I could be wrong but i doubt that a catamaran that size would have a adjustable boom-vang. The only ones I have seen are the fixed boom-vangs. The only way I know how to get the main from one side to the other is to sheet in the main (using the main sheet) until almost centre and then jibe over to the opposite tack. you can then ease out the main to the side of the coach roof you want and then tighten down ward. I don't do runs on cats at all and usually do broad reaches as I don't want to have an accidental jibes. I don't know if the line you are tightening is the down haul or just the main-sheet that is integrated with the traveler? There is usually away to let the traveler out to one side and then tighten the main sheet. Always takes me a few time s to figure out how the traveller and main work...