How low can you go? (an unclassified accounting of the term Test Depth)

Test Depth

Note: No classified material is contained in this unofficial story about submarines and the place in the ocean we call “Test Depth”.

Every time someone hears that I served on submarines, one of the first questions that is always asked is “How deep can you go?” My answer is always the same: Not all the way to the bottom. If I am pressed, I might add a depth that used to be the official depth that we were permitted to tell curious non-submariners.  To be fair, I only thought of depth a few terms. Periscope depth. Normal operation depth. and of course Test Depth.

The words Test Depth are two words that hold a special meaning for every American Submariner that has ever qualified to claim the title. Whether it’s the first gasoline powered craft or the latest deep diving boat, test depth is a place where the theoretical challenges the actual.

There are some laws of physics that apply. The deeper a submarine goes, the more pressure is exerted against the hull of the boat. I was surprised on my first submarine how elementary the depth recording actually was. I won’t go into any detail but there was a need from time to time to “vent” the gauges.

I was fascinated with the actual depth the George Washington operated at during my tours. We were not a deep diving boat compared to today’s modern submarines but you could hear the hull creak and crackle as we went lower and returned to the surface. I am sure there were some limitations based on the fact that the boat had been cut in half in new construction in order to insert the missile compartment.

What I do remember pretty vividly was that past a certain depth, there was a pipe in the overhead of the machinery room where I stood watch for two patrols that leaked. The first time I really noticed it was on the third patrol when I was on watch and a trickle of water went from barely dripping to a fairly steady stream. Salt water entering the people tank is always attention getting but having it increase as the boat went deeper was disconcerting to say the least.

I called maneuvering and asked for someone to come and take a look at it. Not only was the water increasing, but it was getting a lot closer to the top of the Oxygen Generator. The O2 Generator (or as we called it, “The Bomb”) was kind of finicky to start with but I was really aware that salt water and 24 volts DC are not compatible.

My second call must have gotten some attention. May have been the tone in my voice was different. Whatever it was, the Executive Officer came back to see what the fuss was all about. I had placed poly bags over the bomb by now and was using as much EB green tape as I could to divert the growing stream of water anyplace else but where it was spraying.

The XO was on the sound powered phone talking to the Captain. I don’t recall exactly what he said (it has been over 45 years) but there were a few salty words about the Captain getting his head out of his ass and getting the boat back above our limited test depth. When he finished, he slammed down the phone and helped me tape up another few poly bags. I could feel the boat taking on a decided up angle and the water finally slowed down to a trickle again. Clean up took the rest of the watch. I’m fairly certain we did not go to test depth again that patrol.

A lot of workers came on board as we were getting ready to turn the boat over. Kind of glad nothing more serious went wrong.

So a little more about test depth.

“Depth ratings are primary design parameters and measures of a submarine’s ability to operate underwater. The depths to which submarines can dive are limited by the strengths of their hulls. It is important to realize that there is a limit to how high the pressure can build inside the sub, as problems develop.

For example, oxygen becomes toxic at high pressures, thus the pressure cannot be allowed simply to equalize. As a first order approximation, each 10 meters (33 feet) of depth puts another atmosphere (1 bar, 14.7 psi, 100 kPa) of pressure on the hull, so at 300 meters (1,000 feet), the hull is supporting thirty atmospheres (30 bar, 441 psi, 3,000 kPa) of water pressure. (Note: The one atmosphere of air pressure at sea level is balanced by the roughly one atmosphere maintained inside the sub, so it does not appreciably stress the hull.)”

Test depth is defined as the maximum depth at which a submarine is permitted to operate under normal peacetime circumstances, and is tested during sea trials. The test depth is set at two-thirds of the design depth for United States Navy submarines, while the Royal Navy sets test depth at 4/7 the design depth, and the German Navy sets it at exactly one-half of design depth.

(Planesman error sometimes causes submarines to exceed test depth by a few feet or meters during trials; note that a one-degree up-bubble on an Ohio-class boat indicates that the stern is some ten feet or three meters deeper than the bow.)

Crush depth, officially called collapse depth, is the submerged depth at which a submarine’s hull is expected to collapse due to pressure.

This is normally calculated. However, it is not always accurate. Submarines from many nations in World War II reported being forced through crush depth, due to flooding or mechanical failure, only to have the water pumped out, or the failure repaired, and succeed in surfacing again. These reports are not necessarily verifiable, and popular misunderstanding of the difference between test depth and collapse depth can confuse the discussion. World War II German U-boats generally had collapse depths in the range of 200 to 280 meters (660 to 920 feet)

Submarine design has always been heavily influenced by the depths at which we were able to operate.

This section comes from a book about Submarines published in 1958, “The Complete Book of Submarines”, Charles W. Rush.

“At the turn of the twentieth century, the struggle for acceptance of the submarine as a recognized war vessel neared its climax. In the United States, the pressure of Admiral Dewey’s testimony strained against the dams of national inertia and naval conservatism. The Navy Department announced specifications for a submarine, and invited bids.

There were two dominating figures in the submarine design picture at that time. One, John Holland, had been experimenting with submersibles for almost a quarter of a century. The other, Simon Lake, had tried his hand in the game for the first time in 1895. Both had constructed craft which would submerge and — very important — would consistently regain the surface.

The Holland and Lake Theories of submarine construction differed. Holland conceived of the submarine as a vessel which would submerge by diving beneath the surface with its engine power, in a condition of neutral buoyancy. Lake, however, thought in terms of a craft which would descend on an even keel with a slight negative buoyancy. Lake also saw the submarine as a vehicle which would roam about on the bottom of the sea, provided, of course, the water was not so deep that its pressure would crush the hull. As a result, Lake’s first submarine was fitted with wheels. Holland did not conceive of the submarine as a wheeled vehicle at all.

