An ice-breaking ship, to perform its allotted task, must be both weighty and powerful, and capable of travelling at a speed sufficient to give her the required momentum so that she may break the ice by the sheer force of the blow she delivers when she rams it, and she must be strong enough to inflict and not sustain damage by the collision. Further, besides cracking the ice into fragments weighing a few score tons apiece, she must be able to slide upon the ice and crush it by sheer weight. The _Ermack_ is 305 feet long, 71 feet beam, and 42 feet 6 inches deep. She had three screws aft and, when first built, had a fourth screw forward, the forefoot being considerably cut away to allow it to operate between the stem and keel. The idea was that the forward screw would agitate the water under the ice about to be struck and thus lessen the support the ice received from the water, and that it would also prevent an accumulation of ice under the ship’s bottom by creating a current of water towards the stern where the after propellers would throw the ice astern of the ship. This screw was found to be less useful than was expected, or rather it was discovered in practice that as good results could be obtained without it as with it in dealing with the massive Arctic ice, or any ice over a certain thickness, and when the ship was sent back to her builders a few years later to be lengthened, the forward propeller was taken out and not replaced. When the alterations were made the bow was severed in dry dock, and another bow having been built it was launched and floated into the dock and attached to the vessel. This bow is of a different shape from the other and has proved to be even more effective than the old one. Three screws aft are necessary in an ice-breaker of this size in order to give the power for the proper performance of her duties and also to enable her to be steered in very limited areas, greater steering facilities being obtainable by this means than by any other. The _Ermack_ is fitted with three sets of triple-expansion machinery, having cylinders 25 inches, 39 inches, and 64 inches diameter, with a 42-inch stroke of piston, working at a pressure of 160 lb. The boilers are six in number, 15 feet in diameter by 20 feet long, working under forced draught. The machinery develops about 10,000 horse-power. One of the _Ermack’s_ feats was to rescue the coast defence armour-clad _General Admiral Apraxine_, which had got frozen in after stranding in the Baltic.