Outboard racing under the auspices of the American Power Boat Association (APBA) falls under five different categories.

Stock

The Stock Outboard category is essentially spec racing. Engines must be left for the most part as they come from the manufacturer. The only changes allowed are in the interest of safety or to make the engines more suitable for racing. For example, the lower unit has only one gear – forward – and it’s always engaged. And the motors can use pump gas only. Minor technical changes to engines are permitted only after a thorough review by APBA’s Stock Outboard Racing Commission. There are also limits on boat dimensions and total racing wieght as shown in this chart. The idea is to make racing a contest of driving skill. Minimum age for drivers is 14 in A Stock Hydroplane and Runabout, 15 in 20SS Hydroplane, 18 in D Stock Hydroplane and Runabout, and 16 in all other classes.

Modified

Modified Outboards are similar to stock in that they use pump gas and there are some limitations to engine modifications, but not many. Mod racers spend a lot of time at the milling machine and use more exotic exhaust systems. They can also very loud. Class specifications and age limits can be found here.

PRO

Professional Racing Outboard (PRO) refers not to the pay of drivers, but to the fact that the PRO category takes modified to a whole nother level. Any methanol burning piston-based motor can be used within the cubic centimeter limits of the particular class (there are two exceptions, KPRO and OSY 400, which considered entry-level classes to some extent and closely resemble A and C Stock Hydro respectively). The full PRO technical manual (2.5 mb) can be viewed here.