A Suggested Explanation for Soviet/Russian Use of Tandem Submarine Propellers
By Bruce Rule - Jul 18, 2014
A tandem propeller utilizes a pair of conventional propellers fitted on the same shaft in series and rotated in the same direction.
Tandem propellers should not be confused with previously discussed counter-rotating propellers which have two shaft lines, one within the other, to drive two propellers in opposite direction at nearly the same rpm. Examples: USS JACK (SSN-605) and the USS ALBACORE (SS 569) in the last of its many modifications.
Open sources indicate VICTOR, CHARLIE, OSCAR I and the single MIKE Class Soviet/Russian nuclear submarines employed tandem 4-bladed propellers (total of eight blades) on some or all units of those classes. Most of those tandem propellers had a 45 degree angular separation between the eight blades; however, at least one CHARLIE Class SSGN had a 20 degree separation which produced a 70 degree gap to the next set of blades.
The link below shows an image (click to enlarge) of the KOMSOMOLETS (MIKE/Project 685) Soviet nuclear submarine) tandem, 4-glad)ed propeller resting on the bottom in the far northeastern Norwegian Sea at a depth of 5530 feet (1685m). It sank on 7 Apr 1989. Note the four-vaned vortex dissipator on the shaft hub (far left).
Increasing the number of blades on a submarine propeller while retaining the same diameter increases the thrust that can be developed at a given rpm or, alternately, permits a propeller of smaller diameter to develop the same (original) thrust at that given rpm.
Decreasing the diameter of a submarine propeller reduces the rotational velocity of the blade tips, the location where cavitation first occurs; hence, the use of tandem propellers appears to have been an effort to raise the cavitation inception point of the above noted classes of Soviet nuclear submarines for given combinations of depth and speed.
This conclusion is based on the assumption that measurements more refined than those the writer can make ffrom open sources will confirm the diameter (and rotational velocity at a given speed) of a single-plane, 5-bladed VICTOR propeller is greater than the diameter (and rotational velocity) of tandem propellers used by other VICTOR Class units operating a the same shaft rpm.
The extensive use of tandem propellers indicates that, at least as late as the 1970s, the Soviets were most concerned with the detectability of cavitation in a submarine versus submarine environment to the exclusion of other considerations. An independent assessment states: (quote) A tandem propeller on a single shaft creates less noise compared to a single propeller; therefore, the sonar system of a submarine, limited by self-noise at high submarine speeds, can perform better. (end quote)
So, we have - apparently from the Soviet perspective - a two-fold benefit from the use of tandem propellers: (1) reduce the high frequency detectability of the submarine (cavitation) while also improving its capability to detect other submarines. This extensive use of tandem propellers appears to have been another example of "Better is the enemy of good enough." where better later became the ability to design and fabricate the skewed, seven -bladed propeller first employed by the AKULA Class SSN in the mid-1980s..