Depth charges

How close to a submarine did a depth charge have to
be to inflict severe/ fatal damage?

That varied with depth.

A depth charge killed a submarine if the pressure
wave from its explosion exceeded the strength of
the hull.

However, the hull would already be under stress
from water pressure. That pressure could vary
from 15 psi at periscope depth to 300-500 psi
near crush depth (600-900 feet).

When a sub was near its crush depth, a relatively
small additional stress could smash in the hull.

It would also depend on the size of the depth
charge and the type of explosive in it.

The force of the pressure wave would be proportional
to the square of the distance from the depth charge.

Unfortunately, I have neither specific information
nor the ability to calculate the actual numbers.

But here are some speculative calculations.

Assume that the sub's crush depth is 250 meters,
and that a depth charge would be fatal at
10 meters range and 10 meters depth. Then the
kill radius would be 15 meters at 140 meters
deep, 20 meters at 190 meters deep, 25 meters
at 210 meters deep, and 35 meters at 230 meters
deep.

Of course, my assumption about the depth charge
effect may be complete nonsense.

Subs ran deep to avoid detection and also to
evade sinking DCs. (The deeper the DC has to
sink, the more time the sub has to move away.)

--
The real Velvet Revolution - and the would-be hijacker.

http://originalvelvetrevolution.com

news

2013-01-15 05:05:04 UTC

How close to a submarine did a depth charge have to
be to inflict severe/ fatal damage?

That varied with depth.
A depth charge killed a submarine if the pressure
wave from its explosion exceeded the strength of
the hull.
However, the hull would already be under stress
from water pressure. That pressure could vary
from 15 psi at periscope depth to 300-500 psi
near crush depth (600-900 feet).
When a sub was near its crush depth, a relatively
small additional stress could smash in the hull.
It would also depend on the size of the depth
charge and the type of explosive in it.
The force of the pressure wave would be proportional
to the square of the distance from the depth charge.
Unfortunately, I have neither specific information
nor the ability to calculate the actual numbers.
But here are some speculative calculations.
Assume that the sub's crush depth is 250 meters,
and that a depth charge would be fatal at
10 meters range and 10 meters depth. Then the
kill radius would be 15 meters at 140 meters
deep, 20 meters at 190 meters deep, 25 meters
at 210 meters deep, and 35 meters at 230 meters
deep.

so in effect, the deeper the sub dived and DC was set, the more effective
it became?

Post by Rich Rostrom
Of course, my assumption about the depth charge
effect may be complete nonsense.
Subs ran deep to avoid detection and also to
evade sinking DCs. (The deeper the DC has to
sink, the more time the sub has to move away.)

TeoZ

2013-01-15 05:55:22 UTC

I would bet the temperature (colder the deeper the sub was, metal shattering
instead of flexing) would affect the outcome of a close depth charge (also
weld quality and metalergy of that time period).

Bill Shatzer

2013-01-15 06:15:42 UTC

Post by TeoZ
I would bet the temperature (colder the deeper the sub was, metal shattering
instead of flexing) would affect the outcome of a close depth charge (also
weld quality and metalergy of that time period).

There's really no meaningful difference in water temperature by depth -
maybe 25 or 30 degrees Fahrenheit in all but the most tropical of waters.

And, of course, water temperature cannot decrease below 29 degrees
fahrenheit (-2 degrees Celsius). (Salt water freezes at a bit colder
than does pure water.)

TeoZ

2013-01-15 06:45:29 UTC

Post by Bill Shatzer

Post by TeoZ
I would bet the temperature (colder the deeper the sub was, metal
shattering instead of flexing) would affect the outcome of a close depth
charge (also weld quality and metalergy of that time period).

There's really no meaningful difference in water temperature by depth -
maybe 25 or 30 degrees Fahrenheit in all but the most tropical of waters.
And, of course, water temperature cannot decrease below 29 degrees
fahrenheit (-2 degrees Celsius). (Salt water freezes at a bit colder than
does pure water.)

I was thinking of liberty ships cracking in the cold in the Russian convoys,
the metal on US ships was brittle in the cold so mass produced metal used in
German subs (most likely short on key materials late in the war) would not
have been any better.

Padraigh ProAmerica

2013-01-15 15:39:00 UTC

Re: Depth charges

Group: soc.history.war.world-war-ii Date: Tue, Jan 15, 2013, 12:55am
From: ***@neo.rr.com (TeoZ)
I would bet the temperature (colder the deeper the sub was, metal
shattering instead of flexing) would affect the outcome of a close depth
charge (also weld quality and metalergy of that time period).

