....3 screws.....

[corvette1271]

hi i am new this forum and to termonology and engineering of large warships. i am an engineer in the aerospace sector but have been interested in military history/science and would like to learn about features of this story. one of the things that i would like to know is why 2 screws on the bismarck rotate in one direction, and the other screw rotates the opposite direction?

[marcelo_malara]

There was another thread recently about this, but I believe that the outer screws are counter rotating to balance the torque. And as they are odd, the center screw rotate in the same direction that one of the others.
You may know that the propellers of the P-38 are counter rotating too, don´t know if this applies to all propeller driven aircraft.

[ontheslipway]

Depends what torque needs balancing, as far as I can see none. However, co-rotating props on a double shaft ship induce a lateral force.

Also, due to the influence of the wake, vibration and efficiency will differ.

[marcelo_malara]

Sorry Foeth, the turbine does not produce torque as a piston engine do?

[corvette1271]

well i do know that a piper senaca, PA-34 which is a light twin, has counter rotating props.....and it was designed that way to eliminate torque, especially on take-off.

[ontheslipway]

Sure it delivers a torque, but it's not a problem. Notice we have lots of single screw ships which all have torque acting on the ship through the propeller action. This torque is not so important compared to the stability in the ship, small enough to be ignored. I can image that a small airplane is affected, as Corvette pointed out, I wasn't aware of that. Torpedo's also have counter rotating propellers to remove torque. Note that counter rotating propls usually have a higher efficiency as well as thier rpm can be lower than a single screw ship. The only problem for large ships is the construction so you rarely if ever see them.

[marcelo_malara]

So Foeth, the only reason for counter rotating propellers in a multi screw ship is to eliminate the lateral force? How did Iowa´s screws rotate?

[_Derfflinger_]

How did Iowa´s screws rotate?

The Iowa's had four shafts. Each inboard shaft had a five-bladed, 17ft 0in screw; each shaft was housed in a hull skeg, with one of the twin rudders mounted behind the skeg in each of the inboard screw's wash. The two outboard shafts each had a four-bladed, 18ft 3in screw.

Each rudder was 340 sq ft in area.

Total shaft HP was 212,000. At 202 RPM on the shafts, an Iowa BB would step out at 33+ knots.

I can't find a written reference on prop rotation, but from the looks of the few pics and/or drawings I can find of the Iowa's underside at the stern, the two port shafts rotated counterclockwise when looking forward towards the bow, while the two starboard shafts rotated clockwise.

Derf

[ontheslipway]

o Foeth, the only reason for counter rotating propellers in a multi screw ship is to eliminate the lateral force?

No, there are more reasons. The lateral force isn't that large, most single screw ships can sail without much trouble. The flow to the propeller is not homogenous, so going left or right handed can change the efficiency and vibration characteristics. It depends on the hull shape what works best, but reversing shaft rotation sometimes has benefits. The cruise liner Oriana changed her rotation direction after encountering severe cavitation problems. (and changed bearings and gear boxes and so on!).

Counter rotating propellers on a single shaft are efficiency wise quite nice, but cavitation can be difficult to control. Such a solution is difficult to simulate during design.

[corvette1271]

....so we have cavitation factors, hull shape and propeller efficiency as factors as well? the designers must have settled on this arrangement for some particular reason. i have read from somewhere here...perhaps on this website that german engineers wanted a vessel with open water efficiency? (undersanding the mission that they were designing it for)and so this rotation feature was evidently what worked for them. thoughts on this?

[ontheslipway]

Open water efficiency is a standard propeller term, not something special. Open water effiecincy only means you run the propeller in a homogeneous inflow during your model test. With those data you can do a propulsion test. My guess is that the most important aspect is effieciency. You can loose a few percent with a different direction of rotation.

[Bgile]

....so we have cavitation factors, hull shape and propeller efficiency as factors as well? the designers must have settled on this arrangement for some particular reason. i have read from somewhere here...perhaps on this website that german engineers wanted a vessel with open water efficiency? (undersanding the mission that they were designing it for)and so this rotation feature was evidently what worked for them. thoughts on this?

There are other factors. Internal subdivision is one. They were happy with the proven arrangement in their WWI ships. They had to be able to navigate the Kiel canal. There were drawbacks too. 3 screws had to be individually larger than 4 screw arrangements, and I believe this resulted in the stern over the screws being shallower and therefore weaker in that area.

[corvette1271]

...internal subdivision...could you explain, please. and how does this enable navigation in the kiel canal? i can understand them designing in a proven arrangement.....and good to see you are in the neighborhood...

[marcelo_malara]

I think Bgile is referring to a 3 versus 4 screws discussion.

[Chel Sea]

well i do know that a piper senaca, PA-34 which is a light twin, has counter rotating props.....and it was designed that way to eliminate torque, especially on take-off.

I know that this is late in the day (of this thread), but just to confirm your point vis a vis avionics and torque, the Hawker series of "brute" planes based on the Napier Sabre engine (The Hawker Typhoon, Tempest, & Sea Fury aircraft), arguably the most powerful piston engines/aircraft (arguably, because the P-51 Mustang was also a piece of piston engine too) devised.

This series is what I have coloquially called "pure engines with wings, tail planes and a cabin tacked on as an afterthought", were so torquey that the pilots for all three models learned on take-off to instinctively pull to the left in order to counter the fact that the relevent craft would attempt to spin/pull to the right, so much was the torque or single propeller "bite" on the air, also why some later post WWII Spitfires had 2 sets of counter-spinning props, and/ to also chime in with the tropedo pointer above for single screw configurations.

http://www.vectorsite.net/avcfury.html


Great thread, thank you gentlemen