The Submarine Torpedo Boat
Its Characteristics And Modern Development

162 THE SUBMARINE TORPEDO BOAT feet for every minute elapsed between the instant of firing the torpedo and the instant at which it crosses the path of the objective. The radius of the circle of visibility with the periscope exposed three feet above the surface of the water is 4000 yards. At this range it would take about five and a half minutes for the torpedo to reach the target, and therefore even were all the calculations correct, it would still be highly improbable that a successful hit be made, for in so long an interval of time it would be possible for a ship to both considerably alter her speed and her course. The speed of the target may be solved mechanically by measuring the angles of her image in the periscope, with respect to the course held by the submarine, at certain intervals of time and by plotting these points on a specially constructed scale. This method would be only approxi- mate however, as it depends for its accuracy upon the defi- nite range of the target. At long ranges this error might be considerable. At 1000 yard range the conditions are much more favorable for a successful hit. In this case the time elapsed between the discharge of the torpedo and the instant of its interception with the course of the objective is about one minute. An error of two knots in the esti- mation of the speed of a 600-foot ship would, providing all other calculations were perfect, still offer favorable chance of a hit. Other errors might creep in however, and therefore it is well not to disclose the position of the sub- marine by opening fire until within about eight hundred yards of the quarry. This might be considered easy range and in any event there would still be opportunity to send another torpedo from a practicable range should the first one miss.