Holland’s organization, the Holland Torpedo Boat Company, won the contract for the first Navy submarine and the work was undertaken despite the fact that Holland himself believed the specifications would not produce a satisfactory boat. He was correct.

The submarine was to have three propellers and the contract required that it be propelled on the surface by a steam engine, and beneath the surface by electric motors. When the ship was put through its first trials, Holland’s doubts were promptly vindicated. The time required for extinguishing the fire and venting the steam in the boilers made the period of preparation for diving unacceptably long. In addition, the heat remaining in the firebox after the fire was put out made the interior of the boat unbearably hot when it submerged.”

It’s interesting to note that the Navy was set on the value of steam power even in the early 1900’s. On the other hand, I have to wonder if the Naval Board was merely placing another obstacle in front of the early submarine pioneers as a way to dissuade them from their goal of building an operational submarine. Little did they realize that eventually, the power of steam in a new technology would make all of the most powerful surface ships subject to a weapon of unimaginable capabilities.

“In the early days of submarine operations, the known phenomenon of water pressure increase with depth was not a serious factor in submarine design. The chief advantage of submergence was concealment, and as long as the only detecting device directed against the submarine was the human eye scanning from a point of observation no higher than a ship’s mast, the requirement of concealment was satisfied by anything which could submerge to a depth of 20 or 30 feet. At 30 feet, the water pressure on a submarine hull is less than 2,000 pounds per square foot.

But the simple picture became complicated with the advent of World War I. The balloon, blimp, and airplane were introduced into the antisubmarine patrols and observers thus elevated could spot the dark outline of a submarine at considerable depths. In addition, detecting gear on surface ships, based upon the sounds emanating from a submarine operating beneath the surface, replaced the human eye and was far more accurate and sensitive. As the means of spotting the submarine improved, weapons capable of subsurface destruction came into being. And they soon developed into instruments of deadly application.

The submarine was faced with the necessity of going deep if it was to survive.

The answer was to strengthen the pressure hull. Submarine designers are always striving to combine material and design to attain ever greater depth capability. In World War II, for example, a German U-boat was reported to have descended to over 700 feet; at that depth the force exerted against the pressure hull is 44,800 pounds per square foot.”

You may notice that I haven’t said anything about what depths my boats operated at. That is of course by design. Submariners have always been very guarded about certain technical details. Even though there seems to be a lot of information published on the internet that baffles my mind, I still hold to the old rule about not divulging anything that might aid a future opponent.

From the submarine book:          

“Depth capability information is so important that it must be classified. No government will release current data on that aspect of its submarines’ performance, for if an enemy can be sure of the maximum depth at which his opponent’s undersea craft can operate, he has gone far toward the goal of defeating them.

But even classified information sometimes “leaks.” During World War II, a blabbermouth gave out the information that our submarines were surviving Japanese depth-charge attacks in the Pacific because they were sub merging to depths greater than those for which the Nipponese were setting their charges. With the information thus so kindly provided by a man who did not himself venture into the war zone, the Japanese quickly corrected their attack procedure. The grim notation “missing and presumed dead” was entered with shockingly increased frequency after the names of American submariners.

(Mister Mac Note: June 23, 1943, Rep. Andrew May (D-KY) bragged to reporters that the Japanese were setting their depth charges too shallow because they didn’t know how deep U.S. subs could dive. VADM Lockwood said the revelation caused the Japanese to adjust, costing the Navy 10 subs and 800 sailors) U.S. Congressman Andrew J. May served as the Chairman of the Committee on Military Affairs during WWII. In his capacity as Committee Chairman, Congressman May toured the Pacific Theater in the spring of 1943, and received numerous intelligence and operational briefings: …..and the subject of Japanese ASW failures came up in one of them. Upon returning home in June, he did what politicians love to do: ….He held a Press Conference. During this OPEN Press Conference he made this statement; “Don’t worry about our submariners; the Japanese are setting their depth charges too shallow.”

Inside the all-important pressure hull are the men, the machines, and the weapons which make the submarine a mobile fighting ship.”

When a boat goes to test depth, more than the hull is tested. Frankly, it is a way to test the crew as well. Watching the depth gage indicate a depth that you only read about in school is pretty impressive. Seeing all of the other physical signs is also a reminder that there is very little between you and pressure of an enormous magnitude. Magnitude that could crush you in an instant if you were not enclosed in this cocoon of steel.

I have known a variety of types of pressure in my time here on the planet. I have been tested and to be honest am being tested now. But I am glad to have had the unforgettable experience of riding more than one boat to its limits and having the ability to say we survived the encounter. While many people are tested under pressure, only a submariner knows what that unique experience actually feels like.

Mister Mac

3 thoughts on “How low can you go? (an unclassified accounting of the term Test Depth)

  1. Nice article. I’m a fellow A-Ganger and I am proud of my time. But I am ashamed that a fellow Kentuckian was so stupid.

  2. Great Article Mr. Mac.
    We rode down into Davy’s Locker a time or two … I was an RMSA(NQ) driving n diving watch when we scrammed by mistake.
    That depth indicator was ticking of 10’ increments like a stopwatch…
    Another Beautiful Day in Submarines!

  3. EPM operator when we slid backwards thru TD during a drill…. still remember CO on 1MC “Secure the Drill, Recover the ship…NOW!”

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