Radiated Noise Signature of Modern Diesel Submarines

Written by George Papangelopoulos

Japanese Navy Oyashio class SSK performing an
emergency main ballast blow maneuver. Photo: JMSDF

One of the best-kept secrets of submarine manufacturers and operators is the level of their subs’ signatures. As a result, related official published and open source data is mostly limited to general estimations of outdated designs from the Cold War era. The key signatures of a conventional submarine include its radiated noise, its target echo strength, its magnetic characteristics and its snort mast’s radar cross section and radiated heat. As new and more sophisticated naval mine detonators are developed, electric signatures (UEP, ELFE) are added to this list. The present article focuses on the radiated noise of modern diesel-electric submarines (SSKs), in an effort to shed some light on this interesting and highly classified subject. 

Italian Sauro class submarine emerging next to a Durand de la Penne class destroyer

Radiated noise is the total noise emitted by a submarine and can be received by hostile passive sonars. It should not be confused with sonar self-noise, although in most cases both are caused by the same sources. For example, flow noise can be an important contributor to the sonar self-noise even at low speeds, but it is only at high speeds that it becomes a significant contributor to radiated noise. 

 

Russian Kilo class submarine emerging next to Steregushchiy corvette

In the frequency domain, radiated noise comprises three components: tonals, transients and broadband noise. It is dependent on frequency, speed, depth, manoeuvring (e.g. turning), maintenance status and mode of operation (e.g. snorting), and its sources are classified as follows: 

Propeller noise 

Propeller noise is generated by the mechanical vibrations of the rotating propeller blades. The propeller’s oscillating thrust is the dominant noise source in the low frequency range, while the less intense trailing edge noise occurs in higher frequencies. Turbulence in the inflow causes broadband noise (100 Hz - 1 kHz). The propeller’s rotation may also generate strong high frequency broadband noise due to the formation and collapse of bubbles produced by cavitation, a phenomenon of seawater vaporising in low-pressure. Propeller cavitation can be suction-side, tip-vortex and hub-vortex. 

The propulsion system of U31, first German Type 212A submarine. Photo: Bundeswehr

The propulsion system of U36, latest German Type 212A submarine. Photo: Bundeswehr / Björn Wilke

In general, the advanced design of a modern submarine propeller provides a rather large cavitation-free margin of operation (speed/depth) and reduced-level tonals. For example, the HDW Type 212A uses a 7-blade skewback propeller  with an optimised blade shape and trailing-edge geometry for a low acoustic signature, as well as PCBF (propeller boss cap fins) to suppress hub-vortex cavitation and increase propeller efficiency. 

Alrosa's pump jet system uncovered

The sole Project 877 (Kilo class) submarine with pump-jet

The pump jet of Kilo submarine Alrosa

The standard propeller of a "regular" Kilo submarine

Pump-jets can be even stealthier, but they are not suitable for small size submarines. The only known example of an operational diesel-electric submarine equipped with a pump-jet is the Project 877V Kilo-class B-871 Alrosa of the Russian Navy Black Sea Fleet.

Acoustic stealth comparison (broadband quieting). Work based on N. Polmar "Cold War Submarines" - originally on en:Office of Naval Intelligence data. Author Voytek S

Hydrodynamic noise

As a submarine moves through the water, it generates boundary layer turbulence and vortex shedding noise, with both broadband and narrowband components. At low cruising speeds, the broadband flow noise has a relatively small contribution to the total radiated noise. This contribution, however, becomes more significant at higher speeds, as the flow noise power level is proportional to U5 where U is the sub’s speed. The hull, fin, control surfaces and any appendages (e.g. fixed tube for towed array) contribute to the generation of hydrodynamic noise either directly, or by disturbing the wake field entering the propeller disc, which in turn causes the radiation of low frequency noise. 

Collins class submarines of the Royal Australian Navy

The narrowband hydrodynamic noise component is sourced by flow-induced vibrations and may be present even at low speeds. If the frequency of the flow fluctuations is near one of the natural frequencies of a structure, this part of the submarine may vibrate relatively strongly and generate noise. 

Victoria class submarine of the Royal Canadian Navy

Machinery noise 

An SSK’s machinery noise is normally independent of its speed and can be classified as follows:

a. Low frequency transients generated during the operation of the submarine (for example: movement of the hydroplanes and the rudders; opening the torpedo tube door; changing depth). Although of short duration, LF transients can assist a hostile anti-submarine force in classifying and probably also identifying the submarine. 

b. Noise generated by the electric motor, the cooling units, the various auxiliary systems and the air-independent propulsion (AIP). The use of advanced isolation infrastructure, anechoic tiles and permanent magnet propulsion motors can reduce this type of radiated noise. It should be mentioned that in absolute terms, AIP systems do not reduce the machinery noise radiated by a submarine. On the contrary, a submarine using its batteries radiates less noise than a similar one using its AIP system, although the difference is minimal. 

c. Noise produced by the diesel engines and their auxiliary equipment (pumps, valves, etc) during the recharging of the submarine’s batteries. in the course of this process, diesel engines operate under high load so as to recharge the batteries as quickly as possible, generating significant acoustic noise. Despite the passive and the more recently developed active vibration isolation systems and exhaust silencers, the diesel engines remain the dominant source of radiated noise for conventional submarines. 

Walrus class submarines of the Royal Netherlands Navy in formation

In conclusion 

Israeli Dolphin II class submarine

Detecting, tracking and locating submerged SSKs using passive acoustic sensors has always been a challenging task, since diesel-electric submarines, especially in silent patrol mode, radiate minimal noise. In practical terms, this type of sub is vulnerable and easily detectable only during snorting, a process generating a high level of acoustic noise, as well as radar reflections, IR emissions, hull and snort head wake, exposing the SSK to passive and active detection sensors from long distances. An SSK’s total submerged endurance exceeds 100 hours, but it normally has one 15-minute snorting per 12 hours, depending on the mission requirements, to keep its battery in a continuous high state of charge. 

Ula class submarine of the Royal Norwegian Navy

AIP technology greatly prolongs the snorting intervals, enabling an SSK to significantly extend its submerged endurance to more than 400 hours (at 4 knots and with a normal “hotel load”; speeds higher than 8 knots cannot be supported by AIP systems in stand-alone mode). Combined with the latest advances in vibration isolation systems, propeller and hull design, which further reduce the radiated noise signature, AIP provides a decisive tactical advantage and makes the passive acoustic detection of a modern SSK a nearly impossible task, even for an advanced anti-submarine force. 

Okeanos, sole Type 209 submarine upgraded with AIP technology

In light of the above, active sensors and, specifically, low frequency active sonars in multistatic configuration, are probably the preferred solution against a modern diesel-electric submarine. Such a configuration offers a longer detection range, increases the number of detection opportunities per ping, allows higher ping-repetition rates and, most importantly, complicates the tactical situation for the hunted submarine. In the absence of a valuable bearing, range and depth estimation of the receivers, the submarine cannot successfully exploit counter action manoeuvres, such as the “turn tail-on to the source and change depth”.

Dolphin II class submarine of the Israeli Navy Tanin, perhaps the most
advanced type in the Mediterranean today

Bibliography:

• Submarine Design, Ulrich Gabler
• Submarine Hydrodynamics, Martin Renilson
• Sonar, A.D. Waite