Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

the propeller hub and the shaft taper. In in-
terference fit, the hub is machined to be
slightly smaller in diameter than the taper
on the shaft. The old method of installing
these was to chill the shaft with ice and heat
the propeller with a low-temperature torch
(nevera high-temperature torch, such as a
welder’s cutting torch). This caused the hub
to expand enough (and the shaft to shrink
enough) so the propeller could be slid on.
By the time both components returned to
normal temperature, the hub would have
shrunk to an exceptionally reliable fit on the
shaft, with no stress risers created by a
machined keyway.
The modern method is to force the
interference-fit propeller up onto the taper
using hydraulic pressure. Most often this is
accomplished with a Pilgrim nut from Mid
Atlantic Pump and Equipment Company. This

nut is set on the shaft aft of the prop, and hy-

draulic fluid is used to force the prop fully up

onto the taper. This is common on large com-

mercial and naval vessels.

Keeping the Shaft

in the Boat

If a coupling lets go inside the hull, or if the

shaft breaks inside, the shaft can pull right

out of the boat. Though unlikely, such a prob-

lem not only can cause you to lose a costly

shaft and prop, but also could sink the boat

due to water entering through the now open

shaft-log hole. Installing a shaft collar inside

(a solid ring around the shaft, slid on and fas-

tened with one or two setscrews) eliminates

this possibility at virtually no cost. Even a

hose clamp around the shaft inside will be

better than nothing.

Chapter 1: Propellers and Shafts

Figure 1-18. Short

hubs increase

stress

Figure 1-17.

Keyway machin-

ing (Courtesy

Western Branch

Metals)