Sunday Ship History: Mortars at Sea

In the New York Times February 10,1862, appeared something of a mystery:

The powerful flotilla of Mortar-Boats which has been, for some weeks past, fitting out at the Brooklyn Navy -yard, under the superintendence of Commodore Porter, is now nearly ready for sea, and may be expected to start for somewhere during the coming week. Concerning what particular rebel stronghold they are intended to beleaguer and bombard, official reticence, as yet as made no sign. Whether, indeed, Captain Porter himself is precisely aware of the point where is expedition is to make itself offensively felt, is exceedingly doubtful. One thing, however, is certain; if he does know, he keeps his information hermetically bottled up from all access tp inquisitive outsiders.

The article goes on to explain how the mortar boats operate and how mortars differ from the then conventional cannon:

Their use is to throw hollow shells, filled with explosive material, which, when falling on a building or into the works of a fortification, burst, and, with the fragments, destroy everything within reach."

The idea is somewhat akin to a slow pitch softball pitch- the projectile is launched nearly vertical - and descends nearly straight down. This was very different from the cannon of those days i which the shot went out essentially horizontal. The advantage of the mortar is that high walls and deep trenches pose little in the way of obstruction to a shell descending vertically.

Mortars as weapons of war are old weapons, well known to armies engaged in sieges.

Porter's sea mortars were generally 13-inch (measured across the bore) and could toss a 200 pound shell about 2-2/3 miles. Ship cannons in those days had effective ranges of a few hundred yards, maybe. The large mortar shells could also act as anti bunker weapons - their weight could cause them to sink into the ground to some depth before exploding. The NYTimes describes the mounting of the mortars on the schooners and brigs as follows:

...Capt Porter has has the decks of all his vessels underlaid with massive timber beams, interlaced and riveted, and extending (wher the main pressure of recoil occurs) almost through to kelson. The mortars themselves rest upon circular revolving platforms, of novel and ingenious construction. By means of a very simple arrangement of machinery these platforms can, whn the mortars are fired, be lifted from their rollars, and planted firmly in the solid beds beneath and when a change of aim is desirable, they can easily be elevated, and turned in any direction the gunners please. These platforms are a great improvment on those adopted in the British Navy.

You might recall that revolving turrets for guns had just been put to their first use on USS Monitor a couple of months before this Times piece appeared.

We now know that Porter's Mortar-Boats were headed, towed by steam ships, for the Gulf of Mexico, where the U.S. Navy was about to move forward on General Winfield Scott's Anaconda Plan, where Commodore Farragut was looking for ways to get past Fort Jackson and Fort St. Philip and move his fleet up the Mississippi River to link up with forces working with the Army and capture Vicksburg close the Mississippi to the Confederates. Porter had a vision which came to pass:

As it is written in Sea Power: A Naval history (2nd ed., E.B. Potter, ed, USNI, 1981, p.137):

Porter argued that a fleet of mortar boats could wreck forts Jackson and St. Philip to the point where deep-draft naval vessels could pass upriver and threaten the city.

Farragut collected a group of ships and gunboats. Porter had 21 mortar boats with 13" mortars. April 17, 1982 saw Porter moving up the river to get into position, 15 boats began shelling Fort Jackson, the other 6 taking on Fort St. Philip. Wooden buildings in the forts burned, counter battery fire sank some mortar boats, but the fort defenders, under constant mortar fire, couldn't prevent a couple of Farragut's gunboats breaching the river barrier enough for the bigger ships in Farragut's force to get upriver.

When through the barrier, New Orleans was assaulted and fell, and the Navy mortar boats moved up river, eventually participating in the siege of Vicksburg. In later days, some of these mortar boats bombarded Fort McAllister, Georgia (where the Confederate commander noted, "Their mortar fire was unusually fine, a large number of their shells bursting directly over the battery") and took part in several other actions.

In addition to these "mortar boats," the Navy had "mortar barges" built (see here for use along the Mississippi River. Details of these barges can be seen in the picture of model nearby. These barges were towed into place on river bank with range of their targets and left to lob rounds onto the opposing force. A print from the times showing the assault on island No. 10 is nearby. Caption:

Colored lithograph published by Currier & Ives, New York, circa 1862. It depicts the bombardment of the Confederate fortifications on Island Number Ten by Federal gunboats and mortar boats. Ships seen include (from left to right): Mound City, Louisville, Pittsburg, Carondelet, Flagship Benton, Cincinnati, Saint Louis and Conestoga. Mortar boats are firing from along the river bank.

After the Civil War, not much is heard about "Mortar-Boats" until . . . World War II, when the concept was revived in a effort to provide support for amphibious landings.

