The only surviving example of a Brennan torpedo extant today is in the Royal Engineers Museum in Gillingham, Kent. All the photos in this section are by the author, with the kind assistance of Lauren Jones of the museum. The Royal Engineers were involved with the Brennan instead of the Royal Navy because it was the sappers who were responsible for submarine mining, the laying and operating of controlled electrically-detonated minefields to defend harbours and anchorages in the United Kingdom and around the empire. The Engineers would also be more at ease with the elaborate means designed to protect and operate the Brennan, as its complicated and extensive launch stations were usually installed inside existing coastal fortifications, also built and maintained by the Royal Engineers. The section of 7in-gauge bullhead rails in the museum, down which the torpedo would be launched, was recovered from the Brennan station in Fort Albert on the Isle of Wight.
The operating principle of the Brennan is simple in concept, but extremely sophisticated in application. Basically, each of two propellers was driven by a drum, on which was wound a long length of thin wire. When the torpedo was launched down its ramp, the wires were pulled from the torpedo by a winch in the launch station, powered by a steam engine. There was therefore no power source contained within the torpedo body as this was all positioned on land. The torpedo simply contained the transmission system. The two drums were rotated in opposite directions and this caused the propellers also to rotate in opposite directions, without the need for contrarotating gears as in the contemporary Whitehead. The presence of two drums was also used to guide the torpedo. By operating a complex system of pulleys in the shore station, differential tension could be applied to the two drums. A sensing mechanism then applied this to the bow and stern rudders, changing the direction of the torpedo.
Depth-keeping had originally been a problem, as Brennan had relied on fixing the bow planes at a suitable angle of attack to keep the torpedo running at the desired depth, the rear horizontal planes being non-movable. Not surprisingly, this had not worked as expected, and some of the early testing required that Brennan prove his device by allowing it to operate as a surface runner, while he worked on perfecting the depth control. He accordingly added a hydrostatic depth-control device, as on the very first Whitehead. Again this caused erratic depth-keeping, and again Brennan returned to the workbench. Finally his efforts paid off, and the Brennan’s depth-keeping was described in a report by Major Sale, RE, in November 1886 as ‘a wholly novel and strikingly ingenious apparatus for meeting conditions far more complex than are met by the corresponding apparatus in the Whitehead torpedo.’
Examination of the internals of the sole surviving Brennan in Gillingham do not reveal the presence of a depth-control pendulum, which formed the essential part of Whitehead’s ‘Secret’. What is present is a rectangular box, power-driven from the rear wired drum, with levers connected to the hydrostatic valve and, presumably, to the bow depth planes (although the latter connection is missing). The depth mechanism is a sealed unit, which was not kept in the body of the torpedo but in a locked safe in the launch station, only being retrieved and inserted in the torpedo shortly before launch. This formed part of Brennan’s almost pathological fear that his ideas would be pirated and freely copied — not unreasonable in view of the espionage and skulduggery that surrounded Robert Whitehead and his ‘Secret’. But unlike Whitehead, who sold the ‘Secret’ to any and all comers with the ready cash, Brennan, who signed an exclusivity deal with the British government, managed to retain the secret of his successful depth mechanism so well that, even today, we have very little idea of how it worked in practice.
The depth mechanism box obviously acted as a servo mechanism for operating the non-balanced bow planes against the considerable resistance of water pressure. There must be some means of damping over-reactions from the hydrostatic valve, working to the same end as the large Whitehead pendulum — perhaps a weight sliding on a rod. But the box is compact, and unless and until it is either opened or X-rayed, it seems Brennan succeeded in taking his ‘Secret’ to the grave with him, when he was knocked down by a car in his eightieth year. It would be fascinating to delve into the genius of the man who invented a monorail gyro-stabilised railway train, and an early helicopter, amongst his other innovations.
The 1877 full-size prototype torpedo was stated to have been 9ft long, rectangular in section and boat-shaped in plan view, which corresponds with the shape of the small model in the photo. A year later, having proved that the propulsion system worked as intended, Brennan developed the concept into a cigar-shaped torpedo stabilised by long tail fins at top and bottom, with contrarotating propellers and a hump on top to provide for the wire take-offs. The final production model led the two wires through the centre of the concentric propeller shafts, avoiding fouling the propellers. Brennan then added an ingenious device which at intervals released rings from the rear of the propeller shafts, to hold the two driving wires together and prevent them from separating in the water.
The openings in the bronze torpedo body are, from left to right: a rectangular access hole for the steering and depth mechanisms, used to insert the depth mechanism sealed box shortly prior to launch; then the large openings for the wire drums; next the access to the reciprocating mechanisms which moved pulleys back and forth as the wires unrolled, ensuring that they always came off the drums at right angles; finally, the hatch to insert a calcium phosphide canister for practice runs. This latter device gave off smoke to mark the position of the torpedo for recovery at the end of its run. Not visible in the photo is the very first access cover, on the warhead in the nose, allowing the launch operators to cock the inertia pistol, which would arm itself after a set number of propeller revolutions, when well away from the launch station. On top of the body is the folded mast, which would be propped into an upright position before launching the torpedo, carrying a small pennant to mark its course to the observer controlling the steering mechanism.
Top left of the left-hand drum is part of one of the reciprocating pulleys which took the wire off its drum at right angles, moving back and forth as the reel unwound. The reason why the torpedo body is oval in section was to provide room above the drums for these take-off pulleys, and below the drums for the steering connection to the vertical tail rudder. Under the body are the four wheels on which the torpedo ran down the launch rails into the water. Not visible inside them at the bottom of the torpedo body are the four fixed guide projections, which ran inside the rail edges to keep the torpedo on a straight path. The Royal Engineers collection contains a small-scale silver-plated demonstration model some 20cm long. Pulling on the cork attached to the wire emerging from the propeller shafts revolves the drum.
The perfected torpedo entered service in around 1890. The surviving example is some 22ft long, weighing just over one ton. The operator could steer it by observing its extended mast which was the only projection above water. Using 2000yds (1830m) of the thicker wire (0.07in gauge/1.8mm) introduced in 1894 it could reach a top speed of 24 to 25 knots over short distances, 21 to 22 knots over 1000yds (914m), and 19 to 20 knots over 1600yds (1463m). Maximum range was around 2000yds (1830m). Carrying a 220lb (100kg) charge of wet gun-cotton, the Brennan was a reliable and effective close-range defence weapon. Cost was stated as around £300 in February 1887, to which, of course, must be added the considerable costs of building and equipping the launch station.