All catamarans that I know of that are 18 feet or longer have at least a 7 to 1 mainsheet system. The pictures show this mast with diamond wires. I truly doubt that any less would be a bear to sheet in properly.
In this photo you can see that there is a boom vang. That's the blocks and wire that attach from the mast base to the boom. Since your main sheat is not on a track the vang helps keep the sail shape when sailing down wind.
That's the short wire at fron of boom to the bottom of mast? It's there too! Ha, I knew that was my boat, it still has the red, sheet and boom cover...............and the #7 on my sail!
I am looking at the boom in the last photo, it appears that there are 3 boom bales near the end of the boom, if so the mainsheet system is probably just like a Hobie 18, a 7 to 1.
Let me know if you can, I'm just curious.
Thanks
10 to 15 mph is good. When it starts to white cap its a blast. As long as the wind is steady up to about 25 is doable. Its almost always the gust that get you!