Cast cannon (Fig. 1) such as those which would have formed the main broadside battery for Unicorn were manufactured is a series of stages as follows (see: www.wrexham.gov.uk/english/heritage/bersham_ironworks/jw_wales_wider_world.htm & www.wrexham.gov.uk/english/heritage/bersham_ironworks/boring_for_britain.htm
- A sand mould would be created from a wooden pattern and lowered into the casting pit alongside the furnace.
- Once all the moulds were in place, the furnace would be opened to allow the iron to flow into the moulds.
- After the moulds had cooled sufficiently, the casting pit would have been dug out (manually) to allow the moulds to be extracted.
- When fully cooled, the moulds would be opened and the casting removed.
- The bore would then be bored out and the vent drilled.
The roundness of the bore, and of the cannonballs, was crucial to reducing the windage, the gap between the bore and the cannonball through which the propellant gasses could escape (Fig. 2), and hence to the performance of the cannon.
In 1770 the Dutchman Jan Verbruggen, as Master Founder of the Royal Arsenal at Woolwich, introduced a horse-powered horizontal boring machine which improved the quality of the bore. An improved boring machine was then developed In the early 1770s by John ‘Iron Mad’ Wilkinson at Bersham.
Fearing a monopoly, the Admiralty had Wilkinson’s patent voided in 1779 after which Wilkinson went on to develop a precision boring machine for the production of cylinders for James Watt’s steam engines, resulting in a significant increase in their efficiency and performance.
By the mid 19th century, shell guns firing explosive shell had been developed and as guns increased in size, they needed to be strengthened at the breech to accommodate the increased powder charges being used.
Once solution was the Dahlgren gun of Fig. 3. Designed by Rear Admiral John A Dahlgren of the US Navy and used throughout the American Civil War, these increased the thickness of metal at the breech, leading to their nickname of ‘bottle guns’.
However, the increased thickness of metal resulted in problems in manufacturing. With the conventional process as set out above, the casting cooled from the outside, resulting in the formation of tension cracks and voids in the metal, reducing its strength and potentially leading to catastrophic failures. This was to a significant degree overcome by the introduction of a new casting method by Thomas J Rodman of the US Army in which the casting was cooled from the inside.
Using Rodman’s method (Fig. 4) the mould was placed vertically inside a casing and a bore tube inserted. After the metal was poured, water was passed through the bore tube to cool the casting while the external temperature was controlled by the fire pit. For larger guns, cooling would take place over a period of 3 or 4 days before the casting was removed and the bore drilled out to the required dimensions.
With the introduction of rifled, breech-loading guns, casting was proving unsatisfactory and the built up gun was introduced. This had its origin in guns such as the muzzle-loading Parrott Rifle (Fig. 5) of the American Civil War in which the breech was strengthened by shrinking a wrought iron collar around the cast gun. In the fully built up gun, the inner tube or liner is reinforced by a series of tubes or wire windings as in Fig. 6 to provide the required strength. For a wound construction, tensioned wire is wrapped around the liner in a series of diagonally opposed layers before being encased in an outer shrunk tube.
Blog post by Professor Emeritus David Bradley, HMS Unicorn Volunteer