All the German submarines were capable of prolonged endurance submerged. The U-boats could travel under water at the slowest speed for some 48 hours, at about 4 knots for 20 hours, at 5 knots for about 12 hours, and at 8 knots for about 2 hours. They were tested to depths of at least 180 feet, but many submerged to depths exceeding 250 feet without injury. They did not usually lie on the bottom at depths greatly exceeding 20 fathoms (120 feet). All German submarines, except possibly the _cruiser class_, could dive from diving trim in from 30 seconds to one minute. The _U.B. class_ had particularly rapid diving qualities, and were very popular boats with the German submarine officers. Perhaps the most noticeable features of the German submarines as a whole were their excellent engines and their great strength of construction. Prior to the month of February, 1917, it was the usual practice of the enemy submarine in the warfare against merchant ships to give some warning before delivering her attack. This was by no means a universal rule, particularly in the case of British merchant vessels, as is evidenced by the attacks on the _Lusitania, Arabic_, and scores of other ships. In the years 1915 and 1916, however, only 21 and 29 per cent. respectively of the British merchant ships sunk by enemy submarines were destroyed without warning, whilst during the first four months of the unrestricted submarine warfare in 1917 the figure rose to 64 per cent., and went higher and higher as the months progressed. Prior to February, 1917, the more general method of attack on ships was to "bring them to" by means of gun-fire; they were then sunk by gun-fire, torpedo, or bomb. This practice necessitated the submarine being on the surface, and so gave a merchant ship defensively armed a chance of replying to the gun-fire and of escaping, and it also gave armed decoy ships a good opportunity of successful action if the submarine could be induced to close to very short range. The form of attack on commerce known as "unrestricted submarine warfare" was commenced by Germany with the object of forcing Great Britain to make peace by cutting off her supplies of food and raw material. It has been acknowledged by Germans in high positions that the German Admiralty considered that this form of warfare would achieve its object in a comparatively short time, in fact in a matter of some five or six months. Experienced British naval officers, aware of the extent of the German submarine building programme, and above all aware of the shadowy nature of our existing means of defence against such a form of warfare, had every reason to hold the view that the danger was great and that the Allies were faced with a situation, fraught with the very gravest possibilities. The principal doubt was as to the ability of the enemy to train submarine crews with sufficient rapidity to keep pace with his building programme.