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 allowed
2.12 2.55
2.99 3.44
3.90 4.37
additional extendo clearence 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.
