planning at the time also assumed that international treaties would prevent unrestricted submarine warfare, and the main purpose of the submarine would be to attack enemy warships. In such a situation, a commander may have only one chance to attack, so a large salvo was essential. The ten-torpedo salvo of the pre-war T-class boats was the largest ever fitted to any operational submarine. All T-class submarines had six internal torpedo tubes in the bow. These were fitted with bow shutters on early Group One boats to reduce underwater drag; the benefits proved to be rather minimal, the shutters were prone to jamming from flotsam and the idea was dropped in favour of reshaping the torpedo tube orifices for minimal drag. After the loss of due to the unintentional opening of the rear door of a torpedo tube while its bow cap was open, a special safety clip known as the "Thetis clip" was introduced to prevent the rear torpedo tube door from being opened by more than a fraction if the bow cap was not in place. Each T-class boat carried six reload torpedoes in the torpedo stowage compartment for the internal tubes. The reloading process was manual, although a power loading system was experimented with on Triumph in 1939 based on one developed on . This system proved underpowered and the pressures of wartime production led to development being curtailed. The internal torpedo tubes were complemented by four external ("E-type") 21-inch torpedo tubes on Group One T-class boats, all forward-facing. External tubes were used in order to avoid compromising the structural integrity of the pressure hull with too many openings. These tubes could not be reloaded from within the submarine, and it was also not possible to conduct maintenance on or withdraw the torpedo once it was loaded into the external tube. These tubes were angled downwards at a 5° bow angle to ease operations, except on the lead boat Triton. Two of these external tubes were located in the bow, and another two located amidships at the base of the conning tower. Unlike the internal tubes, the bow caps for the external tubes had to be worked manually, requiring a considerable amount of effort. The tubes also proved to be vulnerable to damage. Two of the T-class boats had their bow external tubes omitted during reconstruction: Thunderbolt (ex-Thetis) and Triumph. Prior to the outbreak of war, there had