Optimizing Rig Tension using DMK Box and Expedition Software, Polar Speed and Pressure Percentages
October 28th, 2013
Boat Maintenance

After sailing my boat for many years on the edge of control I decided this summer to start experimenting with increased rig tension in an attempt to increase my boat's pointing ability up-wind and to try to give her some increased control when sailing in more breeze. However the increased tension slowed the boat down a great deal in lighter air and was not helped by my hydraulic backstay adjuster packing it in after 40+ years of service. This weekend I finally got time to set the boat back up with similar rig tension as I had before I started.

backstay vertical sailing performance forestay vertical sailing performance When sailing and making changes this summer I also had a chance to set up Expedition sailing software with my DMK Box and onboard router and the NKE Display App. I used a Cal 40 polar chart in the software to generate performance numbers that I sailed with during my local beer can races. The numbers Expedition generated that I found useful were my Polar Speed percentage, Polar Pressure percentage, and optimum angle of sailing. Even though the polar was not of my boat these numbers gave me a baseline snapshot of my boat's performance. During sailing this year my polar speed and pressure where mostly in the 70-80% range when I dialed the boat in to the optimum angle upwind. These were the numbers I saw when my competition would grind me down and pass us all summer. After moving the forestay pin aft 5 inches to loosen the rig, (the forestay length is already maxed out) I test sailed the boat and found her polar speed and pressure numbers in the 90-105% range. It was a quick test sail and numbers always bounce around but I think less rig tension is the right direction to take my boat and now I have some numbers to back me up.

My setup with a cheap PC and Expedition was relatively inexpensive compared to using some instrument's performance processor. With my wireless network onboard I don't have to plug in any wires, I simply boot up my laptop and start the program and it all works from there. The next step will be to fix the backstay and see if that helps things even more.

NKE screen sailing performance rig tension sailing performance

Above right: NKE iOS app helped with calibration.