Performance Propellers

propeller clearence

            Propellers must have between 15% and 20% of the propeller diameter as clearance between the propeller and any other part of the boat. E.g. a 12 inch propeller should have 1 .8 to 2.4 inches of clearance between the prop and any other part of the boat. Less clearance means noisy and less efficient propellers. Think of the clearence as hydraulic rather than mechanical.

            prop clearance "C" needed by percent of diameter   

                                   

%                                 prop diameter

            9          10        11        12        13        14        15        16

15%     1.35     1.5       1.65     1.8       1.95     2.1       2.25     2.4

20%     1.8       2          2.2       2.4       2.6       2.8       3          3.2

 

If you find a newly calculated propeller is larger than your existing propeller or the clearance is too small. Then the largest prop with good clearance that your boat can accept needs to be found.

 To determine how big a propeller you can fit on your boat.

Determine the shaft to hull distance by adding of the present propeller diameter (D) to the distance from the propeller tip to the hull (C). This dimension can be used in the chart below to determine the clearance for the standard propeller sizes for your boat.                

                                                            prop dia

1/2D +C          11        12        13        14        15        16

6   3/4              11%                                                    

7                      14%                                                    

7   1/4              16%     10%                                        

7   1/2              18%     13%                                        

7   3/4              20%     15%     10%                            

8                      23%     17%     12%                            

8   1/4              25%     19%     13%                            

8   1/2              27%     21%     15%     10%                

8   3/4                          23%     17%     12%                

9                                  25%     19%     13%                

9   1/4                                      21%     15%     11%    

9   1/2                                      23%     17%     13%    

9   3/4                                      25%     19%     14%     10%

10                                                        21%     16%     12%

10   1/4                                                22%     18%     13%

10   1/2                                                            19%     15%

10   3/4                                                            21%     16%

11                                                                    23%     18%

                                                                       

For example if your boat has a 12 inch prop dia (D)  and there is 2 inches of clearance (c) then there is 12/2 +2.0 or 8 inches from the shaft to the hull. From the chart the acceptable propellers would be 11, 12, or 13 inch diameters. The 13 inch is under the 15% but could be used if  additional noise is acceptable.

 

 

Sometimes the best propeller for a motor is larger than will fit on the boat with proper clearance.

If the propeller is in and aperture, then the propeller must be recalculated using a maximum diameter and altering pitch to find an acceptable propeller.

If the prop is in open water, Performance Propellers has available special extended hub propellers that mount on the existing shaft but position the propeller 6 inches aft. This change in position of the propeller, due to the angle of the propeller shaft to the hull, can permit a 2 to 3 inch larger propeller.

Extendo propeller
ext.jpg

 The Extendo design clearance

Sometimes the best standard propeller for a motor is larger than will fit on the boat. If the prop is in open water, The special extended hub propeller mounts on the existing shaft but positions the propeller 6 inches aft. This change in position of the propeller, due to the angle of the propeller shaft to the hull, can permit a 2 to 3 inch larger propeller.

 

angle hull to shaft      10        12        14        16        18       20                   

dia chg at 6"              2.12     2.55     2.99     3.44     3.90    4.37                

radius change at 6"   1.06     1.28     1.50     1.72     1.95    2.18                

 

For example a 12 inch propeller on a boat that has a 16 degree angle propeller shaft to hull can be replaced by a 15 inch extended propeller.

From the chart the radius changes by 1.72, by adding the radius change to the shaft to hull dimension previously found the new hull to shaft dimension is:  8+ 1.72=9.72 about 9 3/4”  The change in propeller location allows 14% clearance on a 15 inch propeller.

 

The extended propeller can make a difference when re-powering from an older gas motor as the Atomic 4 with direct (1:1) drive to a modern diesel with a gear reduction. There are several popular boat models where a more powerful geared motor needing a 14 or 15 inch propeller was placed in a boat designed around a 12 inch propeller than can benefit by using the extended propeller.

Propeller installation information

Propeller Identification

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Performance Propellers


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