Ive owned a
Seawind 1000 - that we raced as well - and now a
Seawind 1160 Lite. Have never had a problem rounding up with either, although we should have as on both
boats, we oversized the
mainsail, used the standard-sized blade
jib, which should have pushed the center of effort back (and caused the
boat to round up). That said, we have a lot of experience
racing and sail trim, and whereas the
Corsair trimarans we owned liked a very flat
mainsail (tight mainsheet), both of the Seawinds sail faster traveled down a bit and with twist in the top of the main.
I dont remember if the
Seawind 1200 has a mainsheet traveler track on the arch like our Lite, but you want to adjust the mainsail shape to keep the entire sail flowing. It may be that you are overtightened on your main and not tight enough on your
jib, and that will push the
boat up into the
wind. We have a set of leech telltales on the main on the top third of the sail, and the standard front and backside telltales on the middle of the main. Let the traveler down (leeward) until the telltales on the backside of the main flow. Then ease the mainsheet until the leech telltales at the top all start to flow, streaming aft. The main traveler controls the mid and bottom of the sail, the mainsheet controls the top.
For the jib, just trim it in until luff telltales are flowing on both sides of the sail (red and green both streaming aft).
If that doesnt alleviate your issue, then i would look at
mast rake. It might be that its leaning too far aft, which again would push the center of effort back, which is your problem. Think of it as a force pushing the stern of the boat to leeward. When that happens, it is going to want to turn up into the
wind.