The WWII version consisted of modified Landing Craft Infantry (see also here). These ships became known as LCI(M)- Landing Craft Infantry (Mortar). According to U.S. Amphibious Ships and Craft by Norman Friedman,

On 18 October 1944 CinCPac requested 30 . . .side-ramp LCI(L)s to be armed with three mortars and one 40-mm gun, the latter replacing the bow 20-mm.
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In all these ships, the mortars were fixed on deck. BuOrd developed the 4.2-in mortar MK 1, on a 3-in gun mount, which could be trained and elevated.

Some ships had their bows "cut down to allow the mortars to fire forward at reduced elevation.

Whatever their design, in February 1945, a flotilla of these mortar ships joined the U.S. Naval force gathering off the Iwo Jima. There they saw action according to a battle plan:

From: Commander Amphibious Forces, United States Pacific Fleet (Commander Joint Expeditionary Force) *** LCI(M) MORTAR PLAN The rapid rate of fire, high trajectory and long range of the 4".2 chemical mortar were considered to have considerable value for beach preparation fire and for deep support subsequent to How-Hour. Consequently, as recommended following the Marianas operation, an effort was made to develop a craft mounting this weapon and capable of proceeding to the objective under its own power. Three 4".2 chemical mortars were mounted on an LCI and experiments successfully covered all phases of loading and firing. Additional conversions were made, further tests conducted and standard plans drawn up for delivery of mortar fire from designated positions on the flanks of the beaches and after How-Hour deep support, behind the beaches. *** The LCS(L)(3)'s and LCI(M)'s supported the landing by fire on the beach, behind the beach and on the flanks before and after H-hour. Some were under control of Shore Fire Control Parties after H-hour. The greater part of enemy fire came from the flanks. The 40-mm. fire of the LCI's at caves and cliff areas on the flanks was effective in reducing enemy machine gun fire on the beaches, while rockets and mortar shells helped to keep down short range enemy mortar fire. LCI(M)'s were very useful for delivering all-night harassing fire. Initially, three divisions were used nightly, gradually tapering off to one or part of one division. These LCI(M)'s usually came under fire from enemy coast guns and intermediate automatic weapon fire intermittently during the night, and it was necessary to detail a cruiser or destroyer to cover each division. None was hit but there were many near misses.

Mortar Support Units No. 1, No. 2, and No. 5, consisting of six mortar LCI's, were assigned to provide scheduled flanking and deep supporting mortar fire as follows: Units No. 2 and No. 5, using plan "A," fired from How-minus-Thirty-five to How-minus-Seven-minutes on the eastern slopes and approaches to Suribachi Yama while Unit No. 1, using the same fire delivery plan, delivered fire on the eastern flank high ground during the same period. At How-Hour Units No. 2 and No. 5 in column, entered and crossed the boat lanes from the west, turned shoreward into line and followed the Sixth assault wave toward shore.

At about How-plus-Twenty-minutes, when 2,000 yards from shore all ships of these two units opened fire using plan B with mortar range set for 3,200 yards and swept a rectangular area 2,200 yards long by 1,000 yards deep as they moved in. Stopping and lying to 1,000 yards from shore, fire was then maintained 1,800 yards inland and parallel to the beach until How-plus-Sixty-minutes. At How-minus-Seven, Mortar Support Unit No. 1 shifted its line of fire farther to the east for safety to troops and resumed fire at How-plus-Ten-minutes firing at a reduced rate for neutralization until 1,300; 17,400 rounds of 4".2 mortar were scheduled for delivery in support of the landing by these three units.
***
On completion of their scheduled fire, Mortar Support Units NO. 1, No. 2, and No. 5 replenished mortar ammunition and joined Units No. 3 and No. 4 in area Roger awaiting assignment. Night harassing mortar fire requests from Headquarters Landing Force were fulfilled by assignment of Units No. 2 and No. 5 to cover prescribed areas throughout the night using standard plan A and varying the line of fire between specified limits. A total of 24,000 rounds of which 20 percent was WP, were delivered by these two units from reference points 1,000 yards off the northwest and southeast coasts of the island. Large caliber enemy counter fire was received by the northern unit, but was not intense or accurate enough to require the withdrawal of this unit.
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One Mortar Unit continued to be assigned each night to deliver harassing fire. Enemy counter fire continued to require the assignment of a destroyer or cruiser to provide support for the harassing unit. On departure of Units No. 1, No. 2 and No. 5 from the area on February 26, Units No. 3 and No. 4 were reorganized into five-ship divisions. Due to the reduced size of the remaining units and their inexperience in plan A fire delivery, night harassing fires were hereafter delivered using plan C. On 28 February, two more mortar LCI's departed the area, leaving two four-ship units available for mortar fire. On 27 February and for several days thereafter individual mortar LCI's were assigned during daylight to provide direct support to battalions designated by Headquarters Landing Force.

