I’ve written about coast artillery a few times.
There’s a few other places I’ve mentioned seacoast defenses.
But one thing I’ve never really talked about was the other half of the seacoast defense program.
When Admiral David Farragut, at the Battle of Mobile Bay, gave his iconic order, “Damn the torpedoes, full speed ahead!” he was referring to naval mines laid in the harbor. The self propelled torpedo of today would not be invented for another decade, and not really practical for another 30 years.
Naval mines are to this day an effective weapon against warships. It should be noted, when Farragut gave his order, one of his ships had already struck a mine and was sinking.
When we think of naval mines, we have a vision of the classic moored contact mine, with horns that, when a passing ship strikes them, causes the mine to detonate.
And to be sure, mines of this sort are still used. In fact, the threat of these floating mines in the Straits of Hormuz is a key Iranian tool in manipulating public opinion world wide.
Such moored contact mines can be used either offensively, to blockade an enemy in his ports or to hinder his use of shipping lanes for commerce or military purposes, or they can be used defensively, to deny the enemy entry into friendly waters. Britain, during World War II, liberally mined the waters north of the English Channel to deny access to German U-boats and surface raiders.
But while the moored contact mine could be used to deny certain open waters to the enemy, it was a poor choice of weapon for defending harbors and ports. After all, the reason to defend those harbors and ports is to allow you to conduct trade and naval operations. And if your own channels are mined, your own ships can’t move.
When we think of moored minefields, we tend to think of minelayers dropping them over the stern to form a minefield. And in the Navy, that’s just how it was done. Several purpose built minelayers, and large numbers of converted destroyers served as minelayers in both World Wars.
But by law, the defense of harbors and the seacoast was the responsibility of the US Army. I’ll leave it to Craig to discuss the history of coastal defenses for the first century of our nation. In 1885, the Board of Fortifications, also known as the Endicott board, recommended a wholesale reorganization of the coastal defenses, and eventually lead to the division of Army artillery into the Field Artillery and the Coastal Artillery.
During the Endicott period, the Army made a massive investment in seacoast fortifications on both coasts. Virtually every economically significant harbor had a series of gun batteries constructed. But seacoast artillery alone was not the entire solution to harbor defense. During periods of limited visibility, artillery would have a tough time simply engaging any enemy forces. Searchlights could pierce darkness, but fog or stormy weather could blind the defenders. Further, the way to sink a ship isn’t by poking holes in the top, but rather in the bottom. Cannon fire can eventually sink most ships, but underwater weapons are, pound for pound, far more efficient.
Accordingly, the Army began plans to lay minefields to guard harbors. But unlike the naval mines so familiar to those of us who have seen countless WWII submarine movies, these were controlled minefields.
Army mines being serviced
Rather than being detonated merely by contact, Army minefields were connected to the shore via a series of electrical cables. Main cables from shore went to a junction box, with each junction box typically supporting 19 mines. DC current was used to monitor and test the mines, as well as signal to the shore that contact had been made. AC current would then be used to detonate the mine.
One of the advantages of a controlled minefield was that a shipping channel could be completely mined, and yet still usable for friendly shipping.
Establishing an effective controlled minefield was actually a fairly large investment in infrastructure. First, while theoretically a minefield could be left in place at all times, prudence would dictate that the field actually only be planted when hostilities are imminent. After all, mistakes can happen. So storage and maintenance facilities ashore would be needed for the mines. Typically, a mine storage shed would hold the mine itself. A separate magazine would be built to hold the explosives for the mine. In an era when dynamite or guncotton (both quite sensitive materials) were the primary explosives, placing this magazine well away from other facilities was a good idea.
Provision also had to be made for emplacing the mines. Wharf space for the mine planting ships had to be built. Then tramway tracks had to be laid to facilitate movement of the mines from storage to the wharf. Storage for miles and miles of electrical cable was needed as well. Since the cable needed to be tested for continuity in a salt water environment, large salt water vats also had to be provided.
Since the mines were electrically controlled, power generation facilities also had to be provided (remember, this was the age where electrification was far from universal in the nation).
