rudder angles

[sean9c]

Ed Hagemann was a big proponent of twin rudders on single propped boats.
http://www.mvstarr.com/uploads/49799/Passagemaker03-04.pdf

FF wrote;
"Sure but the skin friction can only be reduced by shrinking the rudder."

Or the velocity of the water going over it.
Can you imagine a cruiser going 18 knots w a sailboat rudder? Too much drag because there is too much velocity of the water flowing over it.
Also if you doubled the size of the rudder you'd double the drag. But if you doubled the velocity (speed) I think you'd increase the drag by four times.

Just another idea of mine ......
One could conceivably reduce rudder drag on a slow trawler by having two rudders .. one to each side of the propwash but outside of it. The propwash innitially cones inward so the two rudders need'nt be that far apart.

 

[McGillicuddy]

"One could conceivably reduce rudder drag on a slow trawler by having two rudders"

Our USN Utility does have 2 rudders, one is called a backing rudder.

The launch can go 12+K depending on load so the ruder is power boat size , not sailboat size.

The hassle is docking , and since the Utility prime job was taking 150 sailors ashore , a backing rudder is the Navy solution.

It is a cone like a megaphone , installed in front of the prop with a connection to the rudder.

The concept is (I think) that the cone would take prop wash when in reverse and it would provide thrust to the boat, so it could be stopped straight into a slip,with no prop walk. .

Might have worked , but my installing a 32x32 prop totally out walks any thrust from the device.

Internally it does provide a great spot to mount the AP rudder angle unit.

Almost all inland tugs/push boats have a 'backing rudder' normally referred to as a flanking rudder. The set up is two smaller rudders ahead of each prop and one larger rudder at centerline aft of the prop. Works on multiple prop boats as well as singles. There is a separated control lever or 'stick' for the flanking rudders. They are only used when the engines are backing, to give steerage in the astern mode. The most fascinating way in which they are used is down bound with a large tow of multiple barges in a fast moving river. As the tow approaches a bend, the captain backs his engine to slow the tow and to move the stern of the tug to the slacker water on the point of the bend. Then he holds the stern of the tow in this position while the faster current around the outside of the bend sweeps the front of the barge tow around the bend. When the front is all the way around the bend the tug comes ahead with the engines and starts pushing hard to get back ahead of the current. Fantastic to watch.

 

[Sailor of Fortune]

The "backing rudder" on the navy launches (they were on Uniflights too) aren't really flanking type rudders because they can't be independently controlled. They are connected to the steering rudder by a drag link.
Half the people that have used them say they work well, the other half say they do nothing.

 

[Nomad Willy]

sean9c re post 31,
That's almost exactly what I was thinking about. I read about 80% of the article and was pleased to hear someone elese w some real credibility agreed w me about wedge and articulated rudders in that they seem to work well in the harbor but offer little if any advantage in big following seas.

I was disapointed to not find any input on effeciency. Actually my idea was to have the two rudders a bit further apart. On the 75'Starr the rudder was probably still somewhat in the propwash so there was probably little if any efficiency gain. With a smaller prop and a bit more rudder spread the efficiency could be there.

Thanks for posting!

 

[kapnd]

The "backing rudder" on the navy launches (they were on Uniflights too) aren't really flanking type rudders because they can't be independently controlled. They are connected to the steering rudder by a drag link.
Half the people that have used them say they work well, the other half say they do nothing.

I'm leaning to the "do nothing" side, and may remove the backing rudder on next haulout to find out for sure.
So far my observations are that it creates turbulence ahead of the propeller, reducing prop efficiency, and adds quite a bit of friction to the steering system with its additional linkage, bushings, and stuffing box.
It is a real PITA to keep clean, barnacles love the inside of the tube.
My boat always runs in a "light" condition, so prop walk negates any small effect the reverse rudder might have. On a loaded boat, I think it might have more prominent effect, as the boat will take longer to gather inertia, and have a longer stationary period during gear changes to allow the little rudder to do its thing.
I back into my slip, and if the wind direction is with the prop walk, I have to start backing in two slips above mine!

 

[Nomad Willy]

McGillicuddy,
Great copy about the river bend maneuver.
I'd like to see that too.

 

[McGillicuddy]

NW- you would probably also like to hear the opposite of what you proposed: two screws with a single rudder. There were a few ships built with this configuration, most especially passenger, vessels or what we now refer to as cruise ships. One in particular was the P&O liner 'Canberra'. I watched it come down the Mississippi River one day approaching Algiers Point, it almost didn't make the bend as the current swept it to the outside of the turn and it almost hit the wharves. I had a drinks with my friend Dan that night and he told me that he was the river pilot on board and he almost had brown trousers and was calling for tug help before he even got to the bend. He said it steered like a pig.

I always wondered why the designer did that. Then it occurred to me that the ship had originally been built for the UK/Australia immigrant run. It's a long journey with no stops. So having one central rudder was likely quite efficient at sea. It used tugs to assist berthing in Southampton and Sydney. Once passenger jets made that trade uneconomical the ship became a cruise ship which had a whole different set of required maneuvering characteristics. Most cruise ships nowadays have azumuthing main propulsion and multiple bow and stern thrusters and therefore need no tug assist to dock.

 

[cappy208]

The Cruise ship Monarch had twin screw, single rudder. I thought that was THE worst handling ship I've ever been on. As a trainee pilot I asked the pilot if I could pilot her. He looked at me, chuckled and said: ask me that in about 20 years. Humbled, I stood back. It was a handful to get through the Canal. All he said about it later was: I hope I never draw this job again. She was reincarnated as the Bermuda Majesty. Now razorblades. Many methods have been tried. It's hard to beat a rudder directly behind a Prop. But Flankers.... Oh La La.