The remaining area into which night harassing fire could safely be delivered, required the employment of only one mortar LCI on the night of 1 March. Thereafter harassing fires at night by these ships was discontinued. On 3 March all remaining mortar LCI's departed the area.

Ammunition expenditures by LCI types exclusive of pre-Dog-Day expenditures, were as follows:

4".2 mortar 60,000
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MORTAR LCI's

This was the first operation in which LCI's mounting mortars have been employed by the Fifth Amphibious Force. Their primary mission, as conceived in the initial planning, was the delivery of heavy harassing fire at night to prevent the initiation of organized counterattacks. Their support with this fire was most gratifying and materially reduced the demands for harassing fire by destroyers and cruisers.

Two methods of delivering harassing mortar fire at night were employed at Iwo Jima: (1) Plan A of the standard mortar fire plans, in which five LCI's steam on an eliptical track around an LCI acting as reference ship. Ships fire singly in succession during the two minutes' run on the leg on which they are pointed toward the target area. (2) Plan C in which the six LCI's of a division lie to on a line, 200 yards between ships, and fire when the ship's head is between prescribed limiting lines of fire. Both plans have many advantages and disadvantages. Since it is next to impossible to hold an LCI on an accurate heading for a long period when dead in the water, plan C is unsuitable for interdiction fire where continuous and fairly accurate fire along a definite line is required. Harassing fire, which requires irregular volume and rate of fire with an unsystematic pattern and coverage of the area harassed, is especially typical of the fire to be expected of six LCI's dead in the water all on different headings between prescribed limits. Plan A, on the other hand, has all the fire delivery characteristics most suited for interdiction fire and least suited for harassing fire.

The 3,200-yard range limit of LCI mortar fire requires these ships to approach as close to shore as safe navigation permits in order to place their fire as far inland as possible. On a well defended island such as Iwo Jima, this close approach to shore drew considerable enemy fire even at night. LCI mortar ships found good use for their bow 40-mm. in delivery of counterbattery fire in self-protection, but this was found insufficient and it became necessary to assign one of the general support destroyers or cruisers to cover the nightly harassing mortar LCI unit. In making plans for delivery of night (or day) harassing fire by mortar LCI's, the plans should incorporate the employment of a destroyer, for counter-battery protection of the harassing unit. This ship should work with the Mortar Unit Commander on a common frequency.

On request, individual mortar LCI's were assigned to battalions for direct support, generally to those battalions whose flanks were along the shoreline as in the case of gunboat support. Preliminary reports indicate that this support was more in the form of harassing or neutralization fire for the battalion supported. In rough water, the accuracy of LCI mortar fire in deflection is greatly decreased by rolling and cannot be safely called for in areas close to own troops. LCI mortar fire for direct support should therefore only be expected to accomplish harassing or preparation neutralizing fire for an advance into areas within range of the LCI Mounted mortar.

While the accuracy in deflection of mortar fire from LCI's is greatly affected by rolling and variations in ship head, its accuracy in range is quite dependable and relatively unaffected by motion of the ship. It is therefore very suitable for neutralizing fire over the heads of troops when the line of fire is perpendicular to the line of troops. Its high trajectory makes it ideal for use when ships and troops located between the target and firing ships preclude the use of high velocity flat trajectory fire. At Iwo Jima, the neutralization of large areas inland from the beaches was effectively delivered by mortar fire from LCI's on a line parallel to and 1,000 yards from shore. This fire was not provided however, until How-plus-Twenty minutes. Using Plan B with desired modifications, mortar LCI's should be employed in the boat lanes to provide beach neutralization just prior to the time the first wave leaves the line of departure and during its run to the beach. They should precede the first wave by any desired distance, stop and lie to not less than 600 yards (minimum firing range) from shore, and continue mortar fire on the beach until the first wave is about 200 yards from shore. At this time the fire should be lifted about 200 to 500 yards inland and lifted in predetermined steps thereafter according to a prearranged time schedule based on anticipated troops advance. This type of moving close support was provided at Iwo Jima using 5"/38 AA Common fire with 1,200-foot second charges.

More:

From: Commander Mortar Support Group 52.6 (Commander LCI Flotilla Twenty-one)

Plan ABLE consists of sustained fire covering a comparatively narrow target area and capable of extension in range. This plan is best adapted for close supporting fire (flank protection), harassing and interdiction fire. This plan of fire is delivered from a predetermined reference point around which the ships circle, delivering fire when the firing ship is headed toward the target area. The ships circle in both clockwise and counterclockwise movements, depending on the tactical situation.

Plan BAKER consists of a barrage fire covering a wide target area and capable of progressive movement in range. It is best adapted for neutralization fire and close support fire over and beyond our own troops. This plan of fire is delivered from a predetermined point with the ships that are firing disposed on a line of bearing parallel to the desired range median of the barrage or in line abreast formation.

