For more realism a rudder would not affect the angle of the ship, instead it only controls how fast it can turn.
Either way, instead of using the angle of an object, add the signed angle difference to a variable and use that.
A signed angle difference is like the anglediff() expression except it's negative when counter-clockwise.
Using that you can limit the turn of the wheel to something like 3 rotations either direction. Also it makes it simpler to make the ship turn with a ratio of the wheel.
https://dl.dropboxusercontent.com/u/542 ... ntrol.capx