With any main, you can still sight up the leech to see how tightly the sail is trimmed, then experiment with different settings while watching wind and speed instruments. This is a good way to improve your trimming eye, unless you fall prey to what may be the ultimate obstacle to good trim for a mainsail--a difficulty I encountered last year on a bareboat charter. Standing at the wheel with the bimini up, I couldn't see any part of the mainsail. Sure, I could engage the wheel brake or the autopilot, lean out over the stern pulpit, and have a look, but that got old in a hurry. Fortunately, the solution to this is easy, and it was in evidence on nearly every boat with a pilothouse, hard top, or bimini at last winter's Miami Boat Show: They all had windows above the helmsman's head.