Plan CHARLIE consists of independent or minor concentration fire covering point targets or targets of opportunity. It is best adapted for harassing fire, counter-battery fire, interdiction fire, and incidental destruction fire, particularly that requiring high trajectory. It is conducted by single or several ships whose fire may or may not be coordinated depending on assignment and is delivered from a reference point and bearing from the designated target area.

All target areas and fire plans in connection with the mission were contained in the Operations Plan, up to and including H-hour-plus-60. Subsequently, target fire and smoke missions were assigned.

At the outset of the mission it was assumed that preliminary naval and air bombardment had effectively silenced coastal defense and shore batteries that could possibly interfere with the carrying out of this group's mission. This assumption was subsequently found to be true. Sporadic mortar and machine gun fire was encountered however, which interfered with our operations on several occasions.

Mortars on Navy units reappeared during the Vietnam War when Navy Swift Boats were equipped with 81mm mortars which allowed those small units to provide limited fire support to shore units along the coast areas. A description of this unique weapon can be found here:

These 81mm mortars were unique to the U.S. Navy and U.S. Coast Guard small craft of the Viet Nam and post war-era. The mortar itself is entirely different in design from all other mortars in U.S. service. This mortar is designed for direct and indirect fire. The difference between the Mk 2 Mod 0 and Mk 2 Mod 1 are mostly cosmetic and that the Mod 1 mounts an AN/M-2HB .50 Browning machinegun above its recoil cylinder and a 100 or 400 round ammunition box on its right side.

The mortar itself is mounted on a very robust tripod and uses clamps to control traverse and elevation angles. Unless fitted with NO FIRE zone mechanical stops, the mortar has 360 degrees of traverse and -30 degrees of depression and +71.5 degrees of elevation. Its rate of fire is 18 rounds/minute at 45 degrees elevation in DROP FIRE mode and 10 rounds/minute in TRIGGER FIRE mode. Sights for the mortar are attached to the left side of the elevation arc. Weight of the Mk 2 Mod 0 was 593 pounds; the weight increased to 677 pounds in the Mk 2 Mod 1 (with machinegun). Range of the 81mm (direct) was 1,000+ yards; (high angle, indirect) was 3,940 yards. Maximum effective range of the .50 Browning machine gun was 2,000 yards; maximum range was 7,440 yards.

The Mk 2 was developed by the NavWepSta Crane, Indiana, in the early 1960s to provide offshore patrol boats with a light weight direct and high-angle fire weapon that could engage both surface and shore targets. It was adopted by the USCG in 1962 where it was first mounted on their large WHEC cutters in the Atlantic and Pacific. These mortars were used to fire illumination flares to aid commercial and military aircraft forced to ditch at sea. Tests in the Caribbean showed the 81mm mortar illumination round was more effective than the 3"/50 gun's star shell. Not only was the illumination of the 81mm round better, but it fired at a higher rate and had less fouling problems than the 3"/50.

In mid-1964 the USCG recommended the fitting of a .50 Browning machinegun in "piggyback" fashion above the mortar's recoil cylinder. The prototype was built by the USCG at its Curtis Bay, MD, yard and it worked very well. The mortar's tripod mount was more than adequate for taming the .50 Browning's recoil. In late 1964, the Navy fired the Mk 2 Mod 1 at its Dahlgren, VA range. The Mk 2 Mod 1 was successful in its tests. Two Mk 2 Mod 1 units were then taken to sea aboard USCG 95-foot cutters for demonstration and operational evaluation. Both units passed with flying colors.

The Mk 2 Mod 0/1 was deployed by the hundreds aboard many kinds of craft in Viet Nam. A short, but by no means exhaustive list would be: APLs and YRBMs (non-self propelled barracks barges); USCG 82-foot cutters; USCG 95-foot cutters; Navy 50-foot coastal patrol craft (PCF); Navy 75-foot fast patrol boats (PTF, "Nasty"-class); Navy 95-foot fast patrol boats (PTF, "Osprey"-class); Navy patrol gunboats (PG, "Ashville"-class); miscellaneous riverine craft which were mostly converted LCM-6 landing craft: MON (monitor); CCB (command and control boat); Zippo (flame thrower boat); ASPB (assault support patrol boat); HSSC (heavy SEAL support craft); and advanced tactical support bases such as SEA FLOAT/SOLID ANCHOR (Nam Can) and BREEZY COVE (Song Ong Doc).

Photo of 81mm mortar from here. Caption:

CS2 Bob Burton loads the 81mm mortar of the CGC Point Jefferson WPB 82306, during a 1968 fire mission. (Photo donated by CSC Robert O. Burton, USCG (ret).

I believe the navy 81mm mortar is still in service.

As you can see, mortars in the Navy have a long and worthy history.