gear needs to be somewhat customized to
handle the added travel.
OTHERFISHTAILRUDDERS Hydrodynamic
fishtail rudders are commercially available
for large vessels from companies such as
Schilling (the Schilling rudder, which may
predate the MacLear Thistle), Ulstein, and
others. Many of these rudders have propor-
tionately fatter (even bulbous) sections
with more balance and a blunter, round
leading edge than the MacLear Thistle
rudder. These rudders are optimized for
even better response at still higher rudder
angles—as much as 65 degrees. Primarily
intended for ships, these large-balance,
fatter-section fishtail rudders pay a slightly
higher penalty in drag at small rudder an-
gles for nearly true stern-thruster response
during maneuvering.
For both fishtail and articulated-flap rud-
ders (see below), rudder-blade balances as
high as 40 percent have been used, with
35 percent being common. This means that
the rudder projects forward of the rudder
stock more than on a standard balanced rud-
der, and adequate clearance must be checked
and may require modifications on a retrofit.
This larger balance is also one of the causes
for sometimes problematic steering on high-
speed boats with articulated rudders.
SIZING FISHTAIL RUDDERS The MacLear
Thistle rudder, with 20 percent balance as in-
dicated, should be sized according to the
standard rudder area formula; however,many companies size fishtail rudders (and
articulated-flap rudders) as rectangles, with
their height slightly greater than propeller
diameter, and the chord between 70 and 80
percent of propeller diameter. This should
give good results as long as the propeller size
and installation are within the normal range
for a vessel of the type that it’s driving.FLAT-PLATEFISHTAILRUDDERS In Chapter 10
we described the general construction of flat-
plate rudders. Though we saw that flat-plate
rudders are acceptable, we also found that
they give less steering response and create
more turbulence than any other rudder type.
We learned that flat-plate rudders are really
a cost-saving option.
There is a way to get flat-plate rudders to
perform better. It is to weld a fishtail to their
trailing edge. This is particularly effective
when used in conjunction with endplates.
There are a few advantages: The fishtail
increases rudder response at higher angles
than a standard flat-plate rudder could toler-
ate (as well as slightly improving course-
keeping, low-angle rudder response). The
endplate (as it does with any rudder foil
form) increases the lift generated from a
given rudder area. At the same time, both the
fishtail and the endplates are fabricated of
flat plates themselves, simply welded onto
the flat-plate rudder blade, and so construc-
tion remains inexpensive. Such flat-plate fish-
tail rudders (Figure 13-8) are really only
appropriate for displacement vessels, as thePART FOUR:RUDDERS AND STEERING SYSTEMS
Figure 13-8.
Flat-plate fishtail
rudder section