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ROYAL NAVY
By David K. Brown, Royal Corps of Naval Constructors
Mines are easy to deploy and difficult to sweep, making clearing efforts both per sonnel and resource intensive. Here, seasoned Royal Navy divers prepare to enter the water during minesweeping operations in the Persian Gulf.
by mines, as was the USS Samuel B. Roberts (FFG-58). During the liberation of Kuwait, the USS Tripoli (LPH- 10) and the Princeton (CG-59) were both damaged by mines, and the threat of more mines imposed limitations on the movements of the Coalition task force.
Mines are readily available from a number of sources, and quite sophisticated devices can be made in simple workshops. Many of the world’s major trade routes have choke points where the water is quite shallow and favors the use of ground mines. The potential threat is worldwide, which poses some difficult problems in the disposition, deployment, and cruising speed of mine-countermeasures vessels (MCMVs).
Many naval officers still see mine warfare as a rather undignified task and would echo the exasperated comment of Admiral David Farragut when he entered Mobile Bay on 5 August 1864. (He used the word torpedo instead of mine, as contemporary usage dictated.) Recent conflicts have shown, however, that disposing of even very simple and old-fashioned mines may require the deployment of extensive resources.
Shipping movements in the Red Sea were disrupted bY the mines laid by the Libyan ship Ghat in 1983. Later, the Iran-Iraq War saw merchant ships severely damaged
Types of Mines
The simplest mine, first used by the Imperial Russian Navy off Kronstadt in 1855, is the buoyant contact mine.' It is triggered by the impact of the target vessel. In both world wars, such mines were found easy to sweep, and their potential has been discounted in recent years. As a result, countermeasures such as the paravane are no longer widely available and the threat has reappeared.
Under international law, a mine that breaks free of its moorings and floats to the surface is supposed to be rendered inert. Maverick governments ignore this rule, and the floating mine has been met frequently in the Gulf. In
many cases, mines have been deliberately set adrift. Floating mines are hard to pick up on sonar and are not easy to see, even in daylight. In calm weather, the best precaution is visual observation from a helicopter, but a serious floating-mine threat may stop night movement.
There are variants of the moored mine, such as the antenna mine—used extensively by the U.S. Navy in World War I —and mines designed to float freely, oscillating between two preset depths (Leon mine).2 Antisubmarine mines, such as the U.S. Captor and the Soviet rising mine, also could be adapted against surface ships. Moored acoustic and magnetic mines were used during World War II in deep water.
Ground mines are activated by one or more of a ship’s signatures: acoustic, magnetic, pressure, electrode potential, and others. For all practical purposes, mines with pressure actuation cannot be swept. More complicated combination mines—e.g., acous- tic/magnetic—can be swept only by the most advanced equipment and, even then, with less than total confidence that they have all been removed. It is unlikely that the signatures of major warships can be reduced sufficiently to give total immunity, though degaussing and noise reduction lower the risk considerably. There is no prospect of giving even such limited protection to merchant ships or many naval auxiliaries.
The mine designer can play a number of tricks to make mine clearing more difficult or more dangerous. These include time delays, ship counters, and sensitive, anti- MCMV mines. The mine can be given an anechoic coating to reduce its sonar echo or can be disguised as a discarded refrigerator, automobile, or other modem coastal detritus. Air-dropped mines can be made to bury into soft bottoms while still retaining their ability to detect at least magnetic signatures.
Sweeping
The Royal Navy (UK) carried out its first minesweeping operation the day after the first Russian mine exploded in 1855, and, by the end of World War I, sweeping for moored mines was quick and reliable. Simple magnetic mines, such as those used by the Royal Navy in 1918 off Belgium and in 1919 against Soviet ships, were fairly easy to sweep, as were the early German magnetic mines of World War II. Similarly, early acoustic mines could be exploded by a simple noisemaker.
Combination acoustic/magnetic mines demanding spatial and temporal synchronism and relying on the rate of change of signatures rather than their absolute values—perhaps looking for ship-specific frequencies—complicate sweeping to the extent where it is no longer a viable method of mine clearance. It could require a mix of bravery and ignorance to accept a sweeping operation as ensuring a mine-free channel. A light precursory sweep ahead of a mine hunter may be worthwhile to eliminate the contact mine threat.
Hunting
Virtually all mine-hunting systems use a sonar first to detect a suspicious object and then to classify it, often using a higher frequency in the classification mode. During classification, the ship usually will maneuver to examine the object from different directions. Most early sonars, such as the British 193M, were hull-mounted, but the new generation, such as the U.S. Navy’s SQQ-32 and the Royal Navy’s 2093, are variable-depth, carried in a body streamed below the vessel.