==============R E P L Y ============

VERY doubtful. The water isnl;t cold enough to make steel 'brittle'- you
need to get to around -40 degrees for that to happen.

Metalurgy was standardized (I'm centering on US fleet boats). The
BALAO-class boats had thicker and sturdier hulls than GATO-class boats
(post-war, USS CHOPPER made an unexpected 'excursion' to below 1,000
feet and survived).

--
"A man who can own a gun is a citizen. A man who cannot own a gun is a
subject."--

Unknown

Geoffrey Sinclair

2013-01-16 14:21:05 UTC

Post by Padraigh ProAmerica

Post by Padraigh ProAmerica
Group: soc.history.war.world-war-ii Date: Tue, Jan 15, 2013, 12:55am
I would bet the temperature (colder the deeper the sub was, metal
shattering instead of flexing) would affect the outcome of a close depth
charge (also weld quality and metalergy of that time period).

VERY doubtful. The water isnl;t cold enough to make steel 'brittle'- you
need to get to around -40 degrees for that to happen.

The answer is steel can be brittle at higher temperatures, given it is an
alloy its properties change according to composition.

The experiences of the US merchant ship building program in WWII
showed water temperatures around the freezing point of water were
common when the, mainly liberty, ships developed significant cracks.

Post by Padraigh ProAmerica
From a previous post.

In the end the metallurgists came up with the answer. The Cambridge
(UK) team was lead by Dr Constance Tipper. The term notch
brittleness is used to describe a plate having a higher stress area, a
notch, and when it would break in a brittle manner at low temperatures.
No tests were done for this at the time and it turned out US steels with
their lower manganese to carbon ratios showed the problem more than
UK steels. In almost all cases it required temperatures so low that
they were rarely encountered however some of the US steel batches
were so sensitive they could crack in tropical waters.

It explained why welding was not a problem, why UK welded ships
were not giving the same amount of problems, why it was worst in
winter and why temperatures when under construction seemed to
matter.

Post by Padraigh ProAmerica
Metalurgy was standardized (I'm centering on US fleet boats). The
BALAO-class boats had thicker and sturdier hulls than GATO-class boats
(post-war, USS CHOPPER made an unexpected 'excursion' to below 1,000
feet and survived).

The Chopper was near vertical in the water, bow down, when the bow
was at around 1,000 feet, the stern was around 700 feet, the attitude
shifted to near vertical bow up when all the emergency measures cut in.
Note the pressure hull stops short of the bow. The result of the dive
and subsequent broaching meant the ship was no longer fit to serve as
a submarine.

With 35 pound steel as the hull material the Balao class onwards
hoped for crush depth was around 900 feet.

Geoffrey Sinclair
Remove the nb for email.

Rich Rostrom

2013-01-15 19:08:08 UTC

Post by news
so in effect, the deeper the sub dived and DC was
set, the more effective it became?

Yes.

But there are advantages for the sub in running deep.

If there is a thermocline in the water, it can
block sonar waves. (A thermocline is an abrupt
change in water temperature and therefore density.
They can be surprisingly dramatic. I've put my
hand through a thermocline while scuba-diving, and
the change in temperature was _sharp_.)

The deeper the sub is, the harder its propellor
noise is to pick up on hydrophones.

As mentioned before, the further the depth charge
has to sink, the more time the sub has to move
away.

Also, the depth charge has to be set to a particular
detonation depth. If the attacking ship guesses
wrong about the sub's depth, that adds to the
distance between the sub and the exploding DC.

In relatively shallow water, sonar reflections
from the bottom can mask the sub's reflection.

All this can help the sub get away. The penalties
are that depth charges have a greater kill radius,
and that it becomes harder for a sub to surface
in an emergency.

--
The real Velvet Revolution - and the would-be hijacker.

http://originalvelvetrevolution.com

Padraigh ProAmerica

2013-01-15 20:56:14 UTC

Re: Depth charges

Group: soc.history.war.world-war-ii Date: Tue, Jan 15, 2013, 2:08pm
From: ***@rcn.com (Rich Rostrom)
"news" <***@newsfe12.iad.highwinds-media.com> wrote:
so in effect, the deeper the sub dived and DC was set, the more
effective it became?
Yes.
But there are advantages for the sub in running deep.
If there is a thermocline in the water, it can block sonar waves. (A
thermocline is an abrupt change in water temperature and therefore
density. They can be surprisingly dramatic. I've put my hand through a
thermocline while scuba-diving, and the change in temperature was
_sharp_.)
The deeper the sub is, the harder its propellor noise is to pick up on
hydrophones.
As mentioned before, the further the depth charge has to sink, the more
time the sub has to move away.
Also, the depth charge has to be set to a particular detonation depth.
If the attacking ship guesses wrong about the sub's depth, that adds to
the distance between the sub and the exploding DC.
In relatively shallow water, sonar reflections from the bottom can mask
the sub's reflection.
All this can help the sub get away. The penalties are that depth charges
have a greater kill radius, and that it becomes harder for a sub to
surface in an emergency.

