Enter the propeller's pitch, the engine rpm, the gear ratio and the speed you measured, and the calculator gives the theoretical speed and the slip, in knots or mph.
The formula
A propeller with no slip would move forward its pitch once every turn, like a screw in wood. Omni gives the theoretical speed as:
Theoretical speed (mph) = pitch (in) × engine rpm ÷ (gear ratio × 1056)
The 1056 turns inches per minute into miles per hour: 63,360 inches in a mile, divided by 60 minutes. For knots, the calculator converts with NIST's definitions, a knot being 1,852 m an hour and a mile 1,609.344 m.
Slip is the share of that speed the boat doesn't make:
Slip (%) = (theoretical speed − measured speed) ÷ theoretical speed × 100
What's normal
Omni puts slip at 10 to 20% at top speed, and higher as speed falls. BoatTEST explains why: slip drops as the boat speeds up and peaks while it climbs from displacement onto the plane. So compare readings taken flat out, with the rpm reached at the time.
Treat it as a comparison, not a grade. Steve Sandler notes in Power Boating that slip doesn't define propeller efficiency, and that manufacturers don't measure pitch identically.
On the sea trial
Measure speed by GPS on a run in both directions and average them. Note the rpm and check it against the engine's rated range. A high slip with low rpm, or a slip that's changed a lot since the last survey, points at the propeller, the bottom or the running gear: damaged or bent blades, fouling on the bottom or the propeller, a worn cutless bearing or a misaligned shaft. Ventilation, where air gets into the propeller, is a different problem that Sandler warns is often mistaken for slip.
Citing it in a report
"Propeller 19 in pitch, gear ratio 2.08:1. At 5,600 rpm, theoretical speed 42.1 knots, measured 38 knots (GPS, average of two runs), slip 10%." Give the rpm, the pitch you read off the propeller and how the speed was measured.