Diving and Surfacing

Submarine sailors didn’t talk about “sinking” their own submarine. Sinking usually meant putting a vessel on the bottom of the ocean and that was reserved for enemy ships. Sub sailors spoke of “diving” or “submerging” their own boat.

The simplest description is that they filled the ballast tanks with water to make the boat heavier. That would get the submarine near to neutral buoyancy. In addition, they used the bow and stern planes to put a down angle on the submarine so that their forward motion took them deeper into the water. When they reached the desired depth, they took the down angle off the boat.

However, there was much more to diving the submarine than that. Many different steps had to be completed, mostly at the same time, to dive the boat quickly:

1. The officer of the deck on the bridge sounded the diving alarm twice and announced “Dive, dive” over the 1MC announcing system.
2. The chief of the watch opened the ballast tank vents. This allowed the ballast tanks, which were open on the bottom, to flood.
3. The engines were shut down
4. The engine exhaust valves were closed.
5. The air intake flapper valves in the engine rooms were closed.
6. One of the engineers or machinist mates moved to the control room to man the trim manifold.
7. Another of the engineers or machinist mates moved to the control room to man the air manifold.
8. The electricians took the generators offline.
9. The electricians switched the power source to the battery.
10. The lookouts came down to the bridge from their perches on the periscope shears and proceeded down the hatch through the conning tower to the control room.
11. The lookouts rigged out the bow planes and manned the bow and stern planes.
12. The bow and stern planes were set to full dive position as long as the propellors are not raised out of the water by the down angle on the boat.
13. The officer of the deck verified that no one was left topside.
14. The OOD then dropped down into the conning tower and pulled the bridge hatch shut behind him.
15. The quartermaster dogged (secured) the hatch shut.
16. The OOD reported to the new officer of the deck, submerged, that the bridge was clear. He then proceeded to the control room and became the diving officer.
17. When the engines were shut down, the chief of the watch shut the main induction valve hydraulically. This was the 36-inch valve aft of the conning tower that fed air primarily to the engine rooms.
18. The cook or mess cook locked the main induction valve shut once it was closed.
19. The chief of the watch monitored the “Christmas tree” looking for all the red lights (open valves or hatches) to go green (closed).
20. The chief of the watch reported the status of the Christmas tree to the diving officer. Normally, it was already a green board by the time the diving office arrived in the control room, indicating that all critical valves and hatches were closed.
21. When the boat was at or passing through periscope depth, the chief of the watch would “blow negative to the mark.” The negative tank, which was full on the surface, was forward of amidships and provides momentum down, particularly forward.
22. Once the boat was submerged and leveled off at the desired depth, the diving officer worked to get the boat to neutral buoyancy or neutral trim quickly, so the speed of the boat could be decreased if desired. Otherwise, higher speed might have been needed so the planes had more effect in keeping the boat at the desired depth. At neutral buoyancy, the planes weren’t needed to maintain depth and could be at 0 degrees, with no rise or dive.

All of this happened at once or in rapid succession. The goal in WW2, and after, was to submerge the boat to periscope depth within 30 seconds and to be able to be at 150 feet within one minute. Most of the above steps had to be completed in 15 seconds or less to allow the boat to be at periscope depth in 30 seconds.

Modern submarines take much longer to submerge. A YouTube video shows a missile submarine taking about six minutes to dive. However, modern boats normally only dive once after leaving homeport and they do so in safe waters. As a result, there is no urgency like there was during WW2 war patrols.

When submarines were in port, the configuration of many of the valves was different. For example, some ballast tank valves were secured closed. That prevented accidental opening and submerging the boat at the pier. This was for the safety of the boat and the crew.

When going back to sea, that configuration needed to be reversed. The routine was that a sailor would use a checklist for a given space and set everything to the configuration that allowed to boat to submerge safely. Then one of the officers would use that same checklist to confirm that everything was set properly. At that point, the officer would notify the control room that the space was “rigged for dive.” Every space on the diving officer’s checklist had to be confirmed and marked before the boat would submerge.

Neutral buoyancy is the state where the buoyant forces pushing the boat up equals the forces of gravity pulling it down. In this condition, the boat hovered at the current depth, neither rising nor settling in the water. This was the preferred state when submerged since it was easier to reach and maintain the desired depth safely.

The same way that musicians get to Carnegie Hall – practice, practice, practice. Experienced crewmen had probably submerged submarines hundreds of times. They trained the new submariners in how to do their tasks so all of these steps were done quickly and efficiently.

“It was only when they did this thing as a sort of second nature and in teamwork with everybody else that they can be called a first-class crew. Such a routine as that was not learned overnight nor out of books.” (“Battle Below” by Robert J. Casey, Bobbs-Merrill Company; New York; 1945.)

