Folks that like rules of thumb ,
A shaft spinning at 1000RPM can use the prop pitch in inches to get the boat speed in Knots , with a prop with no slip.
No slip is not possible , even on hydroplanes, but with the rule you can easily work out how much slip the boat has.
1000 shaft RPM 22 inch pitch 22K theoretical would see 11K at 50% slip.
50% slip is not uncommon.
Great fun for those that enjoy marine mental masterbation.