Wayne writes about Ted's schilling rudder:

"Looks good and its easy to see why it should work well. One small
point though, you didn't make a hole in it to pass the prop shaft
through it should that ever become necessary. It was there in your
bronze rudder."

Many of the advantages of the Schilling rudder can be achieved by
bolting 10 degree wedges to the aft third of a bronze rudder plate. The
wedges serve like flaps on an airfoil and let the rudder be effective
at a greater angle. The main problem with powerboat rudders is that
they are too small to control the boat properly at marina speeds and
rely primarily on deflecting the thrust of the propeller. Compare the
size of a powerboat rudder to a similarly sized sailboat and you will
see what I mean.
Larry Z

Snip:"...Just as important for the rudder is its leading edge, but kinda the
opposite of the tailing edge. The leading edge should be a bullnose shape
which transitions into the foil shape with very fair lines.

But, as been mentioned, it is often cheaper to weld on end plates, and wings
on the tailing edge, than it is to develop a foil shape over the entire
rudder. ..."

A variant of the shilling rubber (fish tail) has been standard equipment on
Great Harbour displacement trawlers for four years and has been a desired
retro fit item given the "fish tail" rudder's greater performance in greatly
reducing the turning radius and increasing helm responsiveness.  They are
foil shaped like an airplane wing with hydrodynamic endplates at the top and
bottom. These rudders are built of dense, steel-reinforced fiberglass.

To quote a much more educated person than I,"...This rudder design for the
Great Harbour Trawlers is really an old one, and is derived from historical
designs that sought to both extend the stall free range of rudder angle and
develop high lift at all rudder angles. The end plates top and bottom
increase the rudder's effective aspect ratio. In layman's terms, they force
the water into behaving as though the rudder is far deeper than it actually
is by preventing water from flowing over the ends of the rudder. This would
rob lift producing pressure differential and create turbulence to increase
drag. The fish shaped cross section, though, is really the key to success.

The rounded leading edge delays the stall angle, as does the wedge shaped
trailing edge on the low-pressure side. The wedge on the high-pressure face
greatly increases lift from the back end of the airfoil, which is typically
quite ineffective. And it shifts the center of pressure of the rudder aft a
bit, allowing the rudder stock to located closer to the center of the
rudder, which in turn forces more of the rudder blade into the propeller's
slip stream.

The first trial results have been outstanding. On side-by-side tests of
identical boats fitted with the old and new rudders, the new rudders turned
in one half the turning diameter of the old at low RPMs and about 60 percent
of the diameter at high speed. The turning diameter at full RPM is now about
three boat lengths at lower rpm's the new rudder cut circles at 1/2 the
diameter of the old; at mid-range rpm's approximately 60 per cent and
slightly less at the high end. Of course how the rudder is set may make a
difference...."

Bottom line they work very well for shallow draft displacement speed
trawlers like the Great Harbours. Backing is a function of centering the
rudders and using differential throttle (and or bow thruster if needed and
equipped) to control the boat's direction in reverse.  This rudder
effectiveness also facilitates a more effective 90 degree sideways motion
when splitting plants with rudder over.

Joe

Carolyn Ann  GH N-37

'Lo All,

First, I am not an aerodynamic (can I say that here?) or hydrodynamic
engineer, but I studied aerodynamics when I designed a two-seat
airplane - I made a model of it which was squashed during the Alaskan
Earthquake. Being in the USAF and moving an average of every 11
months, I never had a place nor time to actually build it. I sent the
plans to the Experimental Aircraft Association for evaluation as it
did not follow the usual paradigms of the day - and a few years
later, an aircraft emerged that was strikingly familiar. Also, I have
owned two airplanes. (All the above to show my credentials - or lack there of.)

The Schilling Rudder simply uses the widely known and accepted rules
of aerodynamics, or, more acceptably in this case, hydrodynamics. An
airfoil, designed for the desired speed range is little different
that a hydrofoil, designed for its desired speed range.
Unfortunately, boat design never progressed as far as
aircraft/aerodynamic designs because the payoff was much less, except
for special niches, such as sailboat racing, and to a lesser extent,
powerboat racing. Look at how long it took shipbuilders to add
bulbous bows...which significantly reduces fuel consumption, and for
tugs to employ variations of Kort nozzles which significantly
improves maneuverability and thrust.

The Schilling Rudder is essentially dual, opposing, fixed trim tabs
on a symmetrical wing. The reason it works better for turning is that
water strikes the trailing edge and is diverted, just as a trim tab
on an airplane diverts the air, causing an additional force to be
generated. When turning, say, hard to port, the port side of the
rudder thrusts the water off the rudder at a greater angle than that
of the rudder, relative to the keel, while the starboard side induces
little to no thrust, as it is in at least partially stalled/turbulent
water and the thrust angle is reduced. (It's the difference between
these two opposing forces that cause the rudder to turn a boat
better.) The leading edge airfoil causes much less drag at normal
operating rudder angles because the water can follow the shape of the
rudder more easily without breaking away. When the smooth water flow
breaks away from the rudder surface, it causes turbulence and thus,
drag. Again, we can thank aerodynamic research, specifically the
National Advisory Committee for Aeronautics (NACA), now NASA, for all
the airfoils they tested and documented. (My airplane design, for
example used the NACA 23012 airfoil at the wing root and tapered to a
NACA 23010 airfoil at the wing tips.)

An articulated trim tab on a boat's rudder would be more efficient
than fixed tabs, such as used in the Schilling Rudder. The most
probable reason they are not widely used is the relatively small gain
in efficiency when balanced against the complexity of the linkage and
the ease of damage when the rudder hits something solid. Of course,
all rudders need to be clean to be efficient. Any hard growth, such
as barnacles and oyster shells dramatically reduce a rudder's
efficiency and increase drag. I have no knowledge of how the
Schilling Rudder would work in reverse, other than to speculate that
there would be a small penalty due to the fact that effective rudder
angle is reduced approaching the leading/front edge from the point of
maximum chord (thickness), thus probably reducing the effective size
of the rudder. That should be off-set somewhat, however, by the
prop's increased efficiency due to less turbulent water. (???) A
smoothly curved face on the trailing edge endplate would also be more
efficient than a "T" shape of bolted-on angle stock. Obviously, there
is lots of room for research to tweak boat design, but at our speeds,
the cost vs gain ratio would be prohibitive except in a case, such as
Capt. Will's, where it was just his own curiosity that drove him to
investigate and document those subjects that were of particular
interest to him.

I hope this explanation is not too far off - please correct me where
I may be wrong....and I hope it helps to clear up some folks' idea of
how it works and why.

Take care and be safe.

Wayne
Celestial
Albin 43 Sundeck
Near Panama City, FL