Most of the water went into the main ballast tanks. These saddle tanks were outside of the pressure hull. Normal diving filled the main ballast tanks completely. The tanks were open at the bottom so that, when the vents were opened, the water flowed in quickly. Main ballast tanks were not subject to sea pressure since they are open on the bottom. Sea pressure equalized as the depth changed.

Some dives did not use all of the ballast tanks. For example, the captain might only want to go to “radar depth”. In other words, he wanted to be able to use the radar while showing as little of the boat as possible. As a result, he didn’t want the boat fully submerged.The radar had to be out of the water so it was still operational. The captain might also just want the decks awash for some operations. In these cases, some ballast tanks would remain dry or only partially filled.

Yes, there were other specialty tanks including the bow buoyancy tank, the safety tank and the negative tank.

The bow buoyancy tank was also outside the pressure hull and was flooded along with the main ballast tanks. It provided added lift when surfacing and added downward momentum when diving since it was located just behind the bow.

The safety tank was inside the pressure hull and held about as much water as the conning tower. It was normally full of water. Should the conning tower be seriously damaged and flooded, the safety tank could be blown dry to get the boat close to neutral trim again. It was also be used to offset flooding elsewhere in the boat. The safety tank was rarely blown dry.

The negative tank was also inside the pressure hull. It was used routinely to submerge more quickly. This tank was located forward of amidships and was full when the boat was on the surface. It helped the crew submerge the boat quickly by providing the additional weight forward, pulling the bow down. It was normally blown dry as they reached periscope depth. It could also be flooded to get deeper quickly, typically from periscope depth, in case of an emergency. It would be blown dry again once the boat was at a safe depth. With this tank, the crew usually did not have to race toward the bow when submerging as the German sailors often did, as seen in the movies.

These were small wing-like structures at the front and back of the boat that move up and down. They were used to change and maintain depth while moving through the water. The bow planes were rigged up to the side of the boat when on the surface. (The starboard bow plane on the USS Pampanito is visible from the pier.) The stern planes were always rigged out and were protected by guards. They were under water and not normally visible.

The bow planes were normally used for small changes in depth. Perhaps the captain or officer of the deck wanted to see something farther away. That meant that the periscope needed to be higher. They couldn’t just raise the periscope or the officer wouldn’t have been able to reach the optics. Instead, they decreased the depth, raising the whole submarine.

Stern planes put an angle on the boat, up or down. Stern planes were used for larger changes in depth, for example from periscope depth (63 feet or so) to 300 feet. That way, the forward motion of the boat generated the change more quickly. It was similar to an aircraft taking off or landing.

They are called limber holes. They allowed the empty space below the deck and above the pressure hull to flood and drain quickly. They allowed the crew to dive the boat in 30 seconds or less and to surface quickly. Without the limber holes, the air and water would push against each other trying to get through the openings in the deck and neither would move efficiently.

Instead, when they submerged the boat, the water flowed into the empty space through the limber holes and the air escaped through the various openings on the deck. When the boat surfaced, the opposite occurred with the water flowing out of the limber holes and the air filling the space through the deck.

When submerged, the limber holes did add drag to the boat. A smooth superstructure would have been more efficient. However, the ability to dive quickly was more important.

Modern submarines do not need limber holes since they dive only once and remain submerged most of the time they are at sea. They also do not have all the open space between a deck and the pressure hull that would need to fill with water.

It was a matter of safety. During WW2, lookouts could spot most Japanese planes at a distance of four or five miles. That meant that, if a Japanese pilot spotted the submarine, at WW2 speeds he could be attacking the boat in about one minute. In that one minute, boat wanted to be at a depth of 150 feet or more. At that depth, it would probably be below the kill radius of the Japanese depth bombs. The boat could still be damaged and would be bounced around, but it would likely survive. In order to get that deep, the submarine would need to be completely submerged in 30 seconds and headed deeper. As indicated above, many things had to be done quickly.

Prior to the war, the diving process took much longer and was significantly safer. In those days, when the order was given to submerge the ship, the bridge was cleared and the hatch to the conning tower closed and dogged. Engines were shut down and air intakes and exhausts were closed. When the Christmas tree was all green, air was added to the boat to verify that everything was closed. If the added pressure held, only then was the alarm sounded and the ballast tank vents opened. This obviously took much longer than the minute that they had in WW2.

Our submarines began to consistently use “crash dives” to submerge during WW2. The change was that the OOD would announce the dive and sound the diving alarm right away. The chief of the watch would open vents immediately and the boat would start to submerge while all the other steps were still being completed. All dives were done in this manner, even if there were no aircraft in the area. It was good practice.