=================R E P L Y=======

In the case of Japanese deprh charges, they could not be set below about
250 feet and be expected to detonate reliably at deeper depths. I
believe that was corrected on later models.
--
The real Velvet Revolution - and the would-be hijacker.
http://originalvelvetrevolution.com

--
"A man who can own a gun is a citizen. A man who cannot own a gun is a
subject."--

Unknown

Don Phillipson

2013-01-15 20:57:19 UTC

Post by news
so in effect, the deeper the sub dived and DC was
set, the more effective it became?

As mentioned before, the further the depth charge
has to sink, the more time the sub has to move
away.
Also, the depth charge has to be set to a particular
detonation depth. If the attacking ship guesses
wrong about the sub's depth, that adds to the
distance between the sub and the exploding DC.

The main tactical limitation was that the sub-chaser
had to (1) proceed on the surface to the place was
the sub was supposed to be (or calculated soon
to reach, (2) drop the depth charges into the water,
(3) wait enough time for the charges to sink to
the depth for explosion set by their hydostatic pistols.
These processes take time, i.e. time for the sub
to manoeuvre, change direction, etc. Even after
the charges explode, it takes yet more time for
the water to settle down well enough for sonar or
other detectors to start working again.

This was why British sub-chasers installed instead
the "Hedgehog" spigot mortar which fired 15 or 20
small contact bombs in a ring ahead of the ship.
The size and spacing of the ring was calculated
so as to bring at least one bomb into contact with
the submarine if any part of the boat intersected
the ring: and one contact bomb was usually enough to
pierce the pressure hull. The bombs that touched
nothing merely sank into the depths without exploding.

This was a further benefit of the Hedgehog that misses
caused no explosions, i.e. the sonar and hydrophone
detectors aboard the sub-chaser continued to work as
normal, and could track the boat's unseen manoeuvres.
As soon as the spigots could be reloaded, the sub-chaser
was ready for another attack. (Reloading depth charges
required setting the depth pistols for each to explode,
and that took time.)

--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)

bill

2013-01-16 16:23:44 UTC

This was a further benefit of the Hedgehog that misses caused no
explosions, i.e. the sonar and hydrophone detectors aboard the
sub-chaser continued to work as normal, and could track the boat's
unseen manoeuvres. As soon as the spigots could be reloaded, the
sub-chaser was ready for another attack. (Reloading depth charges
required setting the depth pistols for each to explode, and that took
time.)

How long did it take to reload a Hedgehog.

Reloading depth charge racks and 'K guns' can take time...
--
"Hopefully the fair wind will resume, or this may well take all day."

Admiral Collingwood on being becalmed under the guns of six French ships-
of-the-line at Trafalgar

--
"Hopefully the fair wind will resume, or this may well take all day."

Admiral Collingwood on being becalmed under the guns of six French ships-
of-the-line at Trafalgar

Mario

2013-01-16 21:00:27 UTC

Post by Don Phillipson

Post by news
so in effect, the deeper the sub dived and DC was
set, the more effective it became?

The main tactical limitation was that the sub-chaser
had to (1) proceed on the surface to the place was
the sub was supposed to be (or calculated soon
to reach, (2) drop the depth charges into the water,
(3) wait enough time for the charges to sink to
the depth for explosion set by their hydostatic pistols.
These processes take time, i.e. time for the sub
to manoeuvre, change direction, etc. Even after
the charges explode, it takes yet more time for
the water to settle down well enough for sonar or
other detectors to start working again.
This was why British sub-chasers installed instead
the "Hedgehog" spigot mortar which fired 15 or 20
small contact bombs in a ring ahead of the ship.
The size and spacing of the ring was calculated
so as to bring at least one bomb into contact with
the submarine if any part of the boat intersected
the ring: and one contact bomb was usually enough to
pierce the pressure hull. The bombs that touched
nothing merely sank into the depths without exploding.
This was a further benefit of the Hedgehog that misses
caused no explosions, i.e. the sonar and hydrophone
detectors aboard the sub-chaser continued to work as
normal, and could track the boat's unseen manoeuvres.
As soon as the spigots could be reloaded, the sub-chaser
was ready for another attack. (Reloading depth charges
required setting the depth pistols for each to explode,
and that took time.)

