Question from novice sailor

Although I have chartered sailing catamarans in BVI numerous times over the years, we are scuba divers and often simply motor from dive site to dive site and then to our anchorage for the evening. We will use the sails a couple of times, usually from North Sound Virgin Gorda to Anegada, and then the return to Jost Van Dyke. The downwind to JVD has always been simple enough, with the mainsail boom extended broadside. This past December we chartered a Lagoon 45 at Dream Yacht Charter on Scrub Island. We had some folks on board that wanted to do some sailing therefore we left Scrub to proceed downwind in the Drake Channel. When we attempted to swing the boom to broadside by releasing the downhaul (corrected to main sheet) the boom then moved not only broadside, but also up in the air. Apparently the only way of moving the boom to either side was to move the short traveler located behind the helm to the side and tighten the downhaul line, which doesn't accomplish much. Is this because this Lagoon does not have a boom vang, or am I doing something wrong?

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...

Correct, I just looked up in my copy of 'Sailing for Dummies'. Not a 'downhaul' as I stated, but the 'main sheet' attached to the short traveler behind the helm. Previous catamarans I have chartered had what appeared to be a 'fixed' boom vang, whereas this Lagoon 45 had no boom vang at all, neither fixed nor adjustable. I guess this was the issue …. nothing to control the tension of the sail's leech? It seems this would prohibit the use of the main sail in a downwind run, and limit the use of the main sail to a broad reach or worse?

https://www.boataround.com/us/boat/lagoon-450-f-fin-tonic

What model lagoon was this? It should have had a traveler spanning the back of the Bimini or a two part main sheet some companies are switching to which has each main sheet anchored near the corners of the Bimini. When you say short traveler can you give us a idea of length? Is it possible the topping lift was adjusted to short forcing the sail into that position? I find they are sometimes need to be released a bit to let the sail hold its proper shape.
G

George … The link above is a picture of the actual catamaran. The traveler attached to the main sheet is only about 5-6 feet in length which only allows the boom to be moved 2-3 feet in either direction. Easing the main sheet releases the boom although wind in the sail seemed to blow out the sail's leech (I'm not good with sailing terms) and pull the boom up, spilling all the air. The topping lift was loose, although not enough to allow the boom to rest on the Bimini, nor seemed to have any affect on the situation. The boat is a Lagoon 450F in the DYC fleet at Scrub. Could it be the location of the mast in relationship to the shrouds, fearing damage to the main sail if eased toward the outside of the boat by us credit card captains?