The optimal characteristics can be sum-
marized as
- a good lift-drag ratio for minimum fuel
consumption - minimum drag with rudder at or close
to zero angle, for minimum fuel
consumption - gradual and steady increase of lift at
low to moderate helm angles (If the
slope of the lift curve is too steep, it
will cause oversteering. This results in
an added small reverse course correc-
tion, which is hard on the helmsman or
the autopilot and causes additional
drag and increased fuel consumption.)
Course-Keeping Comparison
The lift-drag ratio at helm angles up to 10 de-
grees is similar for both types of rudders.
Since the flap on the flap rudder moves to
twice the rudder angle, the lift-drag ratio is a
bit more than a fishtail rudder at angles over
5 degrees. Because the flap angle continues
to be double the main rudder angle, it is more
likely to generate an oversteer reaction at
cruising speed, particularly at higher speeds.
The conclusion is that the fishtail rudder
has a slight advantage over articulated or flap
rudders in course keeping.
Maneuvering
At low speed in close quarters, such as during
docking, anchoring, precise station keeping,
and so on, large, quick, accurate changes in
course are required.
The optimal characteristics for maneu-
vering can be summarized as
- maximum lift coefficient regardless of
drag; fuel economy is not a factor - the capability to redirect the flow from
the propeller slipstream to the maxi-
mum angle possible, to get as close to
true stern-thruster response as
practical
Maneuvering Comparison
At maximum rudder angles, the articulated-
flap rudder produces about 7 to 9 percent
more lift than a fishtail rudder.
Articulated-flap rudders usually have a
total rudder angle of 90 degrees (45-degree
main blade, 90-degree flap). They thus work at
nearly perpendicular to the slipstream. Fishtail
rudders vary from 40 degrees for the MacLear
Thistle rudder to as much as 65 degreesfor the
fatter-section rudders such as Schilling. Fish-
tail rudders thus cover up to 65 percent of the
slipstream, while the articulated-flap rudder
will cover almost 90 percent.
Note that both fishtail and flap rudders
almost always have greater balance than
standard balanced rudders.
Considering the preceding information,
the articulated-flap rudder will be somewhat
more effective for maneuvering.Articulated Versus Fishtail
Rudder Summary
Both the fishtail and articulated-flap rudders
will result in a pronounced improvement in
steering response. The fishtail rudder would
generally be the best all-around rudder, as it
has no moving parts and less drag during
course keeping (95 percent of operation for
most vessels), while at the same time giving
very significant benefits in maneuvering.
Articulated-flap rudders give the maxi-
mum low-speed maneuvering response. For
vessels that will be docking repeatedly in
difficult conditions, boats that have very pre-
cise station-keeping requirements, or for
craft whose great windage makes them diffi-
cult to handle, articulated-flap rudders have a
slight edge over fishtail rudders.
There is one important exception: when
going astern, articulated-flap rudders stall
earlier than even conventional rudders. This
is because the flap—at twice the main rudder
blade angle—stalls in sternway conditions
very quickly.
The difference between the two rudder
types isn’t large, however, and both accom-
plish nearly the same results.A Hierarchy of High-Lift
Rudders?
It is possible to very roughly create a hierar-
chy of high-lift rudders based on the relative
size of the turning circles they make possi-
ble at maneuvering speeds. The following listChapter 13:Unusual and Special Rudders