Greenlake,
Last year was my first experience with spinnakers of any sort. So, obviously I'm still learning.
The UPS though three-quarter ounce lightweight cloth is try radial cut with large reinforcements at the attachment points. They say that it is designed to take considerable sheet load, like 18 kn of apparent close reaching. I think that it holds shape well even under load. The determining factor is horsepower. At a certain wind speed you just don't need it… or want it. It seems to be able to sail pretty close upwind in light air, I think possibly as close as with the standard jib, because in light air you can't sail that close anyway. They say, and it seems to be the case, that up to hull speed, I can sail as fast or faster than true windspeed, upwind. As the wind picks up and the horsepower increases the movable ballast quickly becomes in short supply.
Last summer I was sailing upwind in ideal conditions. I had a crew of three and windspeed was 8-10 kn and built to 12 with no chop. I was out racing any sailboat that would engage.
There was a J 24 headed up the lake, so of course the challenge was on. Normally, the J 24s just walk away from me upwind, but not that day! The boat was at hull speed or possibly better and we were feathered up pretty close (the sail shaped looked good). Now when it built to 12 kn JoAnne screamed (she warned us she might) and that was it, it was time to roll up the UPS.
Sailing downwind with it when the wind picks up is pretty interesting, regarding apparent wind. Sailing a broad reach in 15 kn of true wind feels like sailing a beam reach at 6-8 kn. In the gusts you sail deeper. I had one ride last summer where it REALLY would have helped to have crew but I was single-handed. Yes, I probably should have rolled it up sooner. When I started across the lake I'd estimate it was about 12 kn and the boat was up on a plane and it felt great. By the time I was about a mile offshore I would guess it had built to 20 kn. So now it's starting to feel like it's a bit too much, exciting… but starting to feel nervous. A gust hits, so I head down to blanket the UPS so that I can roll it up and I realized just how windy it is. The boat slows down and the apparent wind goes WAY up. The UPS is flapping and snapping and making a heckuva racket… I head up a little bit, the UPS fills and the boat pops back up on a plane and things feel calmer again. I do this one more time and then head down deeper and roll up the UPS. It doesn't roll up well so it's flapping still and I go head for the lee of an island to calm down.