The line (at the end of the boom I'm presuming) is there to prevent the car from sliding forward. The outhaul trim line at the front of the boom is there to pull the car towards the back of the boom.

We usually set the outhaul at the beginning of the day for about a fists' width between the boom and foot.

Around the bouys you really don't have much time to mess with the outhaul before you're kareening out of control <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />