Incorrect pockets can generate consid-
erable noise. The most common problem is
that the pockets are too short fore-and-
aft, with an abrupt entry where they start
forward. The result is interrupted water
flow into the upper portion of the prop
disk. In other words, the upper portions
of the blades—when they reach the top of
their rotation—momentarily see somewhat
restricted water inflow compared with the
rest of the blades. This causes uneven load-
ing and thus vibration.
POCKET PROPORTION RECOMMENDATIONS
When you are installing shallow pockets or
are looking at vibration problems in a boat
with pockets, review the pocket geometry.
The pocket entry should probably be longer
than you think. As a rough rule of thumb, I
like at least a 7:1 slope on the entry. The
pockets should retain their full depth about
one prop diameter forward of the propeller
before the taper starts (Figure 1-7). If the
pocket is recessed into the hull about
9 inches (228 mm) at the prop, and the prop
has a 25-inch (635 mm) diameter, then the
pocket ought to remain constant in cross
section until 25 inches (635 mm) forward of
the prop. Then it should taper gradually
through a distance of 63 inches (1.6 m) for-
ward of that—9 in. × 7 = 63 in., or 228 mm×
7 =1,596 mm (1.6 m). Pocket and tunnel
design is a whole subject in itself, so this
recommendation is just a rule-of-thumb
guide. It is a reasonable marker for a possi-
ble problem, however.
Figures 1-8A and 1-8B show an aluminum
ferry that required repowering work. This
vessel had suffered from severe vibration
because the pockets had been designed
tooshort and with too little propeller tip
clearance. The captain explained that the
vibration had been so severe that welds had
broken. Installing smaller-diameter propellers
with higher DARs couldn’t fully correct this
built-in defect, but it did make a noticeable
improvement.SMALLERPROPDIAMETER ANDMOREBLADE
FOR POCKET PROBLEMS If you’ve got a
problem caused by too short a pocket, one
possible solution, again, is to go to a
smaller-diameter prop with more blade area
and wider blades. The additional blades will
raise the frequency and smooth out theChapter 1: Propellers and Shafts
Figure 1-6. Deep propeller tunnels
Figure 1-7.
Recommended
pocket proportions