Liberty ships split rudder

[Kevin]

saw a photo today on another forum that the WW2 liberty ships had a top and bottom rudder, but i dont know why?, was it an adjustable setting to offset a poor build , ie to enable a straight line/course heading?

[allanyed]

Interesting subject Kevin.   I remember studying these ships in the Naval Architecture class and wish I still had my old books, but do not recall this subject (it has been close to 60 years)    I did find this paper which is quite interesting but see nothing about the rudder design to help you, sorry.    https://ww2.eagle.org/content/dam/eagle/publications/company-information/workhorse-of-the-fleet-2019.pdf

[Roger Pellett]

Kevin,

 

That’s called a ContraGuide rudder.   It is shaped that way to straighten flow from the propeller.

 

The propeller works by accelerating a column of water behind the ship.  The reaction to this column of water, Newton’s Third Law, is a force in the opposite direction that pushes the ship ahead. Water, however, is viscous and this viscosity causes the water in the column from the propeller to rotate.   This rotation is lost energy.  The offsets on the rudder are intended to introduce a vector opposite to that of the rotating water column, to recover the lost energy.

 

Roger

 

 

[Kevin]

2 hours ago, Roger Pellett said:

Kevin,

 

That’s called a ContraGuide rudder.   It is shaped that way to straighten flow from the propeller.

 

The propeller works by accelerating a column of water behind the ship.  The reaction to this column of water, Newton’s Third Law, is a force in the opposite direction that pushes the ship ahead. Water, however, is viscous and this viscosity causes the water in the column from the propeller to rotate.   This rotation is lost energy.  The offsets on the rudder are intended to introduce a vector opposite to that of the rotating water column, to recover the lost energy.

 

Roger

 

 

thank you,

 

rereading the reply it wasnt two rudders then with independent movement, it was two welded together offset

 

so how was this solved,

was it better propeller design, two props, twin rudders?

 

Edited January 21, 2022 by Kevin

[Kevin]

4 hours ago, allanyed said:

Interesting subject Kevin.   I remember studying these ships in the Naval Architecture class and wish I still had my old books, but do not recall this subject (it has been close to 60 years)    I did find this paper which is quite interesting but see nothing about the rudder design to help you, sorry.    https://ww2.eagle.org/content/dam/eagle/publications/company-information/workhorse-of-the-fleet-2019.pdf

thank you, i will read that when i get a chance

[Roger Pellett]

Kevin,

 

That’s right, one rudder, with two fixed offset parts.  I don’t know when or if they stopped using them; my Naval Architecture education occurred in the first half of the 1960’s and they were still being fitted to new ships.  

 

Finite element modeling, CAD, and powerful PC’s have allowed Naval Architects to analyze complex fluid flow problems that could not be solved in the 1960’s.  I would, therefore, believe that this has improved efficiency of propellers.

 

It may seem counter intuitive but propulsion efficiency is better for single screw than twin screw ships as the single propeller works in an area where it can capture more energy from turbulent water shed from the hull.  This “extra boost is called the Wake Fraction.

 

Roger