On the official trial, which took place on the Thames on the 17th of October, 1873, the number of revolutions done in the hour was found to be 27,177, and the number required to do a mile in still water was 1578. The distance run in the hour was then, 27,177/1578 = 17·22, or very nearly 17-1/4 miles.
The steam pressure during the trial averaged 85 lbs. per square inch, and the vacuum 25-1/2 inches.
Swedish and Danish Boats.—Boats of the same size and similar in all particulars to the foregoing one—excepting the engines, which are improved by driving the air pump, feed pump, and circulating pumps off the main engines, and abolishing the auxiliary engine, which performed these duties in the case of the Norwegian boat—were made for the Swedish and Danish governments. The result was an increase of speed to 17·27 miles in the case of the Swedish boat, and to 18·06 miles, or 15-5/8 knots, in the case of the Danish boat.
There is no information regarding the armament of the Swedish boat, but the Danish boat was armed with two spindle-shaped torpedoes 12 feet long and 11-1/2 inches diameter, somewhat like the Whitehead torpedo. They were placed on deck longitudinally near the funnel, so as to facilitate launching, and were arranged to be towed from an upright pole 8 feet high, placed about 6 feet from the stem.
A small winch was fixed on either side aft, to pay out the towing line, and to bring back the torpedo. By these arrangements the torpedo could be projected at a large angle from the direction of the boat's motion, and at considerable velocity. The speed of the boat when towing one of these torpedoes is about 10 knots.
Austrian and French Boats.—The next size of torpedo vessel is that supplied to the Austrian and French governments, which is shown at [Fig. 151]. The dimensions are:—length, 67 feet; beam, 8 feet 6 inches; draught of water, 4 feet 3 inches. The guaranteed speed in the case of the Austrian boat was 15 knots in a run of one hour's duration, and in the case of the French boats 18 knots, in a run of two hours' duration. These boats were built of somewhat thicker plating than the 57 feet type, and the armour was extended.
They were divided into six water-tight compartments, and they differed from the Scandinavian boats in having the spaces forward and aft of the machinery permanently decked, instead of being covered with movable steel covers only.
The machinery was somewhat similar to that in the Scandinavian boats, excepting that the engines were capable of developing 200 indicated horse power, and that the air was supplied to the furnace by being forced into an air-tight stoke hole, instead of being forced directly under the fire grate.
The armament of these vessels consisted of two torpedoes attached to the end of wooden poles, 4-1/2 inches diameter and about 43 feet long, connected to the battery by insulated wires, and arranged to be fired either by coming in contact with the enemy's vessel or at any distance from it, at the will of the operator.
The torpedoes themselves were simply copper cases, of sufficient size, in the case of the Austrian boat, to contain 11,000 cubic centimetres of explosive, and in the case of the French boats, to contain 25 kilogrammes of dynamite.