deadwood (ahead of the prop) doesn’t even
have its edges rounded off. At 12, 13, or
14 knots, or so, you can get by with such a
setup (though it’s less than ideal). But when
you push such hulls faster, the propeller will
be starved for water in the shadow of the
heavy, blunt deadwood and keel just ahead.
Even though lobsterboats were originally
displacement boats, as long as a lobsterboat’s
run is fairly broad and flat aft it can be made
to go 25 or 30 knots or more—given the
power—but you should provide the prop
with room to breathe. The traditional full
keel, ideally, should be cut away for 20 to
25 percent of the waterline length ahead of
the prop, and the upper/trailing edge of the
remaining keel/skeg should have a pro-
nounced rounded taper to reduce turbulence
(Figure 2-8). Additionally, the keel/skeg on
these boats ought to be dropped aft to make
room for a larger-diameter prop than fitted
to traditional displacement-speed lobster-
boats. Larger engines need additional blade
area—as we’ve seen—to transmit their
power, and greater diameter is the most effi-
cient way to accomplish this.
Shaft Angle
The standard rule is that the propeller shaft
angle should not be greater than 15 degrees
from the horizontal. Further, the lower or flat-
ter the shaft angle, the better. Broadly speak-
ing, this is correct. On all ordinary designs,
you should keep the shaft angle less than
15 degrees. There are two reasons for this.
One is that with the propeller at a pronounced
angle, the water flow into the propeller disk is
uneven from top to bottom. This essentially
changes the apparent pitch that the water
“sees” at the top of the propeller from that at
the bottom of the propeller. This in turn
causes uneven blade loading, which can
cause vibration. The other reason is that the
thrust line will not be forward, but will be an-
gled down, and so some efficiency is lost.
Another consideration is that almost all
engines are designed to be installed at no
more than 15 degrees. Fit them at a steeper
angle, and the oil sump will collect a pud-
dle in the lower aft corner, which won’t be
properly circulated. This can lead to a seri-
ous lubrication failure. In fact, the more
level you can install the engine itself, the
better for the lubrication system. Remem-
ber, if you have a planing hull with an en-
gine and shaft installed at 13 degrees and
the planing angle is 4 degrees, the real angle
of the engine will be 17 degrees, which will
cause these lubrication issues. The way
around this is to install the engine with
a down-angle gear or with a pair of CV orChapter 2:Struts, Propeller Apertures, and Shaft Angle
Figure 2-9.
Maximum shaft
angle