Performance Propellers

Motor and gear selection notes

      The boat displacement determines the power (HP) required to attain the maximum speed (hull speed) determined by the length of the boat at the water line (lwl).

           

A common mistake is to put the largest motor that will fit in the engine compartment only to find that the boat will not accept the larger propeller needed for the new motor.

 

Motors should match the displacement and hull style of the boat when possible. Each motor manufacturer and marine authority will have a different idea about how much horsepower should be used for a given displacement and style of hull. This chart shows two of the more common recommendations for sailboats.

Displace-

ment

1 hp

Per 550 of displacement

1 hp

per 450 of displacement

 

Displace-

ment

1 hp

 per 550 of displacement

1 hp

Per 450 of displacement

2000

4

4

 

12000

22

27

3000

5

7

 

13000

24

29

4000

7

9

 

14000

25

31

5000

9

11

 

15000

27

33

6000

11

13

 

16000

29

36

7000

13

16

 

17000

31

38

8000

15

18

 

18000

33

40

9000

16

20

 

19000

35

42

10000

18

22

 

20000

36

44

11000

20

24

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Other power losses from gears, bearings, alternator, refrigeration etc. are added to these estimates and rounded to get the final HP selection. Work with your motor supplier for the correct motor sizing for your boat.

 

            Transmissions gear selection directly relates to propeller size should wait until after your propeller consultation.  find out what gearing options are available. The higher the gear ratio the slower the propeller rotates and the larger the diameter of propeller needed. One popular motor supplier offers three transmission ratios for their “20 HP” model. This results in 6 potential propellers:

 

transmission    propeller          2 blade prop               3 blade prop

Gear Ratio       Shaft RPM      Dia.     Pitch                Dia.     Pitch

2.2                   1455                14        9                      14        9

2.6                   1231                16        10                    15        10

3                      1067                17        12                    17        12

 

The diameters and pitches are rounded up and down from the calculated values to have “standard size” propellers.  Note that the transmission selection can change the propeller diameter from 14 inches to 17 inches.  If you have a choice of gears check your boat for maximum propeller size  and contact your propeller supplier before ordering your motor.


Performance Propellers


44 James Street                                     Phone 607-749-3165

Homer, NY 13077                             Fax 607-749-4604

Email performance.props@verizon.net