Since a submarine can attack while hidden underwater, they need a unique solution to tracking their enemy and that is where the submarine periscope comes in. The early designs of the submarine periscope that were used in World War 2 were very simple by essentially having two telescopes pointing towards each other.
Perhaps the greatest difference between a submarine and a surface warship is the preferred method of attack. During the World War II period, surface ships were designed to shoot it out with heavy caliber guns. Destroyers also carried torpedoes, which were normally launched at a 90° angle to the destroyer’s course. Aircraft carriers used planes and bombs in essentially the same manner that battleships and cruisers used their guns, though obviously with considerably greater range and power.
Submarines generally attacked while submerged. Submarines are normally more vulnerable to damage than surface warships. Most of the time, if there is any armor on a submarine, it is installed around the conning tower and bridge, and designed for protection from light caliber weapons, such as machine guns, or from shell splinters. Surface warships generally carry more extensive armor protection. For the submarine, the primary protection comes not from heavy steel plates, but from being able to operate beneath the surface, where the enemy can’t find it to shoot at.
A military submarine’s main weapon is the torpedo. In World War II, the most common type was a 21-inch (533 mm) torpedo, powered by either a small “steam” turbine or by electric motors and batteries. The electric torpedoes were often called “wakeless,” as they did not leave the visible trail of exhaust bubbles that were characteristic of the steam types. The Japanese Imperial Navy fielded a 24-inch (610 mm) “oxygen” torpedo. This “Long Lance” torpedo, which carried a 1,000-pound warhead—almost twice the explosive power of those in other navies—has generally been acknowledged as the most effective torpedo ever built from the standpoint of utility as a ship killer.
In order for a submerged navy submarine to sink an enemy ship, some means of aiming the torpedoes was required. Different navies evolved different methodologies. During the inter-war period, U.S. doctrine favored the use of Sonar for determining target range, bearing, and angle. It was believed that advances in detection and anti-submarine warfare weapons had made it suicidal to expose a periscope in daylight. To this end, the U.S. Navy installed highly advanced Sonar and hydrophone suites in their fleet submarines, along with Torpedo Data Computers that remained significantly more advanced than anything used in any other navy until well after the war had ended.
In fact, Sonar attacks while submerged turned out to be remarkably ineffective under actual wartime conditions. Falling into one of the more common military fallacies, the U.S. Navy developed a theory, then saw test results through the lens of that theory. Any results that seemed to back up the theory were eagerly embraced, while results that failed to back up the theory were put down to “operator error.” This tendency continued well into the war, to the degree that a number of commanders were relieved for “lack of aggression” when the actual problem was that the torpedoes they were firing didn’t work. (The Bureau of Ordinance said that the torpedoes did work, and since they couldn’t possibly be wrong about that, it had to be the commanders.)
While American senior commanders continued to overlook the torpedo problems for some time, the submerged Sonar attack was eliminated very quickly once the war began. The skippers recognized two facts. First, they weren’t hitting anything using Sonar alone. And, second, as long as you were careful it was a lot harder to see the head of the attack periscope than the theorists believed. The upper section of optronic mast was made as slender as possible to reduce observability. The limiting factors were the size of the upper lense, which had to be large enough to ensure decent daylight operation, and the upper prism and its alignment mechanism. The upper prism could be tilted from the conning tower, to allow the field of view to be elevated for air search, or depressed to look close in.
If the periscope designer—and the navy that employed him—was willing to forgo the air search ability and build a periscope with a fixed head, the diameter could be quite small. In some cases not much more than 1/2-inch. The standard attack periscope used in American naval submarines was 1-1/4 inches in diameter at the upper end. Search or “night” periscopes have a larger head for increased light gathering capabilities. Late war American search periscopes incorporated a radar waveguide in the head.
One common term is periscope depth which is defined as the depth needed to extend the scope above the surface of the water.
The image to the left shows a simplified view of the eyepiece and controls of a Kollmorgen attack periscope, similar to the type used in most American fleet attack submarines during World War II. The main shaft of the periscope rests on ball bearings in the heavy lifting collar at the top. The two hoist rods attached to the collar enter hydraulic cylinders located in the periscope shears above the conning tower. To raise the periscope, hydraulic pressure is applied to the bottom of the pistons inside the cylinders. To lower the periscope the hydraulic fluid is allowed to flow out of the cylinders and back into the reservoir. Gravity lowers the periscope once the hydraulic pressure is released.
