Underwater Craft Design: Signatures

A signature is the result of the submarine interacting with its environment and makes it detectable.

• minimisation of target noise level:

  avoiding detection by foreign passive sonar systems

• minimisation of self-noise level:

  reduction of own sonar system interference due to self-noise

• machinery noise - radiated via the submarine structure as:

  • airborne noise

  • structure-borne noise

• fluid-borne noise

• propeller vibrations

• exhaust discharge noise during snorkel operation

• flow noise

  • hull flow noise

  • propeller flow noise

In this context, a distinction is made between:

• continuous noise

• transient noise (lasting less than 1 sec, e.g. during raising / lowering of hoistable masts, rudder operation or weapon ejection)

• generated by pumps and fittings in fluid circuits (e.g. hydraulic oil system, cooling water circuits)

• in open systems (cooling seawater system), sound radiates directly via inlets and outlets into the surrounding water

• the sound level generated by pumps depends on their design and operating points

• the sound level can be reduced by fluid silencers

  
  

The incoming signal at the sonar receiver depends on:

• the physical ability of the target to reflect sound waves = target strength

• the reflections from water surface, sea bed or water layers (salinity, density, discontinuities)

• background noise and transmission loss

The target strength depends on:

• the target geometry

• the absorbing properties of the reflecting surface

• the signal frequency

TS is measure for the ability to reflect incident sound pulses:

Free-flooding areas of the submarine (sail and upper deck, forebody and aftbody) are critical with respect to target strength as sound penetrates the hull structure to a certain extent and may be reflected by components with unfavourable shapes.

• Inclining the sail and upper deck walls to reduce target strength. The typcial sail with vertical walls has a high target strength from abeam over a significant angle range.

• Avoiding secondary reflections from flee-flooding spaces by using a glass reinforced plastic (GRP) fairing for sail and upper deck, and target strength reducing coatings in forebody and aftbody.

• The high target strength of the pressure hull cylinder is usually accepted. If it is required to reduce the target strength to a low level, a target strength reducing coating can provide a considerable improvement also for the hull.

• Temperature changes in the wake due to mixing with (usually colder) water from the depth, pushed to the surface by the propeller. This effect can be reduced by choosing a deeper diving path and a slow speed, and by avoiding any down angle.

• Heat emission from cooling water is insignificant, though submarines with an optimised energy output are superior.

• Heat is released to the water through underwater exhausts during snorkel operation.

• Heat is released to the air through surface exhausts during snorkel operation.

• Ships and boats made of magnetisable materials have their own magnetic fields which disturb the earth magnetic field.

• Changes in the earth magnetic field caused by a moving magnetic vessel can be detected and analysed by magnetic sensors.

• Mines with magnetic sensors

• Magnetic sensors in antisubmarine warfare (ASW) helicopters or in

  • aircraft flying close above the water surface (Magnetic Anormaly Detection, MAD)

• Torpedoes with magnetic sensors

• Demagnetisation of ship/boat

• Active magnetic self-protection

• Use of non-magnetic materials in combination with

  • magnetic item ranging and compensation

  • low-stray-field design of high-current components

UEP = Underwater Electric Potential

static electric field caused by potential differences at the hull

ELFE = Extreme Low Frequency Electric

the potential field is modulated by

• resistance changes in the bearings of the rotating propeller shaft

• alternating-current (AC) supply systems on board

• incidental earth currents