universal joints to get the engine more level
than the shaft.
Having said all this, I recall how I once in-
herited the design project of a 40-foot (12.2 m)
boat with a 17-degree shaft angle. It was a pro-
duction vessel, and the tooling was too far
along to change. So we kept the shaft angle
and built around it. Though I had concerns, in-
terestingly enough the boat ran flawlessly.
Quite a few of these boats were produced.
They all operated smoothly with no vibration,
made exactly predicted speed (with no
allowance for loss due to shaft angle), and were
good sea boats. (A down-angle gear reduced
the engine-installation angle to 7 degrees.) My
feeling is that a bit too much emphasis is
placed on getting flat shaft angles. My intu-
ition tells me that the water flow entering the
propeller disk is somewhat straightened out
by the suction (lift) of the forward side of the
propeller; so the water tends to enter the
blades at closer to parallel to the propeller
shaft than the shaft angle would indicate. It
would be interesting to run systematic, real-
world tests to see what is really happening
in this complex flow pattern.
Regardless, I have never set out to design
a boat that had more than a 15-degree shaft
angle.Shafts Angled or Offset
in Plan View
Shafts, as previously discussed, are almost
always angled in side or profile view; how-
ever, they can also be angled in plan view: i.e.,
angled in and out rather than higher forward
and lower at the stern. This is not common,
and it’s generally best to arrange propeller
shafts parallel to the boat’s centerline, but
shafts can be angled in or out to solve some
installation problems. This practice was
more common in the early days of power-
boats and auxiliary sail. It’s not unusual to
find boat plans from the 1920s and ’30s with
shafts angled in plan.
The primary consideration with such
installations—in single-screw boats—is
that the turning effect of propeller be
taken into account. In fact, one of the rea-
sons old-time boats sometimes had angled
shafts was to counteract this propeller
effect. A right-hand wheel is the standard
on single-screw craft. Such a propeller
rotates clockwise as viewed from astern
(Figure 2-10). It tends to turn the bow to
port going ahead and the stern to port
going astern. If you mount the shaft paral-
lel to the centerline but off to port, thenPART ONE:DRIVETRAIN INSTALLATIONS
Figure 2-10.
Rotation of a
right-hand wheel
(viewed from
astern)