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Bruce Rule, for 42 years, has been the lead acoustic analyst at the Office of Naval Intelligence. In 2003, he wrote the Navy position-paper on the acoustic, dynamic and temporal characteristics of submarine pressure-hull and bulkhead collapse events. In 2009 he provided the Navy with the first reanalysis of acoustic detections of the loss of the USS SCORPION in 40-years which confirmed that disaster was the result of a battery explosion.


Information from open sources (footnoted below) provides the basis for estimating that the new BOREY Class Russian SSBN (Project 955) will reduce the detectability of reduction-gear noise by employing a hybrid propulsion system with a turbo-electric (TE) mode for patrol and low-speed transit operations while retaining a turbine-reduction (TR) capability for speeds above about eight knots.

Note: a Sep 2013 article1, quotes RADM Richard Breck-enridge and others that the OHIO Replacement SSBN will have an electric propulsion system to make them quieter than currently operational US SSBNs. The new system will employ high-speed turbo-generators to power a very large, electric motor to directly drive the propeller ((the motor armature (rotor) is the propeller shaft)) thus eliminating the need for reduction gears and the noise they produce.

It thus appears Russia and the US have, respectively, already taken or will take a similar approach to reducing the acoustic detectability of their strategic submarine assets. The BOREY TE propulsion mode to be employed only during patrol and low-speed transits with the main propulsion turbine (TR mode) declutched in a “ready status” (Russian term). The US design will accommodate the entire speed range.

Discussions of the BOREY Class Russian SSBN Hybrid Propulsion System

The BOREY Class uses a single-shaft steam turbine plant, the GTZA OK-9VM rated at 50,000 shaft horsepower2, the same system installed on all AKULA Class (Project 971) SSNs3 Note: the acronym GTZA abbreviates (and translated as) “main turbine gear assembly” which indicates BOREY and AKULA Class submarines may employ the same design reduction gear.

The BOREY also has a 5,576 metric hp (5,550 hp) motor,4exactly the same rating as the PG-141 dc propulsion motor used by Project 877 KILO Class diesel submarines5. The use of such a large dc motor (volume of about 380 cubic feet) would appear to be limited to a propulsion application on the BOREY with (rectified) power supplied by two 3,200 kW ship’s service turbo-generators3. Unless the motor has been highly modified to accommodate a shaft-centric installation—in which case it probably would have a new designation—the BOREY may employ a single-stage reduction gear to transfer power from the motor to the propeller shaft. Based on the reasonable assumption that the BOREY will have a turns-per-knot value of about 10 at patrol-mode speeds, a reduction ratio in the range of about six will be required to permit the PG-141—or a similar motor – to operate in an acceptable speed range. (The PG-141 has a maximum speed of 500 rpm.) So, there is no free lunch. A reduction gear will still be required for the BOREY TE propulsion mode, albeit a relatively small, single-stage system compared to the much larger multi-stage gear system associated with the TR propulsion mode using the GTZA OK-9VM.


Comparing the Russian and US approaches to the problem of reducing gear noise provides another example of the differing design philosophies. The US OHIO Replacement will be an entirely new design while the Russian BOREY probably will use existing propulsion system components, the GTZA OK-9VM main turbine gear assembly first employed in 1984 in the lead AKULA Class SSN, and the PG-141 dc motor first used in 1963 in JULIETT Class SSGs.

The Soviet-Russian axiom “Better is the enemy of good enough” (often associated with ADM Sergey Gorshkov, Commander-in-Chief of the Soviet Navy 1956-1985) is evident with the off-the-shelf GTZA OK-9VM and PG-141 propulsion systems being good enough for use in the new BOREY Class SSBN.

This approach potentially gains the primary advantage (noise reduction) of a low-speed TE propulsion capability while avoiding the cost of developing new propulsion system components, especially the very large electric motor that would allow the BOREY to retain a speed capability of more than 20 knots but would create significant installation and possibly trim problems.

In an analogous situation, the Soviets reduced the acoustic vulnerabilities (cavitation) of Project 877 KILO Class SS units (created by the use of the PG-141 dc motor to drive the propeller at speeds as high as about 500 rpm) by using the PG-141M motor on Project 636 KILOs. It is assessed the PG-141M (M for Modification) uses a built-in reducer (a Soviet term for a reduction gear built into the motor) to reduce the maximum propeller shaft speed from 500 to 250 rpm. From space and weight considerations, the most probable gear system is a planetary epicyclic star design: sun gear input, planet carrier fixed, ring-gear output. Star systems can accommodate reduction ratios between 2:1 (the Project 636 KILO value) and about 11:1. This was a simple, cheap and effective solution to the Project 877 KILO cavitation problem. The change from a flat-faced six-bladed propeller on Project 877 hulls to a skewed seven-bladed propeller for Project 636 hulls also helped.

This assessment of the KILO is based on the use of the same motor designation (PG-141) by Project 877 and Project 636 KILOs and the fact that a dc motor that produces 5,500 hp at 250 rpm would have approximately twice the volume of a motor that produced 5,500 hp at 500 rpm and would thus be difficult to install on Project 636 hulls from both space and weight standpoints.

If the Russians need to further reduce the acoustic noise levels of the new Project 636.3 KILO, they could use a reduction gear with the three-to-one ratio which was employed by the single BELUGA Class SS. This would result in a maximum propeller shaft speed of about 170 rpm for Project 636.3 units. Note: the BELUGA, which had advanced hull architecture, could achieve 26.6 knots at 170 rpm for a turns-per-knot value of 6.4. (All KILO/BELUGA information open source.)

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