Propeller calculations and clearance
    Because of the complexity of the calculations, propeller calculations are good estimates and refinements to the prop parameters are sometimes needed during sea trials. Most suppliers have suggested propellers for the more popular boat and motor combinations or will do the calculations and aid in the selection of a propeller to fit your boat and motor.  
    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 of the motor 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 is needed to select the correct information and efficiency factors to enter into the propeller calculations. In many applications the most important determinants of the propeller are the motor horsepower at RPM and gear ratio of the transmission.
    There are free prop calculators available on the internet that can be used to aide in propeller selection.  
Typical input parameters are:
Maximum Displacement of vessel in pounds.  This is often 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.
Required maximum speed in knots. Most calculators will use a boat speed calculated from the length of the water line. An often used formula is 1.5 x (the square root LWL.)
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.
Enter number of bearings between gearbox output and propeller. usually 2
Enter Gearbox reduction ratio. Found on the motor or gear box
Number of blades.   Usually 2 or 3 for sailboat propellers.
Prop slip and/or Efficiency factors. Different calculators will use different factors typically a starting place is about 50%  follow the calculator recommendations.
The public calculators use generalized efficiency factors and usually are not as accurate for a particular prop as the prop manufactures that use slip and efficiency factors developed specifically for their propellers.
    The output from the calculator will be diameter and pitch. These are 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 calculated propeller must be checked that the diameter will fit on the boat.   
                                                                                                                     
open water propeller dimensions C and D
    Propellers must have between 15% and 20% of the propeller diameter (D) as clearance (C) 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.
TABLE 1      prop clearance "C" needed by percent of diameter   
%            prop diameter
                 9       10      11       12       13     1 4      15      16
15%         1.3    1.5      1.6      1.8      1.9     2.1    2.2      2.4
20%        1.8     2.0      2.2      2.4      2.6   2.8      3.0      3.2

If your calculated propeller is the same or smaller diameter as your existing propeller and the clearance is correct then it is a good trial propeller. All that you will need is shaft size and direction of rotation and you are ready to buy and try it out on the water. NOTE: It is still a good idea to still have your prop supplier recommend a prop for your application. Their calculations are usually more accurate and the selections may have already been proven during sea trials on a boat similar to yours.

If you find that your calculated propeller is larger than your existing propeller or the clearance is too small then the largest prop acceptable prop with clearance that your boat can accept needs to be found. Determine the shaft to hull distance by adding ½ of the present propeller diameter (D) from the information sheet to the distance from the propeller tip to the hull (C). This dimension can be used in the chart to determine the maximum prop diameter that can be used on your boat to have a given clearance.
propeller clearance percent                       
TABLE 2  % clearance for a propeller for a given centerline of shaft to hull (1/2 D +C)
                    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. If the prop is in open water, there are 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.
  Extended hub propeller the "EXTENDO" propeller

Table 3 diameter change due to hull / propeller angle.
angle hull to shaft         10      12      14        16       18       20        22      24     26      28      30
dia chg at 6"              2.12    2.55    2.99    3.44    3.90    4.37    4.85   5.34   5.85   6.38    6.92  
radius change at 6"    1.06    1.28    1.50    1.72    1.95    2.18     2.42   2.67   2.92   3.19   3.46 


For example a 12 inch propeller with 2 inches of clearance on a boat that has a 22 degree angle propeller shaft to hull can be replaced by a propeller about 4.85 larger in diameter.
Checking for hull clearance: 1/2 old prop dia (12/2=6 ) + previous clearance (2)  + radius change at 6" (2.67 )= 10.67 new shaft to hull. From Table 2 the clearance chart this new hull to shaft dimension of 10.67 (about 10 .50)  allows 15% clearance on a 15 inch propeller.

The extended propeller can make a difference when repowering 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.

Two things more things are needed before a prop can be purchased. rotation direction and shaft diameter.

    Rotation direction is right or left hand. if the prop shaft spins clockwise a right hand prop is needed. if it spins counter clockwise a left hand prop is needed.
    The last thing needed to specify the propeller is the size of the shaft. There are metric shafts with various tapers and some motor instalations will require special hubs as saildrives. The most common USA (sae) shaft diameters are  ¾”, 7/8”, 1”, and 1 1/8". The shaft can be measured just behind the prop or close to where the shaft exits the hull using a rule or caliper. Adapters are available to adapt 1 inch diameter shaft props to fir 3/4" and 7/8" shafts.

You should have all that you need to buy a propeller and give it a try. It is still a good Idea to review your data with your propeller man to be sure that you have all that you need to power your boat.