The bolt that holds the pulley on my whipple broke off while out on the water today. Luckily I was able to idle back to the ramp about 3 miles. Anyway, I was able to get the broken piece of bolt out but am a little confused that the pulley doesn't seem to be keyed. It is just a smooth mating surface. Am I missing something? Is just the bolt able to hold it without slipping?

Yes, the bolt is the only thing that is holding it. It's a friction drive. The newer ones use dowel pins or flats on the pulley adapter. If you have a socket head bolt that holds the pulley adapter on, then you need to switch over to the stud. It's much less likely to break.
Eddie

Great, thanks for the info!!!!

How can I switch to a stud? Can I use a spacer down inside the pulley adapter to take up the space so I can get a socket on the nut?