The knob on the upper right side of the periscope is used to adjust the focus. The black plate, with the eyepiece in its upper half, is the rayfilter assembly. This contains a disc with three colored — and one clear — filters, which can be rotated in front of the eyepiece to aid visibility under different lighting conditions. The filters are red, green, and yellow in color. When the periscope is in use, a double rubber eyepiece is fitted. One side of the eyepiece is blocked off, and only one eye is actually used. The eyepiece is reversible, to allow the captain to use his dominant eye. (In addition to being right or left handed, people are also right or left eyed, though the majority probably have no idea which.)
The outer part of the left handle rotates, allowing the upper prism to elevate or depress. The button on the inner part of the handle is a detent. This allows the rotating handle to click into preset positions so that by starting with the prism at full elevation and moving to the next detent at the completion of a full sweep of the horizon the captain may lower the prism to the next position by feel. Three complete sweeps cover all positions.
The right handle adjusts the optical power of the periscope, which is also, optically, a telescope. The low setting is 1.5 power, and the high setting is 6 power.
The knob below the right handle is the stadimeter control. The stadimeter is a rangefinding device, which allows the captain to plot his attack with much greater accuracy than simply trying to guess how far away the target is located. The distance to the target, in yards, is read from the large black dial at the bottom of the periscope. This dial is duplicated on the back of the periscope, so that the reading may also be taken by the approach officer without requiring the captain to remove his eye from the eyepiece. Also at the rear of the periscope, in the lifting collar, is the bearing ring. When the periscope is aimed directly at the bow of the sub the bearing is zero, as target bearings are given relative to the heading of the submarine.
Target bearing should not be confused — as has happened in any number of movies and television shows — with “angle on the bow.” Target bearing is the relative bearing from the submarine to the target. The angle on the bow is the angle at which the target is crossing, approaching, or moving away from the sub. If the target is heading directly at the submarine, the angle on the bow is zero. If it is heading directly away from the angle on the bow is one-eight-zero. (Bearings and speed are always given as single digits for clarity.) If the target is crossing at right angles from right to left angle on the bow is port 90°. Essentially, the angle on the bow is the bearing to the submarine from the target.
This image shows the view through the periscope with the stadimeter in use. A split prism is used to superimpose a second image of the target over the actual image. The captain adjusts the prism so that the waterline of the second image is set on the masthead of the actual target image. The height of the masthead from the water is entered on the dial, and the reading obtained. The stadimeter actually measures angles, not distance. If the masthead height is entered accurately, the range will be correct. Getting the masthead height wrong gives an incorrect range. (The same principle is used by surveyors, though they have the obvious advantage of basing their ranges on a graduated pole of known length held by an assistant.) In practice, the most accurate ranges were always obtained during exercises, since the subs were operating against units of their own fleet, and masthead heights were always known. Enemy warships and freighters often involved a certain amount of guessing, though recognition books were careful to list masthead heights whenever they were known.
Once a submarine finds a target, the approach and attack is essentially an exercise in geometry. The captain needs to determine the precise angle at which to fire his torpedo so that it will hit the target.
In stationary objects, this is easy. You simply point the torpedo directly at the target and, so long as it travels in a straight line, it will hit it. The problem with this, obviously, is that neither the submarine nor the target is likely to actually be stationary. With the rare exception of attacks on anchored vessels—Prien’s attack on HMS Royal Oak in Scapa Flow being, perhaps, the best-known example—submarines normally encounter their targets at sea, where both the submarine and the target are almost certainly going to be moving.
In this situation, you can’t shoot at where the target is. Instead, you have to shoot at where the target will be when the torpedo reaches it.
In this graphic, the approach has begun. The submarine is moving due north at 2 knots. The target is moving due west at 6 knots and is currently located to the east of the submarine’s track, at a range of four nautical miles. (In order to fit everything into the graphic the distances and sizes of the vessels are not to scale. Also, the submarine is shown surfaced for clarity—it would be submerged if this was actually happening.
First, the captain centers the periscope’s crosshairs on the middle of the target, or on the point on its hull where he wants the torpedo to hit, calling out, “Bearing.” At the moment he has the target exactly centered he then calls out, “Mark!”
The Approach Officer reads the bearing off the bearing ring located on the periscope shaft. This bearing gives the relative angle from the submarine to the target. In this case, 45°. For clarity, the Approach Officer announces the bearing as, “Bearing—zero-four-five.” (Target bearings are always given as three numbers, and the digits are always given separately. “Zero-four-five” is less likely to be misunderstood than “forty-five degrees.” This is particularly so since lookouts call out bearings as “starboard four-five,” using two digits and always referring to the side of the ship the sighting is on. Some navies use “red” for port and “green” for starboard in making sighting reports, these being the colors of the navigation lights on those sides.)
Once the target bearing has been determined, it is entered into the Torpedo Data Computer (TDC). This is a highly sophisticated electro-mechanical analog computer. Two basic types were used during World War II. In most navies, the TDC was an angle solver only, which would give the correct gyro setting for the torpedo based on the data entered at the time of the reading, or at a given time in the future, based on the captain’s best guess of where the target would be. The American version added a position keeper, which was capable of keeping track of the target’s course in real time. This was a significant advance on the older systems and made for much more accurate target solutions by eliminating most of the guesswork.
The TDC will always know the submarine’s course and speed, as these are constantly updated from the master gyro compass and Pitometer log. (This log is the submarine’s speedometer, by the way, and not the book the captain uses to keep track of daily events.) The TDC now also has the target bearing, but still doesn’t have enough information to work out a target solution.
Range to Target
Now the captain needs to determine the range to the target. To do this, he first needs to know just what the target is. Looking through the periscope he can see that it is a large freighter. Submarines carry recognition books which list every enemy warship and merchantman on which information is available. Looking through this book the captain finds the Oyama Maru, a 4,750-ton Japanese freighter, which seems to be the ship he has in his periscope. Since it is mid-1944, and World War II is raging, he decides this is a legitimate target, so he continues with the approach.
Now that he knows—or, at least, believes he knows—the identity of the target, he looks up the masthead height. This is the distance from the waterline to the highest point on the ship. According to the recognition book, this is 100 feet. This figure is entered into the stadimeter in the periscope.
The range may also be determined by using the active sonar on the single-ping setting. This is one of the two most accurate methods, as it doesn’t depend on knowing the target’s masthead height. Late war American submarines also incorporated a tiny radar antenna in the search periscope, which would also give an exact range, at the risk of throwing up more spray than the thinner attack periscope.
This graphic shows what the captain sees through the periscope’s stadimeter. A split prism is used to place a ghost image of the target so that its waterline is sitting right at the top of the masthead of the “true” image. The stadimeter actually registers the angle above the horizontal to the masthead, not distance. Some basic math is then performed which translates that angle into a distance figure.
The way this works is that viewed from any particular distance, an object of a given height will be at a particular angle. If you know that the angle of view is 1°, for instance, and the object is 100 feet high, you can calculate that the angle of view and the top of the object will touch only at a distance of one nautical mile. The stadimeter simply does the math for you.
One disadvantage of this, of course, is that accuracy is completely dependent upon knowing the correct height of the object. (In this case, the masthead height of the target.) In our example—but not in the graphic, where the ship is considerably closer than it would be in a true view—the masthead height turns out to be 1/4° above the horizontal. Using the formula R=h/tan(q) this means that the target is four nautical miles from the submarine. The stadimeter does this internally, without the need for the captain or approach officer to calculate it, and indicates that the target is 8,100 yards away.
This figure is read off a dial at the base of the periscope and entered into the TDC, providing another part of the solution.
Angle on the bow
In order to work out a shooting solution, the captain also needs to know the angle on the bow for the target. This is not the same thing as the target bearing, despite what you might think of from some movies and novels. The target bearing tells you where the target is in relationship to the submarine. The angle on the bow tells you where you are as seen—which you obviously hope you’re not—from the target.
In our example, where the target is passing from east to west directly across the submarine’s bow, the angle on the bow is port 90°. That is, the port (left) side of the target is toward the submarine, and it is at a 90° angle to its course. If the target was coming directly at the submarine, the angle on the bow would be zero. If it was going directly away, the angle on the bow would be 180°. If the target was on a southwest course, the angle on the bow would be port 45°, etc.
The final factor needed is the target’s speed. There are several methods of obtaining this, though none can really be called 100% accurate.
