12-inch coast defense mortar

The 12-inch coast defense mortar was a weapon of 12-inch (305 mm) caliber emplaced during the 1890s and early 20th century to defend US harbors from seaborne attack. In 1886, when the Endicott Board set forth its initial plan for upgrading the coast defenses of the United States, it relied primarily on mortars, not guns, to defend American harbors. Over the years, provision was made for fortifications that would mount some 476 of these weapons, although not all of these tubes were installed. Ninety-one of these weapons were remounted as railway artillery in 1918-1919, but this was too late to see action in World War I. The railway mortars were only deployed in small quantities, and none overseas. The fixed mortars in the Philippines saw action in the Japanese invasion in World War II. All of the fixed mortars (except four) in the United States were scrapped by 1944, as new weapons replaced them, and the railway mortars were scrapped after the war. Today, the only remaining mortars of this type in the 50 states are four at Battery Laidley, part of Fort Desoto near St. Petersburg, Florida, but the remains of coast defense mortar emplacements can be seen at many former Coast Artillery forts across the United States and its former territories. Additional 12-inch mortars and other large-caliber weapons remain in the Philippines.

12-inch coast defense mortar
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The 12-inch Coast Defense Mortar: A Historical Overview

The 12-inch coast defense mortar was a formidable weapon, specifically designed to protect U.S. harbors from seaborne threats during the late 19th and early 20th centuries. Initiated by the Endicott Board in 1886, the plan focused primarily on utilizing mortars rather than traditional artillery for coastal defense. Over time, fortifications capable of accommodating approximately 476 mortars were conceptualized, although not all were installed. Notably, 91 of these mortars were converted to railway artillery during 1918-1919, but they did not see action in World War I. The main operational use occurred during the Japanese invasion of the Philippines in World War II.

The Weapons

Among the various models, the M1890M1 (Model of 1890, Modification 1) 12-inch mortar stood out as one of the most powerful coast artillery pieces of its era. It was primarily designed to fire deck-piercing shells weighing between 700 to 1,046 pounds (318 to 474 kg) with hardened steel caps that were engineered to breach the armor of enemy ships. Each mortar, firing these half-ton projectiles at an elevation of 45 degrees, could reach a distance of 12,019 yards (approximately 7 miles).

In addition to the deck-piercing shells, early models were also equipped with torpedo shells, which were designed to detonate upon contact with a ship’s deck. The M1890M1 mortar was commonly mounted on the robust M1896 carriage, weighing a total of 78.5 tons, which allowed for precise aiming and swift reloading through its advanced design features.

Emplacement Design History

Initially, mortar batteries were designed as Abbot Quads, a configuration developed by General Henry Larcom Abbot. This design typically featured four mortar pits, each containing four mortars. Despite its advantages in simultaneous fire capabilities, the Abbot Quad design faced challenges due to overcrowded pits, prompting a shift towards designs that allowed for individual mortar aiming, ultimately enhancing operational efficiency.

Firing Mechanisms

The mortars could be fired either electrically or manually, with each method employing distinct firing mechanisms. Electrical firing required a series of connections to trigger the detonator, while manual firing relied on a lanyard system that was simpler but less reliable. The firing process, particularly with four mortars in a confined pit, resulted in a thunderous cacophony, producing shockwaves that could damage nearby structures.

Fire Control Systems

The coast artillery fire control system was vital for the effective deployment of these mortars, employing central data booths to relay precise coordinates essential for targeting. Despite their reliance on these systems, the inherent limitations of the mortar’s design left them vulnerable to modern warfare tactics, especially from aerial attacks.

Railway Mortars and Combat Service

As the United States entered World War I, the army experimented with converting coast artillery into railway mounts, allowing for greater mobility on the battlefield. However, only a handful saw combat service, with the most significant involvement occurring in the Philippines during the Japanese invasion in 1942. The mortars at Battery Way and Battery Geary played a crucial role in defense, yet faced extinction from enemy artillery fire and aerial attacks.

Surviving Examples

Today, of the original 12-inch mortars, nineteen remain, primarily situated in the Philippines, while a few can also be found at Fort DeSoto in Florida. The preservation of these historical artifacts serves as a testament to the engineering prowess and military strategies of their time.

  • 4 mortars M1890M1 at Battery Way, Fort Mills, Corregidor, Philippines
  • 4 mortars M1908 at Battery Geary Pit B, Fort Mills, Corregidor, Philippines
  • 4 mortars M1912 at Battery Craighill, Caballo Island, Philippines
  • 4 mortars M1890M1 at Battery Laidley, Fort DeSoto, Florida

Further Reading

For those interested in delving deeper into the history and functionality of the 12-inch coast defense mortar, consider the following resources:

  • Berhow, Mark A. and McGovern, Terrance C. American Defenses of Corregidor and Manila Bay 1898–1945, Osprey Publishing Ltd.; 1st edition, 2003.
  • Morton, Louis. The Fall of the Philippines. U.S. Army in World War II: The War in the Pacific. 1953.