Linda,
there is a simple method to test the fan-up preventer FUP.
On the photo, Ingeborg’s sail is setting four out of seven panels. With only three panels up, the peak of the yard is nearing the boom lift, so risks falling forward of it. The yard extension stick was made to prevent that. Then I added the FUP, as per that article, above.
To verify that it works, I did this:
First, with the FUP line not made fast, I could easily haul on the yard-hauling parrel, YHP and the yard would then be peaked up until it would fall forward of the boom lift. That would actually be a slow motion fan-up.
Then I repeated the exercise, but this time with the FUP line tied half-taut. Hauling on YHP would then not be able to peak up the yard. Both the furled bundle, plus the boom would have to be lifted. Even more, the slightly beefed up boom is held down by a tack line, which is rigged more like a kicking strap with a rubber snubber on it. With this at work, there is no way the boom will swing up. Therefore, that FUP is good.
If you can lock the yard and top section from fanning up on flat water, then a fan-up is unlikely to happen offshore as well.
Friction
Make sure you use a slippery FUP line, say 5mm flag line, and pass this through low-friction thimbles or blocks - and run the line on the outside of the lazyjacks.
Good luck,
Arne

FUP-line working on batten 2.