Thanks for the questions. Sorry for not replying to this before - I only just saw it.
I am away from my PC right now. The way it should work at the moment is that the sideways lift generated by the rudder does not care whether you are going forwards or backwards (it probably should care). The place that the lift acts is interpolated between a particular position forward, at 1m/s, to a particular point aft, at -1m/s. I don't remember how I set up the aft point. The forward point is somewhere around 1/3 of the way from the front of the ship. It's tuned fairly carefully; I want enough instability so that you have to worry about the sail plan, but not so much that tacking becomes trivial. You can move this point around in one of the debug menus, there's a slider called "ardency" or "griping" or "by the head" or something like that.
Why do you think there is too much authority with sternway?