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.