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 ½ 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 |
|
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
return to propeller design and reference information page
