Twisted Rudder design

Stator as rudder?

Rudder Gets New Twist With Composites
http://www.compositesworld.com/ct/issues/2006/August/1408

 

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

 

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

 

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.

 

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.

 

Why are dipping rudders more complicated? The dipping rudders of waterbikes seem simple enough.

Luc