As for sails-simulator (first time looking at this as well), there appears to be, not surprisingly, quite a bit of similarity, but with one notable difference that I can see so far...
One of the goals of my project was to simulate a large number of boats at the same time with realistic large-scale navigation, geographical and weather effects, so there is much less focus on the individual aerodynamic and hydrodynamic sailboat physics (as these can also get computationally expensive). The "boat physics" in my case consists of four lookups from a "wind response" table (using angle from wind and wind speed as inputs) with interpolation between the four points to get the final result (boat speed over water). With some profiling and optimizations, I've found through load testing that it can simulate (on a modern machine) several hundred thousand boats per CPU thread with a simulation timeslice of one second. I'm certain this would be far more difficult to achieve if I was trying to simulate more "real" physics against the surfaces and masses of every boat. The tradeoff then is that the boat controls are very crude (basically just selection of course over water), and you don't have finer controls over sails/rudder, etc.