Defensive Maneuvering

Brian Laxson's Article on Defensive Tactics:

Introduction
There are several commonly known tactics for evasion by World War 2 submarines. This report presents these alongside some more complex tactics. These are intended for use with the computer game simulation Silent Hunter as made use of by the PTC. Various technical points from that game and the dedicated work of the PTC have been referenced.

The tactics presented here are a selection of those for surviving an attack. It assumes the attackers are already alerted, such as by the sounds of your torpedoes shattering the ships they escort. Discussion is based on evading attacks by only surface vessels.

Aircraft are touched briefly in a separate discussion.

Fuel leakage would also impact the tactics significantly. Oil leaking from damaged fuel tanks adds another means by which the enemy can spot you. It provides them with ongoing updates of your position. Loosing fuel adds a new way to lose the boat, if repairs aren’t done in time you might be unable to get back to base.

The report begins with reviews on various reference data about the enemy, their methods, our own boats and the thermal layer. This section draws heavily on data from Silent Hunter and from the work of other members of the PTC. The author has added to this data from his own tests, such as the time it takes for escorts to make another depth charge attack.

Then presented are numerous defensive techniques. The goal is to encourage understanding the strengths and weaknesses of each. Each tactic is described in detail with notes back to the reference data. When to use the tactic is also listed. An informal "Officer’s club" comment provides informal implications of the tactic.

The tactics are:

Tactic #1: Run Silent, Run Deep

Tactic #2: Run Silent, Run Steady

Tactic #3: Snap on Sound

Tactic #4: Exhaust the attackers

Tactic #5: Surface escape

Tactic #6: Ground the boat

Tactic #7: Hide behind cover

Tactic #8: The far hit torpedo

Tactic #9: Sink the attackers

Reference data on depth charge attacks
Depth charges are explosives that are dropped, rolled or fired off an escort into the water. Other studies have determined the rate of depth charge descent as 200 ft takes 18 seconds, 300 ft takes 28 seconds and 400 ft takes 38 seconds.

Damage from depth charges occurs when they detonate within 100 yards of the submarine. The closer the detonation the more damage is done. At father ranges it may shake the boat without doing damage or simply be heard.

Ammunition for depth charges varies by type of escort craft. Typically it is 12 or 18 "firings" of their combined rails and launchers. You can check this figure through use of the Ship ID book.

After an escort makes a depth charge attack they swing pinging with their sonar. Even with the help of other nearby boats it takes typically 8 to 12 minutes for a given escort to swing back in for another attack run. Be aware that if multiple boats are around one of them may attack you sooner than that. There is also a chance that they might happen to be on a course similar to yours which would greatly reduce the time needed to attack again.

Assuming 18 firings and 12 minutes between attack attempts a submarine must remain evasive for 216 minutes. This is roughly three and a half hours.

Reference data on the attackers' means of detection
The particular advantage of a submarine is its ability to become hidden. Understanding the means by which the attack can accurately target you is of great importance. There are five primary means they can use to locate you.

First is to spot the boat visually. This can only happen if at least part of the submarine is above the water. This is normally the conning tower when running on the surface. It is possible for boats, and planes, to spot the periscope and the wake it leaves in the water.

Second is the use of radar equipment. Whether the attackers have radar is a separate target of research. It is basically similar to the issue of visual detection in that at least part of the sub must be above the water.

Third is through "passive" sonar. By passive we mean the attacker does not create any sound "pings". Rather he listens for the noises of your boat. The primary noise on your boat is the propellers. Propeller noise is linked to the speed setting you use for the boat, the faster you go the more noise you make. Secondary sources include the water pumps, crew talking and firing torpedoes. These secondary noises can be silenced by giving the "Rig for silent running" order.

Fourth is the use of "active" sonar. This is the famous "ping". The range of the active sonar in WW 2 is relatively limited. If you hear a reverberating "secondary ping" then you can be assured that they have a lock on your boat. The advantage of active sonar is that it is accurate. It does have difficulty during the last distance before being over the submarine (this is exploited in the "Snap on Sound" tactic). The chance of detection also depends on the silhouette of your boat, with a stern or bow turned toward them making the smallest sounding board.

The fifth means of the enemy locating you is different from the others. Based on their understanding of your boat and a guess as to your tactic they try to estimate your position. Part of this is seen in the types of search patterns that attackers perform based on an estimate of your boat’s underwater speed.

