The weight of the two men was sufficient to just submerge the boat and bury them to their chins when sitting down, while they could disappear altogether by lying flat on the deck. The degree of submersion, therefore, was just right for the beginning of the adventure, and Jack accordingly signalled Macintyre, by means of an electric button, first to back away from the yacht, and then to go ahead dead slow, guiding him at the same time how to steer by means of another button. The torpedo boat, being the most formidable of the three craft, was selected as the first to be operated upon, and the submarine was accordingly headed for her. The water of Havana harbour is unspeakably foul, the sewers of the town discharging into it, and it is almost opaque with the quantity of matter of various kinds that it holds in suspension; it was therefore necessary for the submarine to approach the torpedo boat pretty closely ere sinking any deeper, or it would have been difficult for the adventurers to find their prey in the muddy water, but they managed excellently, approaching within ten yards without being detected. Then Jack, unwilling to court failure by running any unnecessary risks, gave the order to sink slowly, at the same time turning on his electric lamp. Another moment, and he saw the torpedo boat's stern and propeller before him, and the submarine, magnificently managed by Macintyre, who had worked her in all her previous trials, drew gently up until she stopped motionless in such a position that Jack could do just what he wanted. He had decided not to attempt to remove the propeller in this case, lest the violent exertion required to start the nut should cause him to overbalance and fall to the bottom of the harbour, in which case he would inevitably be lost in the deep layer of foul mud which formed the harbour bottom. He therefore took a length of stout chain, already prepared for the purpose, and, having first carefully wound it round the three blades of the boat's propeller, passed the loose ends round the keel bar and rudder where, having drawn them as tight as he and Milsom could draw them, he shackled them together, thus rendering it impossible to move the boat until the chain had been found and taken off. The whole job occupied them a bare quarter of an hour, and could have been done in less had it not been for the hindrance which they experienced from the fish, which--sharks luckily excepted--attracted by their lamps, swarmed round them so persistently that it was almost impossible to do any work for the obstruction of them.