Class rules and Offshore Regs also come into play. Quite often, PFDs are specifically required to be worn.
Lots of OAs like to "emphasize" the point by adding language for PFD requirements to NORs and SIs. As a PRO, all I care about is whether such language conflicts with the "trump" rules, such as class rules or RRS (including Rx). The RRS tells us what can (or cannot) be changed by SIs...
Interesting point about the throwables on 16 foot boats. Most, if not all, Hobies at points regattas fall into this size range, and I think I can count on one hand how many throwables I've ever seen on any Hobie Cat in a points regatta. I've never seen a protest for this at a Hobie event, either. That would be an interesting way to "win" a North Americans...
