When the boats first started running they were designated "U" for unlimited because they were no engine rules at all. The nearest class to them was the SK class which was a "ski boat" type boat class because at the time everything else was either a hydro or what they called "service" boats which crackerboxes fell into along with other front engine, two rider boats fit into. But SKs ran 5 mile races, the "U" inboard v drives ran marathons. 9 hour, 500 miles, 250 miles.
The Unlimited hydros didn't like the U designation on the small inboards, so they forced(bitched) and the name and designation was changed to SC for Speed Classics, but the boats all left the APBA in a huff over the deal and went on their own. Sound familiar Andrew?

When they came back to the APBA they started using the name Grand National. There is also a Grand National Hydro (GNH)class that uses similar engine rules that mostly runs in the mid west.

Now lets see if the young whipper snapper Andrew can tell you why crackers have a "P" designation?