The prewar classes of these submarines were tested to a depth of 250 feet. This included the Porpoise, Perch, Salmon, Sargo and Tambor classes. The boats the U. S. built just before and during the war did go deeper. The Gato class was tested to 300 feet. The Balao and Tench classes were tested to about 400 feet.

Test depth was not the same as design depth. Design depth would be about 50% deeper. In other words, the design depth of the Gato boats would be 450 feet. For the Balao and Tench boats, it would be 600 feet. Fortunately, the design depth was a conservative estimate. Many of these boats would go deeper than design depth, often unwillingly. Fortunately, the boats were built well and were often able to survive the greater depth. Of course, we only know about the boats that made it back to the surface.

The depth of the submarine was measured from the keel or bottom. Periscope depth was around 63 feet. That meant that the bottom of the submarine was 63 feet below the surface but the periscope could still be extended to be able to see above the surface.

Not just from the compression of the hull. The pressure hull kept the atmosphere inside the boat close to what it was at sea level. It was similar to, but the opposite of what you feel in an airplane. The atmosphere in an airplane is less than sea level but you don’t generally notice. The important thing was that sailors did not need to decompress when the boat came up from the depths.

There were times when air was added to the boat and that could be noticeable. For example, if many torpedoes were fired, the air and water needed added pressure to the air in boat. However, it still wasn’t enough to cause the sailors to need to decompress.

It probably did make a little noise when it compressed due to the increased sea pressure. However, it soon became a normal noise. The crew didn’t need to pay much attention to normal noises. It was the unusual noises that raised concerns.

The record during the war was just short of 38 hours by the USS Puffer (SS-268). She was a Gato class boat built by Manitowoc Shipbuilding. She was being tracked and depth charged by Japanese ASW vessels. At 38 hours, the battery was nearly discharged and oxygen was low. The captain decided his choices were to either suffocate submerged or take a chance on surfacing and fighting it out with the Japanese ships. He chose to surface. The boat was fortunate in that the currents had carried it a few miles away from the hunters.  The captain then presented the stern to the hunters, so it showed the smallest profile. He was able to use an island as a backdrop to make the boat even harder to see. The boat was then able to make repairs and escape back to base.

That was the longest submergence during the war and very unusual. Normally, these boats would be on the surface each night. They wanted to recharge the battery, get fresh air in the boat, get a star fix and pick out their messages from the fleet broadcast. The air in the boat was the same air the crew had been breathing since they submerged. It was expected to last 16 to 18 hours. (Since most of the crew smoked cigarettes, we will call that 16 hours.) 

There were some things that could be done to extend the air. There were five or six oxygen bottles on board. There were also multiple cans of CO2 absorbent in the overheads. (Those were the cans labeled “DO NOT PAINT”.) The oxygen and CO2 absorbent would extend the air a few hours. After that, they would bleed some high-pressure air into the boat. It wasn’t fresh air, but it improved the percentages and sustained life.

But air wasn’t the only issue. The other was battery power. How long the battery could last depended on how fast the boat was going. The battery was rated at 48 hours if the boat was going two knots. However, if they doubled the speed to four knots, they had to divide the time the battery would last by eight. It was a cubic relationship. At four knots the battery would last about six hours. Double it again to eight knots, nearly the maximum speed the boat could go submerged, and the battery would last less than one hour. It was called the “one-hour rate”.

Clearly, it was important to use the battery and air conservatively when the boat was being attacked. Slow and silent was the best way to conserve the battery and to escape. To use air more efficiently, put all non-essential crewmen in their bunks. They used less oxygen when resting.

No. Depth changes were done with the bow and stern planes. However, it might have been necessary to add water to the trim tanks or pump it out as a result of the depth change. Differences in temperature or salinity may have made the boats seem heavier or lighter. Trim might have needed to be adjusted to get back to neutral buoyancy.

While there was no gauge telling the diving officer whether the boat was out of trim, there was a clear indication available. The diving officer just watched where the bow and stern planes needed to be, up or down, to maintain depth. Then, through experience, he knew whether the boat was light or heavy, and whether the boat was in trim fore and aft. He would then move water in or out, or fore and aft to get the boat into neutral trim.

The best test for neutral buoyancy was to let the boat drift to a stop. At that point, the bow and stern planes had no effect. If the boat hovered at that depth, and didn’t rise or settle, then the boat was at neutral buoyancy. It was trimmed properly. This was often the first thing that a young officer on a submarine accomplished that was part of the art of submarining.

