Your set base polars are either Predefined, Sail, Power or Advanced.
The polar speed adjustment is a percentage adjustment on your set base polars.
Having it set at 100% reflects your set base polars, however you may wish to adjust these polars.
For example setting the polar speed adjustment to 90% will take 10% off your set polar speeds for your next weather routing download.
Useful for say when you reef your main before night fall to be prudent, or perhaps towards the end of a long passage and the boat is not so heavily laden and performing better you could set it at 110%.
The polar speed adjustment is also useful if we do not have your boat in the Predefined polar list. Selecting a similar boat and using the polar speed adjustment is a good option.
Download a weather route, check the results in the WIND tables, check to see if the results look reasonable for the performance of your boat. Does the speed of the boat look okay for that particular wind angle and wind speed? If not use the polar speed adjustment, download another weather route and check the results again.
Where do I find the Polar speed adjustment?
On iOS/Android device > Found under Weather Routing Preferences > Routing settings > Boat polar
Windows 10 or Mac > File > Preferences > Routing.