The propeller calculator computes any one of the five parameters shown below based its relationship to the other four as described below. The calculator is not a Greek Oracle; it simply computes an answer based on your inputs. Prior to clicking the Calculate button, you must have one field blank. You can select the units for the BOAT SPEED parameter. (If units are changed you must force a recalculate.) The calculator returns advisory messages based on your input.

NOTE: Some features require a browser with Style Sheet capability.

The most commonly calculated value is BOAT SPEED, based on the other four parameters. Set the SLIP value to zero to see what the maximum speed could be. A typical actual value for SLIP would be in the range of 6-10 percent if the propellor is in good condition and running near the maximum speed at which it was designed to operate.

If accurate observations of boat speed versus engine speed have been made, the propeller SLIP can be deduced. This will allow analysis of the propeller performance. Typically values of SLIP will be at least 6% at wide-open-throttle speeds. At lower speeds slip can be much higher, often 50% or more. Generally SLIP decreases as SPEED increases, typically tapering to less than 10%. If a negative value for SLIP is calculated, the propeller PITCH was understated. Add one-inch to the PITCH for any propeller with cupped blades.

If the PITCH of the prop is not known, it may be deduced by observing the other parameters and using various values for SLIP in the anticipated range. Probably the best assumptions about SLIP can be made at wide-open-throttle speeds, where it will probably be as low as 10% or lower.

Your engine manual will contain information on the GEAR ratio. Usually it is around 2:1, but it varies with horsepower. For 200-HP engines a ratio of 1.86:1 is common. For smaller horsepower engines, higher ratios are used, more like 2.33:1.

For more information on the relationship between these parameters, read my articles on propellers in the Reference section.

The calculator is based on the following relationship:

RPM PITCH SPEED = ----- X ----- X [ 1- (SLIP/100) ] RATIO C

Where:

**RPM**a positive number; the crankcase speed in revolutions-per-minute.**RATIO**a positive number; lower unit gear reduction ratio; the number of revolutions of the crankshaft to produce one revolution of the prop shaft.**PITCH**a positive number; blade pitch of prop in inches.**SLIP**a percentage, 0-100; index of propeller performance.**SPEED**a positive number; the boat speed.**C**a constant to convert inches-per-minute of revolution to boat speed; for miles-per-hour, 1056; for nautical-miles-per-hour, 1215.2; for kilometers-per-hour, 656. (See below for derivation.)

By manipulating the elements using algebra, any of them can be computed if the others are known. Hence:

SPEED X RATIO X C SLIP = 100 X [ 1 - (-------------------) ] RPM X PITCH SPEED X RATIO X C PITCH = ( -------------------------- ) RPM X [ 1 - (SLIP/100) ] RPM X PITCH X [ 1 - (SLIP/100) ] RATIO = ( ---------------------------------- ) SPEED x C SPEED X RATIO X C RPM = ( ---------------------------- ) PITCH X [ 1 - (SLIP/100) ]

The constant C is derived from the analysis of the dimensions involved in the calculations. The propeller advance is generally given in INCHES/REVOLUTION and the speed of the propeller rotation is generally given in REVOLUTIONS/MINUTE. The speed of the boat would then be calculated in units of INCHES/MINUTE, an unusual dimension which needs to be converted into something more commonly used for boat speed.

The most common conversion is to MILES/HOUR (MPH), which is derived below:

1 HOUR 12 INCH 5280 FOOT 1056 HOUR INCH ------- X ------- X --------- = -------------- 60 MIN 1 FOOT 1 MILE 1 MIN MILE

Similarly, if the results are desired in NAUTICAL MILES/HOUR (KN):

1 HOUR 12 INCH 6076 FOOT 1215.2 HOUR INCH ------- X ------- X ----------- = ----------------- 60 MIN 1 FOOT 1 NAUT-MILE 1 MIN NAUT-MILE

And if results are desired in KILOMETERS/HOUR (KPH):

1 HOUR 12 INCH 3.28 FOOT 1000 M 656 HOUR INCH ------- X ------- X --------- X ------ = -------------- 60 MIN 1 FOOT 1 METER 1 KM 1 MIN KM

Comments or questions about the prop calculator can be posted in a message thread in the forum reserved for that purpose.

The propeller calculator was inspired by a JavaScript version that was created by Gary Polson.