On moving to catamarans I treated the trimming of the sails in much the same way as I did the monohull rigs that I had grown up sailing in the past. For the most part this was fine in the early days; I didn't think much of the fact that my speed wasn't as good as it should be. I was enjoying the stable, even keeled, sailing that didn't draw too much complaint from family and guests who found the heel uncomfortable, or difficult to leave a drink without it spilling!
It soon I realized that learning how to trim the big Cat sails effectively was important if we wanted to get somewhere with purpose. We have all done it; we have all raced the yachts that seem to be going to the same place you are - you know its important, and you know if despite all your best efforts at trimming they still go faster than you that you put it down to the fact that your rival is, secretly, motor sailing!
On the main the key tools are sheet, traveller, halyard, and in weather, the reefing system. For the headsails it’s about sail selection (assuming you have a selection to make) or, again in weather, the roller reefing system to shrink the genoa size, and then working the sheets, and the position of the sheet cars to ensure that the force on the sheet is in the right direction for the sail position. On our boat we have a roller furling screecher rigged forward of the genoa on a bowsprit. The screecher is larger than the main, and has a fuller cut with depth in the body of the sail for powerful beam and broad reaching, while still being able to point up to 65 degrees off the wind.
The key difference on the main between a cruising Cat and a monohull is that the Cat doesn't have a boom vang to adjust the bagginess in the sail, or the mast bend, and given the beam of the vessel it has a traveller with a very wide span. The mainsheet on a Cat, given the flexibility of the traveller, in some ways takes the place of the vang in flattening the sail while the traveller adjusts the angle of the main to the wind. This helps with twist which when I figure out (!) I will blog more about... Tell tales on the leech of the sail should stream smoothly aft from the head of the sail on down.
On the headsail its all about the tell tales, and to be fail much more akin to monohull sail trimming. Tell tales break from the top of the sail down, and the trick is to find the right balance of the sail off the wind where the fore and aft tell tales at all levels of the sail are streaming smoothly. When the tell tales break the helmsman needs to adjust, either by changing course or sail set. For course, I steer toward the good tell tale. For sheet adjustments play with the sheets till you get a smooth airflow on both sides. Of course, if you are close hauled then that may mean coming off the wind by a few degrees if you are pinching too much.
Given the dynamics of a cruising Cat, and the lack of dagger boards (unless you are in a very expensive boat) upwind sailing is much more challenging than on the monohull. In my Lagoon 450 the best I have achieved is 45-50 degrees off the wind before the headsails start to luff, even when winched close in. Happiness is a broad or a beam reach with the tell tales streaming smoothly against the sails, and 12 knots of boat speed on the log! Running before the wind with my rig is a sloppy and uncomfortable experience, and I prefer to gybe tack downwind to get a decent broad each angle for the sails each side of the course made good.