At some point, soon after the young diving officer mastered neutral trim, the crew conducted a “trim party”. As he should be, the young officer was focused on the planes, the planes operators and the depth gauge. At some point, he noticed that the boat was suddenly heavy forward. He wasn’t aware of any changes. Since it didn’t resolve itself, he moved water from forward trim to aft. Then, a bit later, he would find that the boat was heavy aft. He still didn’t know the cause. After a while he moved water from after trim to forward. After another little while, the boat was heavy forward again.

This would repeat, heavy forward and then aft, until the diving officer turned around and saw the ten big sailors going toward the forward torpedo room and then to the after torpedo room. That was the trim party. It was, and remains, a submarine institution. It accomplished two important objectives. First, it deflated the ego of the young diving officer. More importantly, it was an object lesson about changes in the weight of the boat. For example, if the captain fired all six torpedoes forward, that was about 19,000 pounds ejected from the boat. The water in the tubes helped offset some of that weight. The torpedomen could help by allowing more water into the drain tanks. But the diving officer still had to react quickly to keep the boat from broaching.

We are told that trim parties continue to this day, although computerized controls may have removed most of the harassment value. Since many diving officers today are chiefs, they likely figure out what’s happening sooner.

No, the water was not pumped out.  They blew the water out using high pressure air. That air was stored at 3,000 PSI in bottles in the ballast tanks. To surface, the air was stepped down to 600 PSI and dumped into the ballast tanks. Once the bridge hatch was out of the water, the high-pressure blow would be secured and the low-pressure blower was used to blow the tanks dry. The low-pressure blower was run until bubbles appeared from the ballast tanks. (The low-pressure blower ran at 10 PSI.) This eliminated the need to use any more of the stored high-pressure air.

The air used to surface the boat would be lost when they dove the next time. It was just vented so that the ballast tanks can be filled quickly.  They replaced the air in the storage bottles by running the air compressors as needed when on the surface. There was usually enough air in the bottles to surface the boat about seven times.

There were two 3,000 PSI air compressors in the pump room below the control room. The low-pressure blower was also in the pump room. The high-pressure air manifold and the controls for the low-pressure blower are both in the control room.

It was possible to surface the boat without using any high-pressure air. The boat was driven toward the surface with the planes. Once the bridge hatch was out of the water and opened, the low-pressure blower was used to blow all of the water out of the ballast tanks. It was slower, but it worked. This was done most often when training new diving officers and lookouts to submerge quickly. There was enough high-pressure air in the tanks to surface the boat multiple times, but perhaps not enough for all those training dives.

There usually was no reason to hurry. The OOD (or captain) would take his time in order to surface safely.  The boat would come up to periscope depth carefully. The OOD wanted to be sure that there was no danger of collision with other vessels and, in WW2, no immediate danger of attack by the enemy. Then, when they saw nothing through the periscope and heard nothing on sonar, they dumped air into the ballast tanks.  They also put rise on the planes. That brought them to the surface.

However, if they would be going to battle stations guns in WW2, they would want to surface very quickly. Coming up to periscope depth was still done carefully. Then, when it was nearly time to surface, some air was added to the ballast tanks while the planes were holding the boat down. Then, when the command to surface was given, the planes went to full rise and more air was dumped into the ballast tanks. The goal was to get the first rounds fired in less than a minute after the order to surface was given. The target was likely to be shooting back, so speed and surprise were of the essence.

Many things needed to be done to fire the first rounds in less than a minute:

1. Enough air had to be dumped into the ballast tanks to get the main deck out of the water.
2. Gun crews needed to open hatches and assemble on deck.
3. The gun(s) needed to be unsecured. (They were tied down so they would not make noise when submerged.)
4. Ready storage for ammunition had to be opened.
5. The gun(s) had to be loaded with the ready ammunition.
6. The gun(s) had to be aimed.
7. The gun(s) would be fired when the boat was steady enough.

No. The pressure inside the boat had increased only slightly, if at all, and was hardly noticeable. There was no need for decompression.

There were two air compressors in the pump room, below the control room. They were four-stage compressors that produced air at 3,000 PSI for storage in the air bottles in the ballast tanks. Naturally, they would only run the air compressors on the surface when outside air was available.

The low-pressure blower was also located in the pump room. It took air from inside the boat, compressed it to about 10 PSI and dumped it into the ballast tanks. The manifold was in the control room just aft of the high-pressure air manifold. This allowed them to save some of the high-pressure air used to surface. The process took some of the stale air from inside the boat and dumped it into the ballast tanks, which started refreshing the air in the boat.

The low-pressure blower was usually run until they saw air bubbles coming up from most or all ballast tanks. This didn’t cost any high-pressure air and ensured that the boat was fully surfaced. When the ballast tanks were empty, the boat rode more efficiently on the surface.

