Performance Propellers

Determining maximum propeller size

.

            The design of the propeller mounting will determine the maximum size of propeller that can be used. Propellers must have between 15% and 20% of the propeller diameter as clearance between the propeller and any other part of the boat. Think of this clearance as hydraulic clearance to allow for prop wash rather than simple mechanical clearance.

 

Sail Boats have two styles of Propeller mounting.

          Aperture                                                        open water 

aperture-sketch.jpg
open-water-sketch.jpg

Diameter of prop________________  D                     diameter of prop_____ D

Prop to rudder__________________   A                    Prop to rudder_______ A

Top of prop to hull_______________ C                     top of prop to hull____ C

Bottom of Prop to fairing: __________C

Back of propProp to cutlass bearing___E

 

Using your existing propeller measure C, D, then add C to of d. this is the distance from the shaft centerline to the hull.

This chart shows the clearance % for a propeller diameter and can be used to determine the diameter of standard propeller that will fit on your on your boat with the given clearence. E.G. A 12 inch diameter propeller with 2 1/2 inches of clearence is a 8 1/2 inch dimension. which will allow 21% clearance for a 12 inch prop or a 15% clearance on a 13 inch propeller.

 

                             Prop dia

1/2D +C

   11

12

13

14

15

16

 

 

 

 

 

 

 

 

 

7       

14%

 

 

 

 

 

 

7   1/4 

  16%

 

 

 

 

 

 

7   1/2 

  18%

 

 

 

 

 

 

7   3/4 

  20%

15%

 

 

 

 

 

8       

 

17%

 

 

 

 

 

8   1/4 

 

19%

 

 

 

 

 

8   1/2 

 

21%

15%

 

 

 

 

8   3/4 

 

 

17%

 

 

 

 

9       

 

 

25%

 

 

 

 

9   1/4 

 

 

21%

15%

 

 

 

9   1/2 

 

 

 

17%

 

 

 

9   3/4 

 

 

 

19%

14%

 

 

10       

 

 

 

21%

16%

 

 

10   1/4 

 

 

 

 

18%

 13%

 

10   1/2 

 

 

 

 

19%

15%

 12%

10   3/4 

 

 

 

 

21%

16%

 13%

11       

 

 

 

 

 

18%

 15%

This is the largest size of a standard propeller that would fit on your boat with the needed minimum clearence. Smaller sizes can be used but larger sizes will cause additional noise and be harder to control.
 
Sometimes the best propeller for a motor is larger than will fit on the boat with proper clearance.

15 inch extendo replacing a 12 inch bronze.
stdext.jpg

 

            If your prop is in open water, Performance Propellers  has  available special hub Extendo 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.

 

Angle hull to shaft               10        12        14        16        18        20        22          24        26

Prop Dia chg                      2.12     2.55     2.99     3.44     3.90     4.37     4.85       5.34   5.85

extendo clearence adder  1.06     1.28     1.50     1.72     1.95     2.18      2.42      2.67     2.93

 

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.

 

A more accurate size and clearence can be determined by adding 1/2 the diameter of the existing prop(D) , the propeller clearence (C) and the extendo clearence adder and checking propeller clearence again.  for example the 12 inch propeller with 2 1/2 inches of clearence on a 18 degree angle has a max size dimension of 6+2 1/2 + 1.95 =10 .450 or about 10 1/2 inches. acording to the chart that will allow 19% clearence on a 15 inch propeller or 15% on a 16 inch propeller.

aperature
aperature.jpg

If you are ready to talk to our propeller guy . Gather your information, fill out our  Size Questionnaire, then email Performance.props@verizon.net or call 607 749 3165.
We can help with your propeller choices
 

If your propeller is in and aperture, then the propeller must be calculated using a maximum diameter and altering pitch to find an acceptable propeller. Propellers that substitute pitch for diameter as in this instance are less efficient but will still deliver more horse power to the water. A three blade propeller can some times can help put extra power into the water in the aperature but the three blade prop will have additional drag while sailing.
 
Fill out our Size Questionnaire and give us a call we can help find a 2 blade propeller for you and give you an opinion if a three blade is necessary.

More information about propeller design and selection


Performance Propellers


44 James Street                                     Phone 607-749-3165

Homer, NY 13077                             Fax 607-749-4604

Email performance.props@verizon.net