Self Trimming Wingsail Design 101 (or What I should have known already)

 Self Trimming Wingsail Design - 101

The first sailing trials for Voyager 3 in late 2021 were disappointing.

The boat could not sail a straight course and was uncontrollable.

At first, I thought it was due to turbulent winds caused by the nearby trees, but eventually it became apparent that there was a basic design flaw in the new sail.

The issue relates to the location of Centre of Pressure for a foil.

The Centre of Pressure must be behind the axis of rotation to ensure that the self trimming wingsail will operate correctly and "feather", pointing into the wind.

Various refences indicate that the Centre of Pressure (CP), or Aerodynamic Centre of a foil with established circulation is around 25% back from the leading edge.

Before the circulation is established, the CP will be located close to the centroid of the foil.

Reference: Center of Pressure (nasa.gov)

My sail design has the axis of rotation located 37.5% back from the leading edge. This was chosen, because it seemed reasonable

The image below shows the calculated location of the line of the CP (25%) and the line of the Centroid (50%).

First Wingsail Design for Voyager 3 - CP is too far forward.

The calculated CP is forward of the axis of rotation by about 18mm. That is bad.

The effect of this is that initially the CP will be close to the 50% line (behind the mast), causing the wingsail to feather and commence operating.

Then as the circulation builds, the CP moves forward to the 25% line (forward of the mast). This causes the leading edge to fall away, and foil stalls, and CP moves back to the 50%.

So with these conditions, the wingsail may oscillate back and forth, yielding an uncontrollable boat.

The video below provides examples of unstable behaviour of the sail.





In future, I plan on placing the mast at the 25% line within the foil. Then a tail of almost any size will ensure that the CP is drawn behind the mast, yielding a stable design.

Once the issue of CP being located at the 25% line was understood, a new larger tail could be designed which would draw the CP aft. 

The image below shows the new locations of the CP and centroid with the new tail fitted, which is just over 3 times the size of the original tail.

Updated Wingsail Design - CP remains aft of the axis