Part One
DRIVETRAIN
INSTALLATIONS
CHAPTER 1
An often overlooked aspect of small-craft
engineering is the drivetrain: all the compo-
nents aft of the transmission—the shaft, its
bearings, the strut, and the propeller. Also,
the drivetrain itself is affected by the support
needed to hold things in place—the engine
mounts and beds. It’s important to under-
stand how these many disparate elements
should work, and work together—reliably
and smoothly. Vibration and noise, poor
performance, and even damaged transmis-
sion and broken engine beds can result from
drivetrain problems.
In the following pages we’ll take a prac-
tical look at these many elements and see
how they should be configured, sized, and
installed. We won’t look specifically at the
alignment process, which is a whole topic in
itself. We will, however, see some examples
of what happens when things aren’t installed
or sized correctly.
There are so many different components,
that—to keep things in some sort of order—
we’ll very roughly begin at the propeller and
work our way forward. Some jumping about,
however, will be unavoidable.
Proper Prop Nuts
We’ll start with a minor and somewhat amus-
ing controversy. Two nuts hold the propeller
on the shaft—a full-height nut and half-height
locknut (Figure 1-1). Which nut goes directly
against the propeller hub?
The fact is most boats have it wrong, with
the full-height nut against the prop. It seems
intuitive that the larger nut against the prop
would be doing most of the work and the
smaller nut should go on second. Actually the
smaller nut should always go against the load.
This is because when the second, outer nut is
installed and tightened, it deforms/compresses
the lower nut a smidgen, at the same time
rotating it a tiny fraction of a turn. This effec-
tively unloads the threads of the lower nut and
engages the threads of the upper nut. Thus the
top or outer nut really takes all the load. No
matter how many times I explain this, some
folks still seem dubious. An illustration in
Engineering Drawing and Design(Cecil H.
Jensen and J. D. Helsel, 3rd ed., New York:
McGraw-Hill, 1985) shows that this is standard
practice for all locknuts (Figure 1-2), and an il-
lustration from SAE standard J756 and J755,
for propeller shafts, clearly shows that the half-
height nut is against the hub (Figure 1-3).
If you go to any marina where boats are
hauled for the winter, my guess is that 99 per-
cent of the prop nuts will be on the wrong
way. I wouldn’t worry too much about the
nuts being on backward—as they usually
are—because the wrong way has proven suf-
ficient. But now that you know, you might as
well put the things on right the next time you
install a prop.