Inshore waters are sea areas in which geographic or hydrographic conditions impose restrictions on the movement of shipping and/or submarines. As a broad guide, any sea, which is shallower than 600 feet or is within 50 miles or land may be considered as inshore waters.

Aim of ASW operations in inshore waters

The aim of ASW operations is to deny to the enemy the effective use of his submarines. In inshore waters, this aim can only be achieved by the defense of naval and merchant shipping and shore installations from attacks by submarines, which are already well placed to carry out their mission.

Any sustained anti-shipping campaign by submarines is likely to start inshore where the targets are most numerous and most easily found. Thus, until an enemy has been defeated in an inshore campaign, he is unlikely to use more than a small proportion of his torpedo-firing submarines in deep ocean patrols.

Since surface forces can seldom evade the submarines in inshore waters, these submarines in inshore waters, these submarines remain a threat until destroyed or driven out. This leads to the principle that any submarine detected in inshore waters should be hunted to destruction.

Factors affecting ASW operations in inshore waters

Adverse factors

1. Geographic limitations imposed by the coast or very shallow water.
2. Self-imposed restrictions, such as search channels and own minefields, which make evasive routing difficult if not impossible.
3. Concentration of shipping a he approaches to ports and anchorages.
4. Restrictions imposed by the enemy, such as minefield, and the threat from forces.
5. The depth and bottom topography, which may allow submarines to bottom.
6. Generally unfavorable environmental conditions for all sensors.
7. Limited effectiveness of most homing weapons in shallow water.

Favorable factors

1. The submarine also suffers restrictions to its capabilities.
2. The operational areas are close to ASW bases where concentration of forces and the persistent prosecution of detections is easiest to achieve.
3. Non-ASW aircraft can be used to provide a high density of visual search.
4. Patrol craft and other small coastal vessels can contribute to the harassment of submarines.

Environmental actors in inshore waters

This topic summarizes environmental factors, which are particularly significant in inshore waters.

1.       The sonar environment

Tidal streams are stronger and more prevalent in inshore waters. These tidal streams may produce eddies and may cause sharp changes in temperature, salinity, and aeration over short distances. Such changes will be particularly common at or close beneath the surface. The inability of BTs to detect salinity changes makes recognition of these layers difficult.

The level of background noise in inshore waters is normally much higher than in the deep ocean. The chief causes of this area are:

Bottom reverberations, particularly in areas of rough bottom

High levels of biological noise.

2.       Active sonar detection

In general, the greater the power output and the lower the frequency of a sonar set, the more adverse are the effects of shallow water; scanning sonars will suffer more than searchlight sonars.

The increased intensity of background noise will make it more difficult to recognize submarine echoes, particularly those showing no Doppler effect. It will also result in sonar operators becoming more quickly fatigued.

Modern high-powered sonars are more likely to be reverberation limited than noise limited in inshore waters. When reverberation problems become particularly serious with the higher-powered sonars, reduction in power output to the use of short pulse keying may result in improved ranges and make easier the recognition of submarines echoes.

When operating close inshore, better results may be expected when transmitting on sonar in an offshore direction.

Sound channel effect may be experienced, particularly when using low frequency sonar, and this will cause unpredictable sonar ranges. Skip distance effects are also prevalent in inshore water.

3.       Active sonar classification

Non-submarine contacts from wrecks, bottom irregularities, schools of fish or even from boundaries between water masses will give sonar echoes. Wrecks in particular may remain more convincing contacts despite exhaustive investigation. Detailed wreck charts should always be kept available.

Inevitable these will not be up to date, and accurate fixing and reporting of such persistent nonsubs may save much wasted time and ammunition in future patrols. Persistent unclassified contacts can sometimes be disturbed or broken up by the firing of a single depth charge or ahead thrown weapon.

The use of short pulse transmissions to reduce reverberations is likely to reduce the chances of recognizing Doppler. Doppler is a particularly valuable classification aid in areas where non-sub contacts are prevalent.

There is normally no Doppler effect on echoes from bottomed targets, but in tidal streams the echoes will show Doppler as if the target was moving up tide at the speed of the tidal stream.

Conversely, echoes from a submarine hovering and drifting with the stream will give no Doppler effect. Bottom targets will appear to move on plots, and hovering submarines will appear stationary on them. It is therefore essential that all plots show direction and speed of the tidal stream.

Longer sonar ranges may increase the classification problem since more non-subs may be encountered on each sweep.

4.       Submarine sonar

Passive sonar

In ocean waters, submarines rely primarily on passive sonar for both initial detection and classification. Or both tasks, its efficiency its likely to be degraded in coastal conditions. Long range convergence zone detections will not be possible in shallow water, and the increased background noise, the prevalence of local layers, and so forth will tend to decrease initial detection ranges using the surface layer. Classification may be difficult for two main reasons:

a. Many passive detections on adjoining bearings

b. The mixture of direct and bottom reflected signals being received may make it more difficult to establish the characteristics of the signal.

Active sonar

Modern submarines may make use of active sonar, particularly during the layer phases of attacks carried out from deep.

Their active sonar is affected by the same factors as the active sonar of surface ships; but since they will seldom need it for their initial detections or classifications, adverse conditions are less significant to them and will indeed be welcome for their effect in decreasing ASW effectiveness.

5.       Radar.

Submarine and surface ship radars may be affected by the large number of contacts present in inshore waters. Aircraft, with their much wider areas of radar cover on each sweep, will get proportionately more such contacts.

6.       ESM

In such areas as harbor approaches, the many radars likely to be in use create a very unfavorable ESM environment. The value of the ESM set to the submarine, as a warning of approaching aircraft and to the ship as warning of likely impending attack, are both likely to be reduced.

ASW weapons in inshore waters

Homing weapons

The use of ASW homing weapons in inshore waters is affected by depth of water and high ambient noise levels. These effects are usually worse with active weapons and will depend largely on the roughness of the bottom.

Degradation of performance can be expected with most homing torpedoes once water depth is less than about 400 feet. Torpedoes are unusable in water depths of 150 feet or less when bottom capture of most torpedoes can be expected (the momentum of air-dropped weapons normally carries them to about 120 feet).

Thus, in many inshore areas, the aircraft is limited to depth charges and the ship to close ASW actions.

Fused weapons and depth charges

When water depth is less than 300 feet and sonar depth indication of the submarine is not available, fused weapons and depth charges should be set at half water depth. In deeper water, successive patterns should be staggered in depth. When fuses are set automatically by sonars which may be confused by bottom echoes, ships should be alert to the danger of their being set to a depth greater than depth of water with the consequent risk of damage to the attacking ship.

foto source: http://beta.tnial.mil.id/koarmatim/gallery/KRI402%20NGL.htm