The Coehorn Mortar

A 5 ½ Coehorn mortar is on display in the Army Museum. It was designed by the civil engineer, Goether Kerr Mann, grandson of Lieutenant General Gother Mann and son of Major General Cornelius Mann, both distinguished officers of the Royal Engineers.

Cornelius Mann was a friend of Lieutenant-Colonel George Barney, designer of Victoria Barracks. Gother Kerr Mann was born at Athlone in Ireland and served first in the East India Company Artillery, then the Royal Bombay Horse Artillery. However, as a result of bad health, he retired and he and his brother Joseph Frederick (explorer and surveyor) decided to settle in New South Wales.

In Sydney Gother Mann served for a time as an assistant to the Commanding Royal Engineer, Lieutenant Colonel James Gordon RE, who had succeeded Lieutenant Colonel George Barney. Later he became Engineer-in-Chief on Cockatoo Island and from 1955 was Chief Commissioner for Railways in New South Wales.

Coehorn mortars of various calibres had been in use in the British Army for many years and had been adapted from mortars designed by Baron Van Menno Coehoorn, a Dutch military engineer of Swedish extraction. It is believed that Coehorn mortars were first used at the siege of Grave in 1673 and caused considerable problems for the French defenders.

In 1845 a northern Maori chief, Honi Heke, raided Russell on the north-east coast of the Bay of Islands on the North Island of New Zealand. In an official report about the serious British reverses in the fighting against the forces of Hone Heke at Ohaiawai and Kawiti at Okaihu a plea was made for a weapon possessing portability with sufficient accuracy of range and direction to throw a shell at from 500 to 600 yards in an enclosure of amongst lage bodies of men.

Mann was aware of the difficulties associated with the transportation of field guns and ships' guns in the New Zealand forests and, at his own expense, had arranged for the construction of Coehorn mortars at the foundry of Peter Nicol Russell in Sydney. Russell had government contracts for iron work in New South Wales, including that at Victoria Barracks.

The Governor of New South Wales, Sir George Gipps, KCH (an officer of the Royal Engineers) - on the recommendation of the Commander of the Miliary Forces in New South Wales, Major General Sir Maurice O'Connell, KCH - had approved Mann's plan for the construction of Coehorn mortars for use with the large stock of 5 ½-inch shells which were held in the colony.

On 4 October 1846 Lieutenant Colonel C.B. Despard, commander of the British Force in New Zealand, reported the arrival at the Headquarters of the 58th Regiment in New Zealand with four mortars "….which had been cast in Sydney…". On 4 November he reported the arrival of a detachment of the 99th Regiment with military stores and "…three more mortars…",

When the fighting died down, Mann sought reimbursement for the expenses involved in the production of his mortars. To support his claim, Colonel Despard wrote:

"They were found very useful and very efficacious in throwing shells into the pa at Ruapekapeka when placed in battery at a distance of not more than three hundred and fifty yards, but beyond that distance, when tried, they were found uncertain. On the whole, I consider that they did us good service, and that very few instances occurred in which a shell thrown from them did not fall within the pa, which was not the case on a former occasion when scarcely one in five of those from the Iron 12 Pound Howitzers fell within the enemy's works."

An old chief of the Ngapuhi tribe commented on the mortars:

"These guns [i.e the mortars] had shot which were hollow exactly like a calabash [a gourd], and they were full of gun- powder, and they came tumbling into the pa one after another, and they would hardly be on the ground before they would burst with a great noise, and no sooner would one burst than another would burst; and so they came one after another so fast they the people in the pa could get no rest, and were getting quite deaf.

These guns, however, never killed anyone. They are a very vexatious invention for making people deaf and preventing them from getting any sleep. One good thing about them is that whenever one of the shots does not burst, a considerable number of charges of powder for a musket can be got out of it; and whenever one dropped close to one of the men in the pa he would pull out the wicki [fuse], and then get out the powder…"

By 1847 the Maoris in the Wellington area were subdued and, in turn, the Coehorn mortars were placed in storage. In the wars with the Maoris from 1860 to 1864 the Coehorn mortars were again in use. During the Tauranga Campaign of 1864 Colonel Williams, Commander of the Royal Artillery wrote to Gother Kerr Mann:

"On the 20th April last, I drew your mortars from the store, tried one and embarked four with guns and stores for Tauranga. We had an opportunity of using them in a breastwork about 100 yards from the enemy's work on the evening of the assault. We considered them very useful pieces and are pleased with the accuracy of their fire."


Characteristics of the 5 ½ inch Brass Mortar and cast iron bed:

Content of Chamber 9 ounces (255g)

Effective Range 550 yards (503m)

Weight - Brass Mortar 54 pounds (24.49 kg), Cast Iron Base 70 pounds (31.75kg)

Length of Base 14 ½ inches (370mm)


Army Museum of New South Wales appreciates the assistance of Brigadier P.J. Greveille, CBE in the preparation of this text.