A variable-depth sonar puts the transducer closer to the mine, increasing the chance of detection, particularly against camouflaged and buried mines. It should also be less affected by thermal layers than a hull- mounted set.
Route Surveillance
Route surveillance, in contrast to mine hunting, is not so much to locate mines but to ensure that selected routes are free of them. The technique is to tow a sonar—usu-
ally a high-frequency side-scan set—at a fairly high speed, comparing today’s picture with one recorded earlier, helped by a computer. Route surveillance can only be used with confidence on a fairly smooth, hard bottom whose contours will not change too frequently. On the other hand, Its speed frees the conventional mine hunter for the more difficult areas.
Mine Watching
During World War II, the approaches to British ports Were guarded by mine watchers looking for aircraft laying mines. Nets were stretched across sections of the Suez Canal to give an even more positive indication of aerial minelaying. New holes in the net were a sure sign of a mine below. Modern electronics should be able to produce a portable mine-watching radar for use in areas where nerial minelaying is a threat.
Requirements for an MCM Vehicle
A mine hunter using sonar will always come close to a mine, and there is a considerable risk of passing over ar> undetected mine—especially if using a towed body such as a side scan. Because of these risks, the mine hunter must have very low signatures, and they must be monitored frequently while in service to ensure they do not deteriorate. Since there is still a risk of a mine exploding close to the vehicle, it must be resistant to shock damage.
It is sometimes suggested that an MCM vessel with high signatures will explode a mine at a distance well clear of the ship. This approach was used by both the British and Cerman navies in World War II, using ships with electromagnets weighing several hundred tons. The Royal htavy soon gave this up as too dangerous, but the Germans persisted until, one night, a large number of mines Were laid and set to explode directly under the magnets.
Another fallacious argument is that, since sonar will detect a mine ahead of the ship, low signatures are unnecessary. However, in difficult conditions, when the vessel ls maneuvering close to the mine, the probability of detecting a mine in a single pass is low and the risk to the I vessel is high.
Mine clearance is inevitably slow and, to complete a elearance in a reasonable time requires large numbers of yessels. The task also is hazardous and the numbers must allow for losses. Such losses should not represent too large a Percentage of the force, and the loss in both human and financial terms must be acceptable. The need for large j numbers implies a cheap vessel, but the equipment and Performance needed for the task are bound to be expense. Nowhere is the conflict between quantity and qual- hy more severe than in MCM vessels.
Size and cost are less closely linked than is supposed, ^ut, even so, unnecessary size adds to cost. The MCMV 1 should be fairly small but not so cramped as to affect performance. A margin for new equipment is vital, because '*• 's comparatively easy for the mine designer to add tricks the actuation system, which the hunter has to counter, margin should include space, both on the upper deck
and below, and stability on services. All functions not vital to the task, such as office work and maintenance, should be carried out ashore.
The requirement for a low-pressure signature sets an upper limit of about 800-900 tons, unless operations are to be confined to deep water. A lower limit is set by seakeeping, as motion affects the performance of both equipment and crew.3 Early World War II wooden minesweepers of the Royal Navy were 31.5 meters long, and their performance in open water was degraded by motion in even moderate seas. Later vessels were 38 meters long and were better suited to coastal waters. The more complex tasks expected of the crew of a modern mine hunter require even lower motions and, for use in European waters, a minimum length of about 50 meters seems necessary. Shorter lengths can be accepted by countries that can be sure their vessels will not need to work in rough water—but few fall into this category.
Until now, most countries have thought of mine countermeasures in terms of their own coastal waters or those of close neighbors. As a result, there has been no need for rapid deployment over long distances. The mine threat in distant waters—with the accompanying need for rapid transit—is now seen as more important. A fast MCMV is possible, though it is difficult to keep down all signatures on such a powerful craft. The cost also would be very high, making this an unlikely approach.
The U.S. Navy’s MCMVs were taken to the Gulf on a heavy-lift ship, and this may well be the best approach for conventional vessels. Thought should be given in the design to the strains imposed during lifting, and lifting eyes might be provided. The availability of heavy-lift ships would have to be kept under review. Helicopters are, of course, transported in a carrier of some sort or flown directly to the scene.
MCM hovercraft, such as an SRN-4 derivative, would cruise at about 60 knots, but frequent fueling stops would reduce the mean speed to about 40 knots and increase the dependence on friendly fueling depots.
