Twisted Rudder design

[kach22i]

Stator as rudder?

Rudder Gets New Twist With Composites
http://www.compositesworld.com/ct/is...06/August/1408

Ship rudder design, historically, has been dictated by the need to minimize hydrodynamic drag, resulting in simple, linear designs analogous to vertically inclined wings. It is, therefore, far from obvious why the U.S. Navy's latest development is a rudder that is twisted in such a way that it presents a different angle of attack at different water depths. The reason for this departure from conventional rudder design has to do with the flow of water from the propeller...........................To alleviate cavitation, Dr. Young T. Shen of the Naval Surface Warfare Center Carderock Div. (NSWCCD, Bethesda, Md.) developed a "twisted rudder" design that incorporates a varying angle of attack from top to bottom (see photo, this page) — one that conforms more closely to the actual water flow pattern. During a development and testing program that was completed in June 2001, trials onboard the USS Bulkeley, an DDG 51 Arleigh Burke-class destroyer, using a prototype metal rudder constructed in accord with the new design showed that the proposed solution works well, delaying the onset of cavitation from 23 knots — within the normal operating speed range of Arleigh Burke-class ships — to 29 knots. Tests also demonstrated that the design had no negative impact on speed or steering capability..................... "The composite rudders are produced from molds that allow us to produce a rudder for about half the steel rudder cost."

[mflapan]

It is amazing how old ideas resurface!

I used to be half owner of a very old motor boat called Swan (fitted with an 1897 4 stroke Union Gas engine) that was fitted with a rudder that operated on very similar principles.

According to the story that went with the boat, it was at one time owned by the chief naval architect with the Australian Commonwealth Shipbuilding Board at Cockatoo Island in Sydney. In 1931 the yard built a lighthouse tender called Cape Otway that was half a knot under contract speed during trials. To overcome the problem, someone came up with the idea of a cranked rudder having a shift in the angle of attack in line with the shaft. So instead of a gradual twist, there was a sharp twist having a near horizontal transition plate between. The concept was tried out on the chief naval architect's motor boat and was found to be effective in increasing maximum speed. So they fitted a similar rudder to the Cape Otway and did achieve the contract speed. However, the rudder proved to be problematic in service due to the near horizontal surface causing the rudder to hammer vertically on its pintles as the vessel pitched. I understand it was later replaced.

When I was part owner of the Swan in the late 1970s, she still had the cranked rudder from 1931, and she had a good turn of speed from her 5 bhp one-lunger.

A few years back, I was searching the internet and came across a proprietary cranked rudder design being marketed in Germany, if my memory is correct.

By the way, the factory where they made the Union engine in the Swan was destroyed in the 1906 San Francisco earthquake.

Best regards
Mori

[alan white]

Speaking of efficient rudders, years ago I designed (but never built) a two rudder steering system for a sailboat. The rudders curved right and left respectively, and they dipped to turn the boat. When no steering was needed (as when the boat perfectly balanced), no rudder was in the water.
Steering would involve one curved blade or the other to dip, never both at the same time.
The result was far less wetted surface and consequent friction/drag. Only enough blade dipped to do the job. One could balance the rig and know the hull underbody was truly balanced.

Alan

[rwatson]

Alan - I know the 'dipping' rudder concept is at least 20 years old. I saw a catamaran ferry being built with the two rudders at an angle, and whether the port or starboard rudder, and the depth they were lowered controlled the steering.
As far as using them on a sailing boat, the steering 'feel' and responsiveness would be interesting to test.

[alan white]

Nothing new under the sun, I guess. I think especially with sailboats, and particularly with older more balanced type hulls, the dipping rudder could be practical. It IS more complicated, and that alone is good reason to stay away from the idea, but I think it would make for a faster boat in light air.

A.