Discussion on the attacker’s use of search patterns
Each attacker can cover a portion of the possible courses you can take. Thus the more attackers there are the more chances they have of their search patterns finding you. Let's assume for our theoretical analysis that with their active search equipment each attacker can sweep about 70 degrees (out of 360) of possible courses during their search.

One, two or three attackers (three scanning about 60% of the possible routes) can usually be escaped on the first attacks. Your own passive sound gear helps by allowing you to keep turned away from them. Four attackers (78% scanning) leave you only one half the previously possible escape routes and the gap is sometimes not in an area convenient for you to escape to. Five boats leave only about a 2% gap. In my experience five or more boats can not be easily escaped forcing you into using either "Run Silent, Run Deep" or "Exhaust the attackers".

In all cases the attacker focuses the search pattern on where they think you are. Thus if you can confuse the enemy all these searches take place far away allowing you to escape. Means to confuse the attackers searches include: firing torpedoes at long range, switching from one side of the convoy to another, using sinking vessels to disrupt their search pattern, changing the route of your escape or moving quietly away after jettisoning debris.

Reference data on your boat
The range of a submarine underwater depends greatly on the speed selected. At 5 kts boats are listed as having a range of 60 to 95 nautical miles. This translates to well over 12 hours at 2/3 speed. At flank underwater speed of about 9 kts the batteries on a Gato are drained in about 1 hour 10 minutes. This is around only 10 nautical miles.

Of particular note is that your flank speed duration of 70 minutes is only a third of the possible 216 minutes an attacker may be engaging you.

The diving and crush depth varies by boat. Starting with P-Class you can usually get into depths of 350 feet without being crushed. Later boats can manage even deeper.

The amount of air available in your boat is a little over 24 hours, well over the 216 minutes an attacker can sustain persistent depth charging. This is assuming that no fire or chlorine leaks occur. Either of these dangers consume the useable oxygen at a deadly rate. These will force you to engage the escorts to make it safe to surface the boat.

In the Silent Hunter game it takes about one minute for a Veteran crew to get to a keel depth of 30 feet and a second minute to get to periscope depth.

Reference data on the thermal layer
Registering on the Bathythermograph as a sharp change of temperature it creates massive deflections in sounds. Beneath this you are comparatively "safer" from passive sound detection. Even if your boat lacks the temperature sensor these important layers may, or may not, still be out there.

In shallow waters of under 200 feet depth, such as the Yellow Sea, one does not normally find a thermal layer. It is only in the deeper waters that they are found. Coincidentally these are typically areas where the sub might be crushed if it fell to the bottom.

Thermal changes can come at many different depths - some of the ones reported have been at 350 feet. Thus you may need to go below your intended diving depth to find one. Be careful of when you hear the phrase "We are too deep sir" as you have only a short 30 to 50 feet left before the boat is lost.

Advisory on aircraft situations
Aircraft are a significant complication to any defensive maneuvering. Primarily they force you to stay down under water. However, they can sometimes spot even your periscope. Aircraft cover great distances very quickly as well. Be advised on one last point, aircraft in your area can be replenished quickly by bases or sea going craft.

If for some reason you feel compelled or forced to engage the aircraft keep up a good speed and be willing to keep putting the rudder hard over to make it difficult for them to bomb you. Aircraft cannon fire tends to cause only relative minor leaking with repeated hits needed to really put the boat at risk. There is some possibility of downing them with AA fire or with a deck gun (firing at 5-7 degrees elevation).

Timing the dive of the boat is critical to escape an air attack. If the aircraft is on radar and not in visual range you should be safe to dive. If the aircraft is visible though you may be bombed while diving.

A "safer" dive technique against a nearby aircraft is as follows. Order "Man the AA guns". Consider putting the rudder hard over to make a less steady bomb target. Watch the aircraft, possibly assisting the deck gun crew in AA fire. The AA fire "usually" spooks the pilot into releasing his bombs early. At the moment he turns from his bomb run, or the bombs hit the water, that is when you order the dive.

Be sure to dive to at least 100 feet to avoid clear water leaving the sub visible. It is also advisable to turn the course of your boat once dived. Aircraft will drop additional weapons they may have. The airplanes are also radioing your last known location to any surface attackers available.

In general if there are aircraft get under the water.

Tactic #1: Run Silent, Run Deep
Dive below a thermal layer. Slow to 2/3 speed at most. Slower speeds of 1/3 and the order to "Run Silent" can also help you greatly. Personally I like to run at 2/3 for a short while to gain some distance from where the boat crossed the layer.

