Technical Notes: Calculating Prop Performance

Here is a way to calculate prop performance with different props, a formula suggested to us by technician Peter Arneson:

Props slip, so they're about 85 to 90 percent efficient. Multiply mph by 0.85 to determine final boat speed. Here are the calculations for an example motor and two different prop pitches:

Example engine: 115 hp

Engine RPM: 5,000

Lower Unit Gear Ratio:

13:26 or 13/26 = 0.50

Prop Pitch:

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19 inches and 21 inches

Formula:

(RPM x Ratio) x Pitch = inches per minute

IPM/12 = feet per minute

FPM/5,280 feet = miles per minute

miles per minute x 60 = mph

19-inch-pitch Prop

(5,000 x 0.5) x 19 inches = 47,500 inches per minute

47,500/12 = 3,985 feet per minute

3,958/5,280 = 0.75 miles per minute

0.75 x 60 = 45 mph

0.85 x 45 = 38.25 mph with 19-inch-pitch prop

21-inch-pitch Prop

(5,000 x 0.5) x 21 inches = 52,500 inches per minute

52,500/12 = 4,375 feet per minute

4,375/5,280 = 0.83 miles per minute

0.83 x 60 = 50 mph

0.85 x 50 = 42.5 mph with a 21-inch-pitch prop

42.5 mph - 38.25 mph = 4.25 mph speed increase with a 2-inch increase in prop pitch

Many variables need consideration in prop selection, such as boat-hull design, boat loading, boat use, motor size, motor mounting, and water temperature. Prop calculations are one part of the total equation.