measurement taken from photographs suggest that it's pivot point at around 19% or slightly more.

Saildrone fitted with the Hurricane Sail

Future Study

Future work will involve preparing a test foil which allows for an adjustable pivot point. The aim is to allow the pivot point to be varied between 15% and 30% to allow for testing in a wind tunnel environment.

The likely aim will be to find the position representing the largest percentage away from the leading edge for the axis, where the sail still exhibits stability and "weather vane" behaviour.

 8th Bass Strait Voyage 

Just after dawn on April 13, 2025 suitable weather conditions lead to launching Voyager 2.8 from Flinders, Victoria. The intended destination was Torquay, but rougher than expected conditions later that cut the journey short.

Launch at Flinders Beach, after dawn.

Voyager departed Western Port Bay well pushing against a flooding tide, with Northerly wind of around 10 knots building to over 20 knots.

Voyager successfully exited Western Port Bay into Bass Strait, with northerly wind building to 20 knots.

Wing Sail damaged several miles out, approaching Cape Schanck waypoint.

Recovery from the rocks near Cape Schanck several days later.

Voyager lost control and drifted on the rocks during the next day, and was recovered two days after that.

She suffered a lot of damage on the rocks. The vent/antenna tube was broken off and the equipment housing filled with seawater, destroying the electronics. 

But the SD Card was still good !!

Analysis of the logs on the SD Card provided hints about what happened at sea.

It appears that the rough conditions with over 20 knots of wind combined with tidal movement caused breaking waves that tumbled the vessel, and damaged the tail on the wing sail.

It appears that the vessel lost drive from the sail, most likely due to the tail being damaged.

This is the second time the vessel has come ashore in Bass Strait due to suspected sail damage.

The logs showed that all of the electronics continued to function until a few hours after coming ashore on the rocks.

Conclusions

Its time to change the Wing Sail construction. Printed ABS components, with PVC film covering is ok in protected waters like lakes or Port Phillip.
But it is not suitable for Bass Strait.

Next Steps

My plan now is to develop new Wing Sail design and use new construction methods.

1. I've always used NACA0018 foils. I plan to test designs using Eppler 169 foils due to their improved behaviour with the low Reynolds numbers encountered with small sailing drones. (calculated based on 12 knots of wind, over a chord length to 300mm to 450mm)
2. The Tail assembly is fragile. I plan to develop a design where the tail is removed and the trim tab is included within the trailing edge of the wing sail to improve the robustness of the assembly.
3. Effectively eliminating the tail will require the pivot point of the sail to move forward. A typical symmetrical foil is balanced about a point approximately 25% of chord from leading edge. 
   When a separate tail is added it ensures the  assembly will "weather cock".
   Without a tail, it will be necessary to move the pivot forward to ensure the wing sail continues to "weather cock". 
   I plan on commencing with 20% of the chord as the pivot point.
4.  Initially development of the new sail design will be done using the same 3D printed ABS components with PVC film. This allows for rapid construction and is suitable for lake testing.
   The plan is that next time Voyager sails in Bass Strait the wing sail will constructed using foam core covered with fibre glass for increased strength. This is similar to the construction of the hulls, which has been very successful.
   I use 2 ounce fibre glass fabric with West System epoxy resin, over high density construction foam.

 

 Magnetic Coupling for Steering - V3

The magnetic coupling used to transmit the steering servo's torque to the rudder through a waterproof barrier has been quite successful.

But the design has evolved over the years, and it is time revisit the topic to explain the latest design.

I had previously used an array of a 8 magnets, 10mm x 3mm, per disk.

Magnetic Coupling Version 2 - new Magnet Layout

This worked fine on the smaller 1.2m vessel, but the larger 1.8m Voyager 3 was experiencing steering interruptions when the magnetic coupling would be overcome, and break free.

More torque was required in the magnetic coupling for the larger vessel.

Review of the V2 magnetic coupler

These images show the version 2 coupler that was used extensively on the smaller 1.2m sailing drone. 

When used on the larger 1.8m sailing drone, Voyager 3, it proved to provide inadequate torque.

When the relative torque was measured using a spring scale, it broke free at around 200g.

Version 2 Magnetic Coupler - 8 magnets 56mm diameter - magnet side.

Version 2 Magnetic Coupler - 8 magnets - other side.

V2 Magnetic Coupler - 12 Magnet

To increase the torque that could be transmitted by the coupler I tried adding an additional set of magnets, totalling 12 magnets per disk, and increased diameter of 76mm.

This worked ok, but in practice the increased diameter of the disk was going to require more clear space in the equipment bay, which would require shifting other components, and became too difficult.

When the relative torque was measured using a spring scale, it broke free at around 320g.

Version 2 Magnetic Coupler - 12 magnets 76mm diameter - magnet side

Version 2 Magnetic Coupler - 12 magnets - other side.

V3 Magnetic Coupler - 2 Magnets 

The next step was to employ larger magnets, 20mm x 4mm, on the same size disk of 56mm diameter.

This configuration yielded a significant increase in the torque that could be transmitted, within the same space. 

When the relative torque was measured using a spring scale, it broke free at around 525g.

This increase in performance yielded a practical result for the Voyager 3.

I have now retrofitted the smaller Voyager 2 with the same design of Magnetic Coupler. 

Version 3 Magnetic Coupler - 2 magnets 56mm diameter - magnet side

Version 3 Magnetic Coupler - 2 magnets - other side

View of 2-Magnet Coupler in the Voyager 3 Equipment Housing

View of 2-Magnet Coupler in the Voyager 3 Equipment Housing

View of 2-Magnet Coupler in the Voyager 3 Equipment Housing

The new coupler design has proven so good, it has been retrofitted the smaller Voyager 2.

View of 2-Magnet Coupler retrofitted to Voyager 2, shown partially installed

Compass Interference

Of course stronger magnetic fields on a small sailing drone create problems for the magnetic compass.

The only solution is to increase physical separation until the magnetic interference with the compass reduces to a tolerable level.

The following images shows a simple test set up to observe the effects of the new Magnetic Coupler on the magnet compass located within the equipment housing.

It showed the interference dropped to reasonable levels once separation was increased by around 100mm to 200mm.

Testing for Compass Interference

 This was handled in Voyager 3 by adding an additional compass on the deck well forward of the equipment housing, but not too close to the magnetic disk used for the wing angle measurement, as shown in the following image:

New compass mounted away from magnets to reduce interference.