Under what circumstances does the boat round up. I like this boat too and would like to know more. When motoring, at hull speed do you have good control? What kind of keel do you have?
This to me looks like a hull form issue and less to do with sail trim.
Ive attached a picture of the catalina 18 with a First 18. They look similar, but the first has a longer rudder, and a centerboard. The sail plan looks similar, the hull shape is similar. Woulds be helpful to know if anyone on the board had been on both boats or atleast the First 18.
Both boats have fat round hulls. When those heel, the shape pushing through the water goes from a long oval to more egg shape, basically, the forces countering rotation around the keel is less. like standing in a pool, with arms out paddling water it hard to turn, bring arms in and irs easier to turn.
One other issue when heeled (with pressure on sails) is the lenght of that keel on the C18. The flow coming off that is turbulent, more so if stalling sideways, the rudder may not have clean flow to work with. With a higher aspect centerboard or keel there is more lift at speed, less sideways slipping, less turbulence from the keel and hull form.