Blade Area
Naval architect Uffa Fox said:
“The final answer to all engineering effort
in any vessel is in the propeller.”
This is not an overstatement. You can
build the most beautifully shaped hull and
install the most magnificent and powerful
new engines, but if the prop is wrong, the
boat won’t go. This isn’t the place for a
discussion of props, but a properly sized,
properly balanced, and properly installed
propeller is critical. One of the first things
I check—whether I’m starting a new design
or trying to solve a vibration problem—is thepropeller. In this regard, the most overlooked
aspect of propeller sizing is blade area. All
too often just the diameter, the pitch, and the
number of blades are specced. This is not
enough information—blade area is critical.
Too little blade area means the blades are
overloaded (even if diameter and pitch are
just right). Overloading leads to cavitation,
which means loss in performance, possible
blade erosion, and vibration.Disc Area Ratio
Blade area can be defined in terms of
absolute area—so many square inches or
square millimeters. But for propellers, it’s
usually more convenient to define blade area
in terms of disc area ratio(also called blade
area ratio)—DAR or BAR. This is the per-
centage of the area of all the prop’s blades
relative to a solid disc of the same diameter
as the prop.
A standard 3-bladed propeller has a
50 percent DAR, and a wide-blade 5-blader
has a DAR of about 120 percent. There’s a
huge difference in the power that these two
props can absorb, even at the same diameter.
By the way, a 120 percent DAR is about as
close to the theoretical maximum as you canPART ONE: DRIVETRAIN INSTALLATIONS
Figure 1-1. Thread
loading of a nut
and locknut
Figure 1-2. A jam
nut, applied under
a large regular
nut, is elastically
deformed against
bolt threads when
the large nut is
tightened