Once in this mode, below a layer and running quiet, you can usually escape. Be alarmed if any escort pings reverberate meaning they have detected you. If so you will need to accelerate and prepare to disengage them again.

If you can find a thermal layer this tactic is your primary means of defense.

When to use: Any time you can find a thermal layer. Officer’s club: "Most reliable one we got."

Tactic #2: Run Silent, Run Steady
Due to the need for the escorts to search for you they run various pattern techniques. If you hold to a steady course you can manage to escape their search pattern. A 1/3 electric speed is generally going to be to slow for this, unless you have a layer to hide under.

In Run Silent, Run Steady make 2/3 or standard speed and move in one direction. Listen and watch for signs of the escorts closing again. If they do have you make a course change, make a major heading change or use the Snap on Sound tactic.

Once the attackers make their attack take a new heading for Run Silent, Run Steady. The importance of the new heading is that the escorts may try to predict where you are going. If so they organize their pattern search to scan more in that direction, such as your last general heading, and less in others.

The other use of this tactic is to seek out a better depth of water for you. In most cases this will be to get to deeper water in hopes of finding a thermal layer.

When to use: Good in most cases and is usually combined with other methods. Officer’s club: "If you drive in circles you stay in the search area. Bug out and get home."

Tactic #3: Snap on Sound
Of relative importance is that a boat at 9 kts needs a little over 30 seconds to travel 100 yards. One at 5 kts needs about 60 seconds. Consider this 100 yard distance, the damage range of a depth charge, and the 28 seconds it takes a depth charge to sink to 300 feet.

There is one important clue to watch for: the sound of the escorts propellers. By initiating this maneuver you add further yardage to hopefully secure a beyond 100 yard detonation. This assumes of course the attacker was actually aiming at the correct spot.

To perform a "Snap on Sound" at the moment you hear the sound of destroyer propellers put the boat to full or flank electric speed. Make a manual very hard turn until you turn 90 degrees from your original course. Maintain the high speed until your tail is clear of the path.

Your boat is roughly the same 100 yards that is the damaging range of the depth charges. It is best that you keep the high speed until you are a half or full boat length away from your original course. Add to that initial displacement the distance you will make, at whatever speed, from when the depth charges are dropped to when they detonate. This maneuver has almost always made the destroyer depth charges fail to inflict damage. Nothing is certain but it is roughly equivalent to jumping to the side in a game of dodge-ball.

Assisting destroyers will make attacks as well. Due to the interference from the first depth charges they tend to not have updated their firing position. The next few minutes you can usually get away without repeating the maneuver. After a few minutes it is best to assume that any new propeller sounds overhead mean they have picked you out again.

When to use: When you are at immediate risk of being depth charged. Officer’s club: "Deemed to be effective."

Tactic #4: Exhaust the attackers
In this tactic the goal becomes to be able to safe the boat away by having them run out of depth charges. Keep in mind that while the ammunition of depth charges is limited the escorts sill have masses of ammo fire power in their cannons. Surfacing near them is still a death sentence for the boat.

Usually this tactic is not the first choice of a Captain. Large numbers of escorts, like 6 and above, are almost always going to reacquire the submarine after each depth charging. Such a group could in fact force you into this tactic. Wide shallow areas, such as the Yellow Sea, also may mandate selecting this tactic.

First, as always, make use of as much depth of possible. This will help you exploit the time it takes for depth charges to sink.

Second make use of the Snap on Sound tactic described above. If you leave your boat at high speeds your batteries will be depleted before their ammunition.

After each depth charge evasion drop your speed to 2/3. A 1/3 speed will generally make it to hard to accelerate or maneuver your boat, unless you are switching to "Run Silent, Run Deep". Make use of high speed to make the turn then slow down.

You are pitting your battery voltage against their ammunition supply. It is suggested to try keep track of the number of depth charge volleys fired.

When to use: When you have no place to escape to or many attackers around keep acquiring you. Officer’s club: "Rough work but better than dying. Turn fast then slow to conserve your battery."

Tactic #5: Surface escape
On the surface, in good seas, a submarine can make close to 20 kts. Escorts can make 24 to over 30 kts. As you can see surfacing within their sight will undoubtedly lead to you being caught.

In poor lighting conditions, especially fog, it is possible to bring the boat up without being spotted. Range is usually 1,000 yards or farther out (2,000 – 5,000 yards) before attempting this.