Some years ago, I led the design of a utility mine hunter—virtually all equipment was packaged in a number of containers that could be fitted out, tested, and dropped onto a simple hull. One possibility that was considered was to build more hulls, which were very cheap, than container sets and to preposition hulls in various areas. The containers could then be flown to where they were needed and plugged into the ship. Since magnetic signature varies with latitude, any distant deployments would require that the MCMVs are ranged on arrival and corrections made.
Conventional Vessels
The great majority of navies rely on displacement monohulls for their MCMV forces, and the performance of the Royal Navy’s Hunt class in the Gulf has shown that such craft can deal effectively with a large-scale and quite sophisticated threat. There are some alternative vehicles, but these must show significant operational or cost advantages if they are to replace or even supplement the
HMS Sandown, the Royal Navy’s third-generation MCM vessel, is built of glass-reinforced plastic and has a Voith- Schneider propulsion system. Similar vessels are being built for Saudia Arabia.
well-proven displacement craft.
The Hunt class has a relatively elderly sonar; the new HMS Sandown, with a better sonar and improved maneuverability, should be even more successful—particularly in difficult conditions—and a little cheaper. Vessels such as the Tripartite and the Italian Navy’s Lerici are similar in mission, though perhaps incorporating less experience with glass-reinforced plastic (GRP) MCMV technology than those of the Royal Navy.
These vessels have GRP hulls and equipment with a minimal ferrous content to reduce their magnetic signatures. Care is also needed to avoid loops of conducting material that would generate eddy currents and associated magnetic fields when the ship rolls. Grounding electric equipment and screening electronics present difficult problems that require considerable experience to solve. Additional equipment, particularly if it is clumsily wired up, can cause rapid increases in magnetic signature, making strict control essential. Even then, regular monitoring on a range is necessary.
The problems of low noise signature are not dissimilar. Machinery noise is reduced by careful balance of moving parts, by ensuring a long noise path to the water, and by flexible mounting, probably on a raft. It is all too easy to short circuit a mounting system, and care is needed in installation. Regular ranging in service also is essential.
Though GRP is an almost universal choice for the hull, there are several ways it can be used. The Royal Navy has used a framed shell in all three generations of GRP vessels, and this has been proven with many full-scale tests of structural strength and resistance to shock and fire. Such tests have shown both what to do and what not to do. In the earlier generations, difficulty was experienced in resisting the tensile phase of the shock wave, and, to overcome this problem, bolts were used. The use of small amounts of “flexy” resins, not available earlier, has enabled the Sandown to meet the highest shock levels without needing bolts.
Framing is labor intensive, and tests have been carried out on a full-size hull stiffened by longitudinal corrugations, rather than by conventional frames. Tests show this form of construction can be just as strong as the more conventional design and significantly cheaper to build.
Other forms of GRP construction also have been successfully tested, and the choice between systems may be based on cost. Variations in labor costs may change the order of merit from one country to another. Sandwich construction, with two thin GRP skins separated by a thick foam filler, is cheaper and a few types have passed shock testing. The problem is that, after a severe shock or fire, external examination cannot tell if the filler remains sound.
Fears are sometimes expressed over the effect of fire on GRP hulls. Such fears are groundless; in fact, the fire resistance of a well-built GRP hull is an advantage of that
material. In the United Kingdom, tests have been carried out on large panels and test sections, but the most convincing demonstration was a major oil fire in the engine room of HMS Ledbury, during which the temperature was about 900°C for four hours. Damage to systems was serious but that to the GRP structure was localized and did not significantly affect hull strength. The GRP bulkheads were such good thermal insulators that the outsides of the bulkheads exposed to the fire were only warm.
It is possible to build a strong hull from laminated wood, but it will be heavier than one of GRP because weaker cellulose fibers replace the glass fibers. Nonmagnetic steel construction calls for uncommon skills, which means difficulty obtaining repairs, and there is no evidence in open literature that the eddy currents problem has been solved-
Maneuverability is vital to a mine hunter, as the bracket between the first detection of a mine and that at which damage to the ship may occur is only just over 100 meters. In classifying a contact, the MCMV will have to maneuver within this bracket. It has been estimated that, when close to a mine, the vessel is maneuvering for about 10% of its operational time. The maneuvering devices must be both powerful and quiet. The efficient production of hydrodynamic thrust is best achieved by moving a large mass of water slowly, which is also a first step to silence.
Big propellers and large bow thrusters are needed, and the “active rudders,” with high-speed, small propellers, used in the old Royal Navy Ton class and in some other navies, should be avoided. Bow thrusters present particular problems, e.g., if used while moving ahead, the rotor blade sections are bound to work at an unfavorable angle of attack.
