Effect of minefields on inshore operations

The mine is one ASW weapon whose potency has been in no way reduced by the increased speed and depth of modern submarines. A submarine operating in inshore waters will be aware of the danger from declared defensive minefields of either moored or ground mines, and accurate navigation is therefore essential. The submarine must also face the possibility of undeclared minefields of the “deep trap” type, where mines are laid in focal areas deep enough to allow the safe passage of surface ships over them but at a depth chosen to catch submarines. Such minefields will probably use moored mines, which are detectable by mine detection sonar. However, the submarine will hesitate to use mine detection sonar since it is an active set and could betray its presence. Submarines are most unlikely to have sets capable of locating ground mines.

The enemy submarine

Any of types of submarines described may be encountered in inshore areas. Unless adequate intelligence is available, it will not be possible to forecast the type of submarine that may be operating in any given area.

Throughout an inshore campaign the enemy can vary the method of control, the intensity, and the areas of the threat. At the start of a war, almost all inshore waters must be considered submarine probability areas, while local areas and major harbor approaches may be areas of very high probability.

Enemy employment of submarines

Enemy submarines operating in inshore waters may be used in any of the following operations:

1. Torpedo attacks on shipping.
2. Mine laying.
3. Attacks on shipping in ports or anchorages.
4. Missile strikes.
5. Clandestine operations
6. Reconnaissance (particularly prior to a major mine laying effort or an amphibious assault.
7. Navigational beacons
8. Rescue duties.

Submarine maneuverability in speed and depth in shallow water

Depth limitations

The submarine is unable to go deep and take evasive action because of the depth limitations of the bottom.

Speed

The submarine would prefer to operate at slower speeds both in closing a target and in evasion. The nearness of the bottom limits its maximum safe speed because of the venturi effect (when very close to the bottom) and the possibility of temporary loss of depth control. These factors affect all submarines and are particularly inhibiting to the nuclear submarine.

Bottoming

Diesel-electric submarines may bottom to conserve battery power or as an evasive measure. This tactic is less likely to be used by modern submarines, particularly those with “chin-mounted” sonar domes. Such submarines will lose the use of this sonar while bottomed and may easily inflict permanent damage to it. Bottoming also involves some risk of slight structural or propeller damage and consequent increased self-noise.

Submarines are unlikely to be able to fire their bow tubes from the bottom; stern tubes may perhaps be usable, particularly if torpedoes are discharged by the “swim-out” method. If a submarine does choose to bottom, it will probably do so bow tide. Areas with little or no tidal stream and with a gravel or sandy bottom are preferred.

Navigation

The dangers from wrecks, shoals, uncharted bottom irregularities, and mine fields require the submarine to navigate with particular care in inshore waters. Tidal streams will complicate the problem. The submarine, therefore normally require frequent periscope exposures for navigational purposes.

In general, navigational and pilotage factors make it hazardous for submarines to operate in waters less than 120 feet depth. They prefer at least 120 feet of water below their keels at periscope depth (that is, 180 feet depth) to minimize the danger from ground mines and to allow some evasion in depth if attacked.

Submarines on clandestine missions or waiting outside a harbor may penetrate into very shallow water for short periods for a specific mission or attack.

Use of ASW ships in inshore operations

Guiding principles in the use of ships

The following broad principles are a guide to the use of ASW ships in inshore waters.

1. At least one escort, even if it is a small ship of very limited ASW capability, should be provided for each convoy for administrative purposes.
2. Other ASW ships can be used either as screening units or in support groups. As a broad rule, the more sustained air ASW effort available, the greater the proportion of ships that should be allocated to support groups.
3. Support groups will be formed to operate in particular areas and prosecute any contacts gained within their area. They may also provide some degree of prosecution against submarines with objectives other than attacks on shipping, such as attacks against ports and shore installations or mine laying. Such support groups may be ordered to provide support to particular convoys passing through their areas.

Conduct of ASW search by ships

The following points are particularly relevant in inshore waters.

Area searches

1        Submarine expected movements

There may be probable directions and/or speed of escape caused by geographic features, tidal conditions, or known or suspected mine fields. Submarines already in good patrol positions may attempt to evade the search either by bottoming or by moving into water recently searched by the ships. Gambits such as deceptive sonar policies, stopping to listen, so forth are worth considering under these circumstances, particularly on dark nights when sonar conditions are bad. Once such a gambit is decided upon, it is essential to ensure that the use of all sensors be coordinated.

2        Search speed

The use of low search speed may improve sonar ranges by reducing background noise and may reduce the information available to the submarine on its passive sonar.

Such speeds require the use o bold evasive steering if ships are not be easy targets or aimed torpedo. Since this future reduces the sweep rate, the best search speed will usually be the optimum sonar speed.

3        Non-submarine contacts

Numerous non-submarine contacts are likely to disorganize a search by ships in formed state. Under such circumstances, independent search may be more efficient, provided units remain within mutual support distance.

Bottom searches

If it is suspected that the submarine has bottomed in the vicinity of a datum and environmental conditions are such that ASW ships are able to search in a formed state, an ASW bottom search plan can be used.

A ship equipped with a bottomed target classification sonar can be of great assistance to the search ships. Obtaining conclusive identification with such a set may take some time and leave the ship operating the set an attractive target for submarine torpedoes; therefore, a second ASW ship should always be present for support. Before such classification runs, the target may be attacked either with a single charge or full salvo. Once obviously under attack, it is most unlikely that a modern submarine would remain bottomed.

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