Heel Angle
I've owned and sailed six boats in the past 30 yrs (currently sail a Hunter 28.5). All had different characteristics, but as a generalization, I would say that optimum heel angles were up to about 15 degrees. Beyond that point, a lot of boats will start to develop excessive weather helm and want to start rounding up. Also, you will have a lot of unwanted leeway. It can be fun to sail my boat up to about 35 degrees of heel, but its tough on the boat and you really are spilling so much wind that you are not gaining any useful speed or forward motion. You know the old adage about reefing: "its time to reef when you first think about it." When to reef is sort of like heeling, it differs with the characteristics of each boat. Depending on the size and type of sails you have up at the time, another generalization I would offer is that many fin keel production boats need to have sail area reduced when the winds begin to exceed 15 kts. Sure, some boats can go far beyond that amount of wind before they need to reef (and many sailors tend to overestimate wind strength), but that's the general neighborhood to start gving it some thought -- and taking some action. Hope this helps.