Recently, several designs have used Voith-Schneider (V-S) propellers, which can produce thrust and turning moment in any direction. Early designs of V-S propellers were noisy, both mechanically and hydrodynamically, but these problems have been overcome, at least for the loW speeds used in mine hunting.
It has already been pointed out that MCM Vs should be cheap—implying some limit on size—but with adequate space for new equipment. The renewed threat from contact mines reemphasizes the old lesson that mess decks should not be too far forward or too low in the ship.
The catamaran, such as HMAS Rushcutter, has attractive features, with a spacious deck and a big reserve ot stability. Such a configuration facilitates changes in equip' ment through the use of portable containers on deck. On the other hand, the high stability of the catamaran leads to very rapid rolling. The roll angles will not be large, but the short roll period causes high accelerations and forces
at the deck edge. This problem can be reduced, to some extent, by “waisting” the hull at the waterline.
Alternative Vehicles
The U.S. Navy uses the MH-53E helicopter to tow a variety of sweeping gear. As sweeping is of diminishing importance, in this role, helicopters are of value only as precursor sweepers for conventional MCMVs. The MH- 53E can also tow an AQS-14 side-scan sonar for route surveillance. Even these large helicopters can tow only one sonar, which inevitably leaves a gap in coverage under the body. This gap can be filled only if the helicopter can fly a second parallel path with great precision.
Helicopters have a fairly high underwater noise signature and it is said one was lost in the Haiphong clearance when it detonated an acoustic mine underneath itself.
Hovercraft have been tried extensively by the Royal Navy and have proved capable of carrying out all sweeping and hunting tasks. Their advantages center on the way the air cushion of a fully skirted hovercraft decouples
JtOyAL AUSTRALIAN NAVY
the craft from the sea. This gives very considerable protection from underwater explosions, as demonstrated by full-scale mine explosions against the obsolete SRN-3. She resisted an 1,100-pound charge just clear of the skirt with all machinery and equipment intact. The air cushion gives inherent noise reduction that does not depend on mountings and cannot deteriorate in service. The pressure signature is low, and the cross-channel car ferry, SRN-4, only narrowly fails to meet magnetic requirements in commercial form. (Simple and cheap changes would satisfy this requirement.)
The air-propelled hovercraft with propellers on rotatable pylons is extremely controllable, and full-scale trials in high wind and sea showed that the commercial craft could hold track even better than a Hunt.
Comparing the whole-life cost of ownership of a hovercraft with a conventional MCMV is not easy. Hovercraft are simple vehicles and the initial cost can be low, although development costs are high. On the other hand, running costs are high: air propulsion is inefficient, and, hence, fuel consumption is heavy. Skirt maintenance is reduced in later designs but still is quite costly.
In wartime, hovercraft could work from a large number of suitable beaches, particularly those with good road access for fuel tankers and other supplies. Big hovercraft, such as the SRN-4, could even carry the containers of the support unit. In peacetime, the situation is almost the opposite: big hovercraft generate substantial airborne noise and a squadron of MCM hovercraft would be unpopular neighbors.
Hovercraft are always thought of as fast, but some of the most commercially successful are used for moving heavy loads at very low speeds. An MCM hovercraft, however, is likely to be light in weight, and high speed can be achieved much more cheaply than in conventional craft.
Though hovercraft can carry out all MCM tasks, their most suitable role probably is route surveillance, where their speed, near invulnerability, and low signatures can be used effectively and economically.
The surface effect ship (SES), or sidewall hovercraft, falls between a displacement vessel and a true hovercraft. It is not fully decoupled from the sea, and, hence, though quieter and more shock-resistant than a conventional ship, it lacks the advantage of the true hovercraft. In particular, it is likely that noise and shock mounting of equipment, with regular monitoring, would be needed, offsetting the SES’s lower vehicle cost.
Remote-control mine hunters seem possible with today’s technology, but would only be a little smaller and not very much cheaper than the Sandown. Human life is valued more highly than in the past, making the robot hunter attractive, but recent operations suggest that risk to life is quite small. Bottom-crawling vehicles may be needed to deal with the most advanced mines, buried or camouflaged, making clearance efforts even slower.
Catamaran mine hunters such as HMAS Rushcutter offer spacious decks and generally good stability, but they can be subject to rapid rolling under certain conditions.
Support
The ships are only a part, albeit the most important part, of an MCM operation. They require support and maintenance facilities, noise and magnetic monitoring ranges, and, most of all, trained and experienced crews.
