GETTING THE RANGE FOR THE TORPEDO
In order to fire its torpedo with any certainty, the U-boat had to get within a thousand yards of its victim. A torpedo travels at from thirty to forty miles per hour. It takes time for it to reach its target and a target which is moving at, say, fifteen knots, will travel five hundred yards while a thirty-knot torpedo is making one hundred yards. And so before the U-boat commander could discharge his torpedo, he had to know how fast the ship was traveling and how far away it was from him. He could not come to the surface and make deliberate observations, but had to stay under cover, not daring even to keep his eye out of water, for fear that the long wake of foam trailing behind the periscope would give him away. All he could do, then, was to throw his periscope up for a momentary glimpse and make his calculations very quickly; then he could move to the position he figured that he should occupy and shoot up his periscope for another glimpse to check up his calculations. On the glass of this periscope, there were a number of graduations running vertically and horizontally. If he knew his victim and happened to know the height of its smoke-stacks or the length of the boat, he noted how many graduations they covered, and then by a set formula he could tell how far he was from the boat. At the same time he had to work out its rate of travel and note carefully the course it was holding before he could figure where his torpedo must be aimed.
There was always more or less uncertainty about such observations, because they had to be taken hastily, and the camoufleurs were not slow to take advantage of this weakness. They increased the enemy's confusion by painting high bow-waves which made the ship look as if it were traveling at high speed. They painted the bow to look like the stern, and the stern to look like the bow, and the stacks were painted so that they appeared to slant in the opposite direction, so that it would look as if the vessel were headed the other way. U-boats came to have a very wholesome respect for destroyers and would seldom attack a ship if one of these fast fighting-craft was about, and so destroyers were painted on the sides of ships as scarecrows to frighten off the enemy.