The speed of a sailing vessel greatly depends on the direction and strength of the wind. This means that there is a different sailing speed for every course to the wind and every wind speed. For simplicity and the sake of comparability this wealth of data is usually displayed as polar diagrams. Apparent wind speed, apparent wind direction, speed over ground and heading (and ideally compass heading for the drift angle) need to be recorded for all courses between upwind and downwind as well as for all kinds of reasonable wind speeds. Collecting the data for these diagram requires very labor intensive test runs und real conditions that may span over days or weeks to cover all wind speeds.
For the “Transitioning to Low Carbon Sea Transport” project in the Marshall Islands (implemented by GIZ and in collaboration with Waan Aelon in Majel) we developed a data logger to record the speed, course, and wind speed/direction simultaneously. The recorded parameters can be used to create polar plots to compare the performance of different sailing vessels. The device is open source, arduino based and works off-grid:
The logger can be mounted to any vessel. A detailed documentation and manual of how to build one can be downloaded here. One of the first tests was done on Proasis back in 2021:
Unfortunately, the conditions were not favorable with off-shore winds and lots of shifts in strength and direction (the wind shifts can be seen as the wavy course lines on the upwind tacks) and we only had time for one test run.
Due to the bad conditions and the limited number of data sets recorded, the resulting polar plot only represents a very rough estimate of Proasis performance:
As can be seen in the polar plot, the upwind performance, especially the tacking angle, is not very impressive. The poor performance can be mainly attributed to two main reasons:
- The sail we use was a 30€ second hand polytarp sail with lots of wrinkles, and a shape far from ideal. With 15 sqm it was to small, too. We already made a nice 19 sqm sail that works much better.
- The leeboard was built 30% smaller than designed due to a measurement error. During the spring refit in 2023 a larger one was built and for the season of 2024 the setup was changed to two leeboard on the centerline to optimize the helm balance. This greatly improved upwind sailing, the tacking angle is now 90-100°.
Currently we are busy recording new data for an updated polar plot. An improvment of at least 30% is expected.
Top Speed
The top speed of Proasis was recorded at 13.1 knots so far. Speed above 10 kn is frequently logged in a good breeze, even in cruising mode. This video shows sailing at 8-9 kn, which is something we score on a regular base in a good breeze.
The limiting factor for the speed is usually the sea state, not the stability. The Baltic Sea is known for very steep waves in winds of 20 kn and up, causing huge loads and a lot of spray at high speeds. Usually we slow the boat down to 8 kn at 20-25 kn of wind for comfort reasons.