HMS Unicorn was built under the supervision of Sir Robert Seppings, the Surveyor of the Navy 1813-1832, at a time when the shortage of timber and the availability of iron was beginning to affect the way ships were built.
HMS Unicorn represents the last great flourish of wooden shipbuilding and illustrates the birth of the iron steamship.
Seppings took advantage of the availability, strength and compactness of iron and introduced new methods of construction which greatly strengthened wooden ships.
New Methods of Construction
A truss of ‘diagonal riders’ stiffened a ship’s hull with iron straps.
A ‘circular stern’ was much stronger and allowed the guns a better arc of fire.
Iron ‘knees’ secured the deck beams to the ship’s side frames, in place of conventional wooden knees which had to be cut from specially grown timber.
The complete ‘Seppings’ system integrated several other structural innovations and was so effective that it allowed wooden ships to be built strong enough to stand the weight of heavy steam engines, boilers and coal, and also long enough to provide the extra space needed for these.
When a wooden ship had been at sea for any length of time, its whole structure would work loose and distort under the action of the waves. The ship would then become leaky and unsafe, and its sailing qualities would be reduced. Seppings devised an elegant system of diagonal strengthening straps, or ‘riders’ which he first tried out in the HMS Tremendous in 1811, and which were generally adopted in 1813. These diagonal riders were of thick iron, securely fixed to the inside of the ship’s hull, and produced a far more rigid structure.
Ultimately this integrated approach to ship design allowed ships to be built long and strong enough to carry the extra weight of engines, boilers and coal.
The Circular Stern
Two great weaknesses in the construction of the old men-of-war were the square bow and transom stern. The stern consisted of elaborate but lightly built windows for the officers’ accommodation, and the bow was strong up to middle deck level, but above that there was only a flimsy bulkhead. Both these points could be easily penetrated by heavy weather or enemy shot, and it became a standard manoeuvre for a ship to attempt to ‘rake’ her opponent by firing a full broadside in through the enemy’s bow or stern. Each shot could then travel the full length of the enemy ship, leaving a terrible wake of destruction. No effective retaliation was possible, as the bow and stern were not even strong enough to withstand the recoil of heavy bow or stern chasers (guns fired ahead or astern).
Consequently when Seppings had to ‘cut down’ the Namur from a three to a two-decker in 1805, he decided to leave the bows fully planked. By 1817 he further developed this idea, when building the ASIA, into his famous circular stern, which was fully framed and planked, and had only small windows. This not only kept out enemy shot more effectively and was strong enough to withstand the recoil of heavy stern chasers, but also allowed a wider arc of fire. In a sense it was the first move towards the modern gun-turret.
The traditional method of joining the deck beams to the side frames of a wooden ship was by immense curved wooden ‘knees’. Each of these had to be fashioned out of a suitably curved piece from the fork of an oak tree and this ‘compass timber’ had become extremely difficult and costly to obtain. By contrast, iron had recently become plentiful and cheap and again Seppings pioneered a new method of construction, using much smaller, yet stronger, wrought iron knees.
Robert Seppings incorporated all these innovations into Unicorn’s design, along with several lesser ones, and the ship now occupies a place of unique importance amongst surviving ships. Externally she appears a traditional ship, similar in many ways to Nelson’s famous flagship, HMS Victory, yet structurally she clearly shows the first signs of the immense upheaval of the Industrial Revolution which in the space of a bare 25 years was to result in steam-powered, iron-built ships. No other ship in the world survives to bridge this gap so effectively.