No, it's not a reference to a rifle caliber.  The Thirty-Thirty rule is a quick way to estimate how much horsepower a boat needs.   Frst add up the total weight of the boat.  That includes everything - hull, motor, batteries, kicker, fuel, passengers, ice, downriggers, anchor, crab davit, lunch and tackle.  Don't include the trailer. 

Now divde the total weight by 30.  That's the horsepower needed to make the boat go 30 mph.  Here's an example.

• Total weight = 3,000 pounds.     3,000/30 = 100 hp    You need a 100 hp motor to reach a top speed of 30 mph.

It's a rule of thumb, but experience shows it's remarkably accurate.  Deep V hulls need a bit more power, and flat bottoms need a bit less. But for the average hull, the rule is pretty close.

Granted, 30 mph is not especially fast.  But the Thirty-Thirty rule is a good estimate of a practical minimum horsepower for a fully loaded boat.   If you divide by 25 instead of 30, you'll get the horsepower required to go about 35 mph. 

Another way of looking at is 30 pounds of weight "uses up" 1 horsepower.  Let's say you ditch one passenger and some gear, saving 300 pounds.  That's like magically gaining 10 extra horsepower.

Thirty-Thirty!