Once you get on the surface the escorts typically remain committed to doing an underwater search. This has them focused on their sound gear that is aimed downward. Secondly they expect your underwater speed to be around 5 kts while you dash away at four times that speed.

If you can get on the surface and run without signs of pursuit for three minutes you will have added an additional 2,000 yards to where they thought you were. At this point even an escort accidentally heading on your course at 10 kts faster than you (his 30 kts minus your 20 kts) would need to keep his course for 6 minutes to eliminate this lead. Given the 12-minute return to attack cycle this 6-minute closure is unlikely to accidentally occur.

If you have radar take particular care to watch for pursuing escorts. You can also use the radar to locate fleeing merchants for follow up attacks.

If you do see an escort closing directly at you, or general closure for more than six minutes, you must consider the possibility of having been spotted. It is recommended you dive the boat to shallow depth, drop to a quieter speed of 2/3 or less and watch to confirm a closing attacker. If confirmed that they are coming after you switch to another tactic.

One technique for this maneuver is it gives the possibility of follow up attacks on escaping merchants or wounded warships. This is aided greatly if you have a surface search radar.

When to use: When visibility is poor or when chlorine gas prevents remaining submerged. Officer’s club: "If you can pull it off for a few minutes you’re clean away."

Tactic #6: Ground the boat
Sometimes things have gone on too long or damage is limiting your endurance. In this tactic you bring the boat onto the bottom and come to a full stop. You are attempting to hide on the bottom until the attacker has decided you won’t come up again.

Do not attempt this tactic if you have signs of flood water accumulating in the batteries. The rate at which chlorine gas will consume your oxygen means it is very important to get off the bottom and prepare to "Sink the attackers" or conduct a "Surface escape."

Also check your amount of compressed air. Make sure you have some available. If you do not this tactic will leave you stranded on the bottom.

In setting up to use this tactic you must break contact with active sonar. If while sitting on the bottom you hear a reverberating ping you must get up and moving.

Selecting where to ground the boat is vital. Try to find as much depth as possible while remaining above your crush depth. If possible get under a thermal layer as well. Generally speaking if you have a thermal layer you can probably use "Run Silent, Run Deep" instead.

In lowering the boat do the main descent as normal. If you have a reading of depth under keel make use of it. Slow things down when you near the bottom. About 15 feet from the registered bottom the boat will be halted from motion. At this point you can choose to sit where you are.

If by your choice or by flooding the boat drops lower there will be a resounding crash. Sometimes if this is done at speed the boat will suffer more damage. This damage is the key reason to bringing the boat to halt prior to "hitting the rocks".

Once down ensure that your engines are ordered to stop. Due to drag of the mud it is in fact possible for the electric motors to be draining the batteries without any forward motion. You will need your power later on.

While it may make sense to issue the "Rig for Silent Running" order be aware that Silent Hunter will set the engines to 1/3 and thus drain your batteries. If you positioned away you will normally be sufficiently quiet to not need to use this command.

Once settled with engines stopped your crews can set about repairing. Be watchful of the following dangers and get back maneuvering if you see them. First is sign of flooding in the batteries, which leads to the deadly chlorine gas. Second is a reverberating active ping from an escort meaning they have an active lock. It is possible that the escorts may be maneuvering near you.

When the time comes use the following efforts to get off the bottom. First try just changing the depth using the depth gauge. If that doesn’t work you will have to "Blow main ballast tanks". The Silent Hunter game puts the boat to flank speed, billows out compressed air and other efforts guaranteed to make a lot of noise.

When to use: When running low on batteries or heavily flooded. Best done at some distance from the attackers. Officer’s club: "Sit silent, sit deep… just not too deep. Also called playing possum".

Tactic #7: Hide behind cover
This tactic can be used to assist the others. By itself it will not ensure permanent safety of the boat. In this tactic you drive near or around an obstacle. The ones most commonly available are sinking wreckage.

The noise of the sinking vessel interferes with their tracking you. Most importantly the fear of a collision makes them have to take a different than optimal course. During the time they are off course you have a short time to initiate another defensive or offensive tactic.

A variation on this technique is to dive under another non-attacking vessel. This could be any sinking ship or a steaming merchant (that lacks depth charges). Be careful to use a depth of at least 80-100 feet or else you might break your periscope even if it is not raised.

This is a temporary solution at best. Some attackers may be able to swing in close enough to still attack you. You might ram your periscope against the obstacle. This tactic is here to assist you when close in with the enemy.

