Performance Propellers

propeller calculations

What is done with all that information?

Once sizing questionnaire is filled out the processes of selecting a propeller a boat can begin.

First we check if your boat, motor,gear ratio combination is in our data base. We have suggested propellers for the more popular boat and motor combinations including combinations lot listed on this web site.

If your combination is not in our data base then we enter your data into our Propeller calculator to find a propeller that matches your needs.

Because of the complexity of the calculations, these calculations are good estimates and refinements to the prop parameters are sometimes needed during sea trials.

Sailboat propellers are estimated by calculating diameter and pitch needed using continuous motor HP @ RRPM, transmission gear ratio, and boat water line length and boat displacement.

Water line length and displacement determine the maximum speed of the boat and the power required to attain that speed.

The HP, motor RPM, and gear ratio are used to determine the diameter of the propeller needed to transmit the hp, and how far the boat must travel with each rpm (propeller pitch) to attain the desired speed.

Experience and judgment are needed to select the correct information and efficiency factors to enter into the propeller calculations. Each manufacturer will use efficiency factors developed for their propeller and often disagree on the propeller for a particular boat and motor combination. In many applications the most important determinants of the propeller are the motor horsepower at RPM and gear ratio of the transmission.

The prop calculator uses these input parameters:

Maximum Displacement of vessel in pounds:     These are adjusted by using the boat manufacturers specifications and adding allowances for gear, water, fuel, installed extra equipment etc.

Waterline Length of vessel in feet. (LWL):      Manufacturers specification always shorter than boat length E.G. a 30 ft boat may have a LWL of 25 ft.

Engine Horsepower (max):     generally max continuous, or 80% of max rated.

Engine R.P.M. (max):    usually rpm at horsepower specified in the previous step often taken from curves in the motors manufacturers data.

Number of bearings between gearbox output and propeller:     Usually 2 Used to estimate bearing power losses

The Gearbox reduction ratio:   Found on the motor or gear box

Number of blades:      Usually 2 or 3 for sailboat propellers.

The output from the calculator will be diameter and pitch. This is raw data as diameter 14.750 and pitch 11.423, and must be rounded to get a practical propeller, e.g. dia 15 pitch 11. Not all diameters and pitch combinations are stocked so these must be matched to the available propellers. Sometimes a propeller can be modified to these specifications, or an equivalent prop of a different diameter and pitch may be found. This is a first pass propeller choice other factors may change both the pitch and diameter. The calculators will also calculate maximum boat speed from the length of the water line. An often used formula is 1.5 x (the square root LWL.) This is useful during sea trials to determine if the boat is getting up to full speed.

The calculated propeller must be compared to the boat dimensions to assure that the propeller has the proper clearance and will fit on the boat.

Now Propeller clearance must be checked

Performance Propellers

44 James Street                                     Phone 607-749-3165

Homer, NY 13077                             Fax 607-749-4604