Because MCMVs must be as small as possible, they will have only a very limited capability for self-maintenance. The Royal Navy has built three forward support units (FSUs), based in some 16 standard containers, with workshops, stores, offices, power supply, communications, and living quarters. The unit is quite flexible and only containers relevant to a particular operation are deployed; for example, the habitability containers were not taken to the Gulf because the unit was based on board a ship.
The complement of an FSU is about 40, mainly chiefs and petty officers, and artificers in mechanical, electrical, and weapon systems. They can carry out major tasks— such as changing a main engine—as well as rapid servicing. The Gulf FSU was able to give full support to the five Royal Navy Hunt-class MCMVs, as well as assisting the four U.S. Navy vessels present.
Since there were no services—water, electricity, or sanitation—ashore in the northern Persian Gulf, the containers were installed in the Sir Galahad, a Royal Fleet Auxiliary tank-landing ship, mainly on the vehicle deck. The Sir Galahad is a very versatile 8,500-ton ship, which has her primary role in amphibious operations; she also can deploy an FSU and support clearance diving teams or, alternatively, can operate six large helicopters.
Both magnetic and acoustic signatures can deteriorate because of unauthorized additions or changes or system failure. Magnetic signature is also a function of latitude, hence, frequent monitoring is essential; mobile ranges are necessary if MCM operations are to be carried out away from home bases.
Above all, the success of an MCM operation depends on the training and experience of officers and enlisted personnel. There has to be a nucleus of MCM careerists who will develop doctrine, initiate the requirements for new material, and fight the MCM comer in departmental debates. The larger number of nonspecialists will learn to appreciate the importance of MCM and the difficulty in carrying it out.
Conclusions
Mine warfare, or even the possibility of mines, poses a worldwide threat to seaborne operations and trade. The first countermeasure is to prevent or reduce the extent of minelaying. Such measures are unlikely to be 100% effective against a determined aggressor, and even a few suspected mines may require the deployment of a large MCM force.
Mine clearance requires a large number of units, and, therefore, the individual unit should be as small and as cheap as is consistent with capability. The risk to such vessels is high and should be reduced by ensuring that they have very low signatures as built and that these are regularly monitored to prevent degradation. Even so,
MCMVs are likely to be exposed to shock from repeated underwater explosions, and the hull and all equipment must be able to resist this shock.
The vessel must be able to follow a course accurately, despite the effects of wind and tide, and must be highly maneuverable without any unseemly increase in noise levels when turning. Good maneuverability and control in high winds requires a balanced aerodynamic profile, while these small ships must be good seaboats.
The equipment fit will include a very accurate navigation system, a location and classification sonar (preferably variable-depth), and a mine disposal weapon. These will be used intensively and should be reliable and easily maintained. The integration of the separate components of the weapon and sensor fit is not easy and requires experience, as does the prevention of electronic interference within a plastic hull.
Support is required from mobile ranges and from a mobile maintenance unit. This could take the form of a depot ship, but the Royal Navy’s containerized FSU offers more flexibility. The overall MCM operation should be controlled by dedicated MCM officers having both training and experience in this task.
A Personal View
I would suggest the following use of resources for a navy comparable in size to the Royal Navy and having worldwide responsibilities. The bulk would be spent on fully capable, conventional mine hunters such as the Sandown, developed from experience with the well-proven Hunt class, the prototype GRP ship Wilton, and the earlier, wooden Ton class. If money and manpower permitted, these vessels would be supplemented by a cheaper MCMV of limited capability, which would carry out the easier tasks, such as field clearance, freeing the main force for the more difficult work.
Mine clearance is inevitably slow, and I would develop a route-surveillance capability using fully skirted hovercraft. Since it is likely that this approach could be used in a limited number of areas, a small force would be suggested, using their high transit speed to get them to the area of concern.
Development money would go into measures against camouflaged and buried mines, particularly those with anti-MCM settings. A remote-controlled bottom-crawling vehicle also may be needed.
'B. Greenhill and A. Giffard, “The British Assault on Finland,” London, 1988. JH.C. Armstrong, “The Removal of the North Sea Mifte Barrage,” Warship International, Toledo, 2/1986.
‘D. K. Brown and P. D. Marshall, "Small Warships in the RN and the Fishery Protection Task,” RINA Small, Fast Warship Symposium, London, 1978.
Mr. Brown retired in 1988 as Deputy Chief Naval Architect of the Ministry of Defence (UK), having spent his entire career in warship design and supporting research work. Among other things, he was responsible for a number of design studies for both conventional and unconventional MCM vessels. In March 1991, he spent time on board HMS Dulverton, hunting mines in the Persian Gulf.