One offensive use of this maneuver is to keep in amongst steaming merchants, usually only possible if they are damaged, slow or a large convoy. In this role the attackers get deflected by the merchant traffic. In turn you gain time to reload or otherwise initiate new attacks. If you are attempting this variation you may find it more productive to be able to use your periscope. Be ready to dive and lower the scope though if an attacker seems to have an unimpeded run at you.

Sometimes you may find a coastline useful. It is rare that this occurs but it is something to be considered when close to shore. Just watch you don’t strand your boat in shallow water.

When to use: Nearby obstacle like wreckage of a sinking vessel is available. Officer’s club: "Works for a moment then do something else."

Tactic #8: The far hit torpedo
This unusual tactic sometimes pays off. The surface attackers generally make use of your torpedo explosions to help predict your location. In this technique you arrange for a distant torpedo explosion, possibly hitting something.

To do this you will need to already be clear of active pinging getting a reverberating echo on you. Fire the torpedo at long range at preferably a slow or non-moving target. Range should be in excess of 2,000 yards. It is also recommended that you consider using an electric torpedo if possible as these have a less visible wake that would give away your position.

If the torpedo hits the attackers search area will hopefully move to another location.

After firing the torpedo revert to "Run Silent, Run Straight", "Ground the boat" or make a "Surface escape".

When to use: Having difficulty escaping and a distant target exists. Officer’s club: "Wasting one torpedo is better than losing a boat."

Tactic #9: Sink the attackers
In PTC the RoE forbid openly attacking the escorts unless the boat is in definite jeopardy. Situations like oxygen being depleted by chlorine gas or imminent ramming can make it rather mandatory. Remember PTC RoE dictates you do not record them as tonnage sunk. It is better to lose ammunition than the lives of your crew or your own life.

The primary cause for being forced to engage the enemy is the threat of oxygen depletion. It is almost guaranteed that if you get visible flooding in the batteries you will have chlorine gas occur until the leaks are stopped and the water pumped out. It is my recommendation that if you see active flooding in the batteries that you must either get clear immediately or (and more likely ) you have to "Sink the attackers".

To "Sink the attackers" you are best off being able to target them. This is best done at periscope depth rather than surfacing the boat.

If you have dived deep though getting to this shallower depth takes time. During this time to get to attack depth the escorts will be maneuvering to engage you. Two major disasters can happen if this is not done right. Most dangerous is that the escorts make a successful depth charge attack on you. Second is that when you raise the periscope it gets knocked out, either by being rammed or by shell fire from a nearby ship.

What I find particularly useful is to first bring the boat up to a depth of 100 feet. While doing so follow the "Snap on Sound" tactic. Only when you do not hear enemy propeller sounds, or at least that the sounds are fading, do you bring the boat up to periscope depth and raise the scope. Unless forced by low oxygen I have sometimes delayed until a depth charge attack has confused the attacker’s sound sets before that final raising to attack depth.

Once up you will have to deal quickly with the escorts. If they are not near you they can be baited into running at you. In this case a torpedo fired at a range of around 500 yards with a depth setting of 6-8 will be very effective.

The other targeting on an escort is a broadside firing. This is possible when they are making their turns to make a repeat attack after a previous depth charge attack. Two key things to keep mind are that your torpedo will need to travel 300 yards to arm and you may have a dud detonator.

One particular matter to concern yourself is to save your periscope. It is possible for it to be lost to ramming or gunfire. If you feel it is threatened lower the scope and if the danger is ramming dive the boat deeper.

While you do have a deck gun you will be seriously outgunned by any escort or warship. An enemy submarine will likely not be sunk by the few hits you could score before they dive. Use the deck gun only in great desperation, such as if you are forced to use "Sink the attackers" and you have lost your periscope.

There is one other possible desperate move. Your sound heads give you a bearing on the enemy but not a range. You can in fact turn the boat to roughly an intercept bearing and fire. If attempting this rather unlikely maneuver fire a spread of torpedoes and do try to center the spread off in the direction the sound contact seems to be drifting. Unlikely as it is this is at least one last thing to try.

When to use: When faced with clear and unavoidable danger. Officer’s club: "Sometimes a mans gotta do what a mans gotta do. No tonnage for the escorts but better than the enemy getting your tonnage."

'''Copyright © 2004 by Brian Laxson. Used with the author's kind permission.'''

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