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 if negative SLIP is calculated. SLIP is not particularly important in propeller selection: good boat speed trumps bad SLIP values.
The PITCH used in the calculator is the distance of advance of a screw in a solid with a particular pitch of threads. Propeller blades are much more complex shapes, and they typically have a pitch that varies across the blades. These days propellers also typically have cupped blades, which tend to add to the effective pitch. As a result of cupped blades and progressive pitch shapes, the effective pitch of a particular propeller is a function of many elements, and the manufacturer often takes this into consideration when designating a propeller as having a particular pitch value. It is impossible to predict if all manufacturers will assign a pitch value to their propellers in a consistent and uniform manner, so with some propeller designs the effective pitch may be greater than the stated pitch. Allowance for diffrences in effective pitch and stated pitch must be made when comparing propellers from different manufacturers or even among different models of propeller from the same manufacturer. The calcaulator cannot provide specific guidance about these allowances; it must come from experience with the actual propellers and the results they produce.
If a propeller has been reworked after its original manufacture to add more cupping to the blades, a good rule is to add 1-inch to the stated pitch the propeller was originally given.
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
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.
A calculator that was recursive in its design and could calculate any one of several factors in a fixed relationship was first noticed by me in financial calculators. This method is a good approach to the problem of propeller calculators as it allows the user to use the calculator in many ways and to calculate any one of five factors used in the relationship. There are many similar propeller calculators now available on-line, but this one is unique in that it completely explains all of the relationships between the parameters and the forumulas used to calculate them. Its use of HTML style sheets in presentation of the data is also rather unusual and seldom seen.