Randy Scott's "Mad Scientist" Responds
By Bruce Rule - May 24, 2014
If having the intellectual curiosity to look beyond the data and ask “why” is a hallmark of insanity, then I take Randy's comment as a compliment.
The current question those concerned should be asking is why the BOREY Class Russian SSBN has a pump-jet propulsor while the new YASEN Class Russian SSN has an ”open” seven-bladed propeller as clearly shown by Internet images even though the screw was canvas-covered when imaged from the stern on a building way.
One advantage of a pump-jet propulsor is that the stator directs thrust developed by the rotor along the axis of the submarine which reduces the tangential thrust which, in turn, reduces torque on the hull which allows the stern area control surfaces to be smaller than would be the case with a conventional screw. This permitted the BOREY to use the stern section – including the control surfaces of a much smaller Project 971 AKULA Class SSN.
In fact, both the bow and stern sections of the lead BOREY Class, the YURI DOLGORUKY (K-535), were originally constructed for the COUGAR (K-337) which was never finished. Why was this cannibalization necessary? - because, as a result of the collapse of the Soviet Union, the Russians lost their capacity to produce the required steel; the plant was in the Ukraine.
I seem to remember there is a very slight difference (one degree?) in the angle of the port and starboard stern planes relative to the long axis of a single-shaft submarine when viewed from astern. This to compensate for the above mentioned torque developed by “open” propellers. Such torque is not an issue for submarines with two shafts because they turn in opposite directions: outboard.
So, one reason (the only reason?) the BOREY has a pump-jet is that that type of propulsor allowed the use of existing AKULA hull sections. (If a pump-jet had other important advantages, one would have expected to see it used with the YASEN.) 0ne might say that, as finally built, the BOREY represents a major compromise of the original late 1970s concept which was a twin-shaft submarine close to the DELTA IV. The BOREY appears to be yet another example of “Better is the enemy of good enough,” and, as noted earlier, using AKULA hull sections and main propulsion power train was "good enough."
Oh, by the way, Internet sources confirm the BOREY has a “double reduction planetary” gear system and a left-turning propeller shaft.
Also note that using an AKULA bow section means the BOREY has eight torpedo tubes: four 650mm tubes and four 533mm tubes. That's pretty nasty for an SSBN. All other Soviet/Russian SSBNs had either four or six tubes.