According to Mike Krantz you can only be protested if the SI's specifically state that a "USCG approved PFD" is necessary, If the SI's only require a "PFD" then you are OK. Mike is driving to FL right now so maybe he can chime in when he gets settled.
There's a problem when the class' rules require it, though.
For example, in the IHCA (Hobie) Class Rules, it states in the "General Rules" section:
8. SAFETY EQUIPMENT
8.1 Each person on board shall carry a lifesaving
device or Personal Flotation Device (PFD) approved
by their country’s national authority or by
the national authority having jurisdiction. The
IHCA strongly recommends that life jackets or
PFDs be worn at all times.
Sailing Instructions can only change a class rule by stating the rule number and the change (RRS 86.1(b))
But since you guys rarely/never race Hobie One-Design, you don't have to worry about it. mmadge does. All it takes is one jerk who wants to improve his score at your expense. Guaranteed the protest won't come until the last day of an event.
