additional drag of the flat-plate fishtail and
endplates is too great at planing speeds.
The drawing shows a section through a
typical flat-plate rudder with fishtail and end-
plates, courtesy of naval architect Charles
Neville. The flat-plate blade thickness is
determined using Formula 10-4 for flat-plate
rudders, as are the stiffeners and the stiffener
spacing. Alternately, you can use multiple
endplates as stiffeners all along the height of
the rudder blade (at the calculated stiffener
spacing); however, endplates are only
required at the top and bottom of the blade.
Note that where the MacLear Thistle rud-
der had the rudder stock at 20 percent of
chord, the flat-plate fishtail rudder has the
stock even farther back at 24 percent of
chord. This can be done only with the angled
fishplates installed. If they are not, such a
high balance (about 24 percent) may cause
the steering problems described earlier.
Endplate thickness =75 percent of rudder-
blade thickness
Angled fishtail plate thickness =50 percent
of rudder-blade thicknessArticulated-Flap Rudders
I’m partial to the MacLear Thistle rudder and
related fishtail rudders because they are
more effective (for displacement hulls) than
conventional rudders, and they are also very
simple—no moving parts. It is possible to get
even more steering effect during low-speed
maneuvering and even more pronounced
stern-thruster effect (as good as the Kitchen
rudder, though without reverse) with articu-
lated rudders (articulated-flap rudders).
Instead of a single-piece blade, articulated-
flap rudders have one or more movable flaps
(sometimes rotating drums at their leading
edge as well) to increase effectiveness at high
rudder angles, at low speed. If you don’t mind
a bit more complexity (and these articulated
systems can be quite rugged) and you have a
displacement vessel that requires very precise
handling at low speed, an articulated rudder
may be the answer. (A Kitchen rudder is really
a very different form of articulated rudder;
however, the term articulated rudderusually
refers to a rudder with flaps, so they’re also
termedarticulated-flap rudders.)
It is possible to use articulated rudders
on fast boats, but it takes very careful engi-
neering and setup or there will be problems.
An example of this was a commercial vessel,
Te Kouma,chosen for use by the Royal New
Zealand Yacht Squadron. This was a 25-knot
boat and the following, from NZ Professional
Skippermagazine, January/February 2004, is
a typical experience for higher-speed craft:Te Koumawas originally fitted with an artic-
ulated rudder for low-speed maneuverabil-
ity. But this feature proved to be unmanage-
able, creating very tough steering on the
hydraulics once she was up to her cruising
speed. Had power steering been installed,
this would not have been a problem. In try-
ing to combat the stiff steering problem,
[Harold] Bennett removed the articulator
attachment to the rudder. On subsequent sea
trials this has removed all the stiffness from
the rudder operation, while maintaining good
maneuverability at high speed, with little dif-
ference in her low-speed maneuverability.Note that the trailing-edge flap on the
articulated rudders moves the center of pres-
sure considerably farther aft. For this reason,
most such articulated rudders have well over
24 percent balance (unacceptable for stan-
dard rudders). Balance as high as 40 percent
may be used, with 35 percent being common.
This high balance can be one of the causes
of potential problems for articulated rudders
at high speed, though it can usually be
tweaked to work. It’s not clear, however, that
the extra cost and complexity of an articu-
lated rudder makes sense on a typical twin-
screw planing hull, which has good maneu-
verability at low speed via backing and
forthing the opposing propellers.
The idea behind articulated rudders is
not new and has been around in one form or
another since the 1890s. Since rudders are
foilsthat generate lift in fluid flow, articu-
lated rudders are also properly termed
variable-geometry foils. They have been
used not only on boats, but on airplanes as
well. Some of the best-known manufacturers
of articulated marine rudders are Rolls
Royce/Ulstein (Figure 13-9), Hinze Becker
Marine Systems (the Becker rudder),
Wartsila Van der Velden Marine Systems’
Barke rudder, and Jastram, which offers notChapter 13:Unusual and Special Rudders