First, periscope observation. The periscope optics are marked in degrees in both the vertical and horizontal axes. If the distance to the target is known, it is possible to determine speed by timing the elapsed time required for it to travel a given number of degrees. The problem with this method is, of course, that it is dependent upon an accurate range, since you are measuring the length of time the target takes to traverse a known number of degrees, and it also means exposing the periscope while you do it, which is potentially dangerous if the enemy spots it. (Japanese merchantmen not only carried deck guns, frequently with gun crews who actually knew what they were doing, but also depth charges, and didn’t hesitate to use them.)
Second, general knowledge. Some types of vessels are known to routinely travel at certain speeds. This will usually be the most economical cruising speed. However, since the target’s captain may be in a hurry, or may be moving slower than usual, this will tend to be the least accurate method of determining speed. An experienced captain can often make a fairly accurate guess at a target’s speed by the appearance of the bow wave. (One of the things warship camouflage patterns are intended to do is make it difficult for the enemy to be able to clearly see the bow wave.)
Third, counting revolutions. The sonar operator can listen to the sound of the target’s propellers and determine the number of revolutions per minute. If the submarine’s captain knows the pitch (the distance traveled in one revolution) of the target’s propellers, he can make a fairly accurate estimate of speed. For instance, a screw with a 24-foot pitch should move the ship forward 24 feet for each revolution. One hundred revolutions per minute should, therefore, move the ship forward 2,400 feet, or 800 yards. This would give a rough distance traveled of 1 nautical mile (2,025 yards) every 2-1/2 minutes or a speed of about 23.7 knots. This sort of speed would generally indicate a large warship or liner. Freighters were usually slower, with the fastest generally limited to about 16 knots. (Fuel economy was the major factor—fast ships use a lot of fuel, so high speed was mostly found in passenger ships, where the line could charge extra for a fast passage, specialized freighters like banana boats, which had to deliver their cargo before it spoiled, and warships, where cost wasn’t a major consideration. In any case, if screw pitch is known, and an accurate count obtained, this can give a fairly good speed estimate. Also, a ship’s screws are far from being 100% efficient—the only vessels that manage this are submarines at a considerable depth, where the great sea pressure suppresses wake and cavitation.)
Once the target’s speed has been determined, this is also entered into the TDC. At this point, everyone waits for a few minutes and then makes another observation. If the data was all correct, the target will be where the TDC’s position keeper is predicting. If it isn’t, more observations are taken and corrections are dialed into the TDC. Several observations over a period of 10 to 15 minutes should eliminate the error—or at least reduce it to the point where a hit is more likely than a miss.
As the TDC works out these solutions, the gyro angles it generates are automatically programmed into the torpedoes. The gyroscopic guidance allows the torpedo to be set to travel a particular course, rather than having to point the submarine at where the captain wants the torpedo to go, which had been necessary with early torpedo designs, which could only travel in a straight line.
While all this is going on, the torpedo run depth will also be set. For this particular target, which has a loaded draft of 38 feet, the torpedo is set to run at a depth of 25 feet, so that it will explode well below the waterline. If a magnetic influence fuse is used—which, this late in World War II, it would not have been, since by then even the Bureau of Ordinance had finally recognized that they were unreliable under field conditions—the run depth would be set to 43 feet, so that the warhead would explode directly under the keel, where it would do the most damage.
This graphic shows the submarine and target at the time a solution has been worked out and the captain is ready to fire a torpedo. (In actual practice, he would probably fire at least two.) The target is now dead ahead of the submarine, at a distance of 1,400 yards. At a speed of 46 knots, it will take the Mark 14 torpedo one minute to travel that distance. In that minute, the ship will have moved ahead one-tenth of a nautical mile, or about 200 yards.
In other words, if the torpedo is fired straight ahead, the target won’t be there anymore when the torpedo arrives and the torpedo will pass astern. The TDC’s calculations take this into account and set the torpedo to travel on a relative course of 350°. This means that the torpedo is traveling at an angle so that it’s relative bearing to the target remains constant. Any two objects that maintain a constant bearing in a crossing situation will eventually hit each other. By having the torpedo “lead” the target, it should hit close to the center of the target and have a good chance of sinking it.
Once the torpedoes have been fired, the submarine may stay around to observe the result. Or, if there are escort vessels with the target, it may be more prudent to attempt to slip away and listen for any hits on the hydrophones. The latter has the disadvantage of making it difficult to confirm the sinking but also has the advantage of making it more likely for the submarine to survive to take the credit.