As indicated above, it was possible to surface the boat using only the planes and the low-pressure blower.

High pressure air was used for many purposes on the boat. It was used to:

1. Charge the air flasks in the Mark 14 steam torpedoes to 2,800 PSI.
2. Blow the water out of the main ballast tanks to surface. The air was stepped down to 600 PSI for this purpose.
3. Launch torpedoes from the tubes. The air was stepped down to 300 PSI for this.
4. Roll over the diesel engines to start them. The air was stepped down to 50 PSI.
5. The air could be bled into the boat if needed when the oxygen content got too low or the CO2 content got too high. This pressurized atmosphere inside the boat somewhat and the air was far from fresh. However, it supported life.

That was a bathythermograph. As it appears, it recorded water temperatures at various depths. Submarines changed depths regularly just to see what the temperatures were. They were looking for abrupt changes in the temperature of the water.

Those abrupt changes are called thermoclines or layers. Changes in temperature also meant changes in density and that distorted sonar. If the temperature difference was large enough, sonar was reflected and a submarine could hide below the layer. In either case, anyone above who was searching for the boat probably had an inaccurate picture of where it was.

However, if sound was being distorted or reflected by a layer, the crew may not have had an accurate picture of what was above them. Caution was advised.

The bathythermograph (BT) was first developed in 1934 by Woods Hole Oceanographic Institute. It was first tested for submarine use in 1942. It could locate a “shadow zone” for a submarine to hide in.

(“USS Pampanito, Killer-Angel”, by Gregory Michno, page 66.)

The weight of any ship changed as it was operating. Fuel was used, food was eaten, supplies were used and ammunition was expended. All of that made the ship lighter. Resupplying, in port or at sea, replaced some or most of that weight. The weight of submarines also changed for those same reasons.

However, submarines were different in a few important ways. The most important difference was that the weight of the boat mattered much more. If a cruiser got much lighter, it might just ride higher and more efficiently. If a submarine got lighter, it might have had problems submerging quickly or staying submerged. The boat needed to take on some water to offset the weight loss.

NOTE: Surface ships could have problems if they got too light. They might have had problems riding out a big storm unless they took on some ballast water for stability.

Another difference is that submarines actually got heavier as they burned fuel. That was because the fuel was stored outside of the pressure hull. The fuel tanks were among the ballast tanks. In order to keep the fuel tanks from collapsing due to pressure, sea water was fed into the tank below the fuel. This way, the fuel was stored at sea pressure even when submerged. Sea water weighs about 1.5 pounds more per gallon than diesel fuel. This was very helpful in that the weight gained by burning fuel helped offset the weight loss from other changes.

A third difference is that the salinity of the water also mattered much more to submarines. If the water was less dense, as for example near the mouth of a river, the submarine weighed the same but appeared to be heavier. Again, the same thing happened to surface ships but didn’t usually matter as much.

Absolutely. The change needed to be calculated the night before going to sea. This was done by the officer on duty that last night in port. It was important enough that it was also checked by another officer. Then the duty officer was the diving officer for the trim dive the next day, for added motivation.

The calculation needs to include any crew members - and their gear - coming aboard or leaving the boat. An average number was used for the weight of a sailor and his gear. The calculation also included any added ammunition, food or supplies, and any changes to the number or types of torpedoes. The calculation also needed to include the decreased weight from adding fuel. Again, the fuel tanks had a compensating system that filled in water below the fuel as it was used so that the tanks did not need to be pressurized. A gallon of diesel fuel weighs about 1.5 pounds less than a gallon of sea water.

The objective of a trim dive was to get the boat into neutral trim so that future dives could be done quickly and safely. This was particularly important for the first dive after leaving port to make sure the boat had been compensated for the recent weight changes. There was a trim dive each day while in transit to and from the patrol area. This was needed because of the increased weight of the boat as fuel was used. That was at least partially offset by the food that was consumed, any ammunition used for target practice, supplies that were used, and so on.

This was done so that the operator could tell them apart even if the control room was dark or full of smoke. To qualify to stand this watch, the operator was blindfolded and he brought the boat to the surface. In normal operation, the different shapes also provided confirmation that the operator had the desired valve handle.

This was also true for the vent valve handles at the chief of the watch’s station. Like the valve handles on the air manifold, it allowed the chief of the watch to tell them apart even if the control room was dark or full of smoke. In normal operation, the different shapes provided confirmation that the chief had the desired valve handle.

That was a backup valve for the negative tank. Remember the rule: everything that was important on the submarine had a backup – except for the ice cream machine.