Mine Casemate for controlled minefield
Then there was the fire control aspect of the mines. Generally, Army controlled minefields could be fired one of three ways:
- Delayed Contact
Command detonation was just that. The firing center (known as the mine casemate) would send the signal to detonate a particular mine. This casemate was generally a reinforced concrete structure with switchboards for controlling the various strings of mines, telephones to fire control observation stations (sometimes called the “long base), and plotting tables to track any enemy force, and decide the proper time to detonate any mines.
Fire control for the mines was fairly sophisticated. At least two observation stations would use a pelorus to determine the bearing from their known point to the target vessel. The intersection of these lines of bearing would provide a location. Multiple sightings over even a fairly brief period of time would provide course and speed information as well. With that information, accurate plotting and a stopwatch, the commander in the casemate could specify which mines should be detonated, and when, with a fair degree of accuracy and likelihood of destroying or damaging the enemy target. For night firing, searchlights were used to track and illuminate targets.
In periods of fog or other limited visibility, when the target would likely be obscured, the minefield could be set to fire on contact, much like a traditional minefield. This was also useful if there were multiple targets, and the plotting team was overwhelmed trying to establish accurate tracks on all targets.
The final method, delayed contact, was in many ways the preferred method of firing. The mine itself served as a sensor. It’s DC power circuit would tell the casemate when contact had been made. The commander could then order detonation after a few seconds delay. One advantage of the short delay was the likelihood of the blast occurring closer to the midships section of the target, rather than the bows. The largest spaces on a warship are its firerooms and enginerooms, and thus most vulnerable to flooding. And they’re located amidships.
This delayed command method of firing also allowed the mine commander discrimination in his targets. If a small scouting vessel entered the field, the commander might forego attacking it, saving that mine for a later ship in the main body. That would conceal the minefield and expose a more valuable enemy asset to attack later.
Below is a sketch of a minefield protecting the Columbia River in Washington State.
Right click to embiggenfy.
Via the excellent Coast Defense Studies Group
Emplacing all the mines was a considerable task. Simply rolling mines off the back of a vessel wasn’t sufficient. Each mine had to be placed very specifically in it’s intended spot in the field. In fact, the vessels used by the Army were known as Mine Planters, rather than Minelayers. Each Mine Planter, while a coastal vessel, was still a fairly substantial size. It needed to be big enough to carry several mines, and carry the booms to transfer them over the side (and to recover them as well for maintenance, or when tensions were low enough leaving the field planted was no longer called for).
US Army Mine Planter via Wikipedia
But several other vessels were also required. Smaller ships laid the distribution boxes, while others handled the cables that connected the mines to the shore control station. Ideally, each defended port or harbor would have its own flotilla of mine planting vessels, but in practice, only a few did, and those vessels had to move from harbor to harbor to service the fields. Other stations used whatever civilian vessels were available if mine planters were not available.
Originally, Army mine planters were crewed by civilians, but the War Department felt such duties should be performed by military personnel. And so the Army Mine Planter Service came into being. Rather than having Army officers and gentlemen engage in the nasty business of running boats, the same act of 1918 that established the Army Mine Planter Service under the Coast Artillery Corps also established the grade of Warrant Officer. These would serve as masters, mates and engineers of the vessels. From there, the warrant officer would expand to other fields, especially those that had particular technical requirements.
It was no great secret that our harbors were defended by minefields. Any enemy force that did attempt to attack would be certain to attempt minesweeping operations. From the first, most minefields were co-located with major seacoast gun batteries. Additional smaller gun batteries, such as the 90mm Anti-Motor Torpedo Boat batteries (using a variant of the 90mm anti aircraft gun) had a primary mission of defeating any minesweeping ships.
Though the AMPS would not be officially disbanded until 1950, by the end of World War II, it was plain that legacy coastal defenses in the US were obsolete, and they were removed. Today, several old forts are parks or otherwise historical attractions. Of the mine casemates and storage sheds and other installations, the explorer has to look pretty hard to find any.