"Operational research" became a discipline trying to solve a
similar problem with reference to air attacks to submarines.

Scientists (Blackett's Circus) discovered that a better choice
had been to set the airborne DC to explode near surface rather
than at 100 feet.

I read that in a Jagjit Singh book.
Great Ideas of Operations Research.
(c)1968
http://en.wikipedia.org/wiki/Jagjit_Singh_(writer)

<<While performing an analysis of the methods used by RAF Coastal
Command to hunt and destroy submarines, one of the analysts
asked what colour the aircraft were. As most of them were from
Bomber Command they were painted black for nighttime
operations. At the suggestion of CC-ORS a test was run to see
if that was the best colour to camouflage the aircraft for
daytime operations in the grey North Atlantic skies. Tests
showed that aircraft painted white were on average not spotted
until they were 20% closer than those painted black. This
change indicated that 30% more submarines would be attacked and
sunk for the same number of sightings.[19]

Other work by the CC-ORS indicated that on average if the
trigger depth of aerial-delivered depth charges (DCs) were
changed from 100 feet to 25 feet, the kill ratios would go up.
The reason was that if a U-boat saw an aircraft only shortly
before it arrived over the target then at 100 feet the charges
would do no damage (because the U-boat wouldn't have had time
to descend as far as 100 feet), and if it saw the aircraft a
long way from the target it had time to alter course under
water so the chances of it being within the 20-foot kill zone
of the charges was small. It was more efficient to attack those
submarines close to the surface when the targets' locations
were better known than to attempt their destruction at greater
depths when their positions could only be guessed. Before the
change of settings from 100 feet to 25 feet, 1% of submerged
U-boats were sunk and 14% damaged. After the change, 7% were
sunk and 11% damaged. (If submarines were caught on the
surface, even if attacked shortly after submerging, the numbers
rose to 11% sunk and 15% damaged). Blackett observed "there can
be few cases where such a great operational gain had been
obtained by such a small and simple change of tactics".[20]>>

http://en.wikipedia.org/wiki/Operations_research
http://en.wikipedia.org/wiki/Patrick_Blackett,_Baron_Blackett
http://www.di.unipi.it/~gallo/Papers/VoceEtica&RO_02.pdf

--
H

Mario

2013-01-16 20:59:21 UTC

Post by news
so in effect, the deeper the sub dived and DC was
set, the more effective it became?

There are here abandoned gravel quarries where a little lake
forms, about 10m/30ft deep.
When in Summer the surface layer warms up, somebody goes there
to swim.
It happened that one died because just under that warm layer
water remained very cold.

--
H

Mario

2013-01-15 19:49:05 UTC

How close to a submarine did a depth charge have to
be to inflict severe/ fatal damage?

That varied with depth.
A depth charge killed a submarine if the pressure
wave from its explosion exceeded the strength of
the hull.
However, the hull would already be under stress
from water pressure. That pressure could vary
from 15 psi at periscope depth to 300-500 psi
near crush depth (600-900 feet).
When a sub was near its crush depth, a relatively
small additional stress could smash in the hull.
It would also depend on the size of the depth
charge and the type of explosive in it.
The force of the pressure wave would be proportional
to the square of the distance from the depth charge.
Unfortunately, I have neither specific information
nor the ability to calculate the actual numbers.
But here are some speculative calculations.
Assume that the sub's crush depth is 250 meters,
and that a depth charge would be fatal at
10 meters range and 10 meters depth. Then the
kill radius would be 15 meters at 140 meters
deep, 20 meters at 190 meters deep, 25 meters
at 210 meters deep, and 35 meters at 230 meters
deep.
Of course, my assumption about the depth charge
effect may be complete nonsense.
Subs ran deep to avoid detection and also to
evade sinking DCs. (The deeper the DC has to
sink, the more time the sub has to move away.)

WRT the effect with distance.

In my opinion what is more important is not just the absolute
overpressure, because the pressure hull is made exactly to
resist that; (in theory a compressed circular section hull can
resist any pressure)
Rather I think more important the effect of spatial variation of
pressure that tends to flex the hull causing a sudden collapse.
Like in a compressed pole (or a crude spaghetti) that can
resist a great compression force till it remains straight but
collapses for a tiny flexing.
Or a man that stands easily with a huge weight on his shoulders
but also suddenly falls if somebody hits the back of his knees.

So I think (roughly) there is an inverse cube law rather than an
inverse square law.

Maybe I am wrong.

--
H

Roman W

2013-01-20 00:50:48 UTC

Post by Mario
In my opinion what is more important is not just the absolute
overpressure, because the pressure hull is made exactly to
resist that; (in theory a compressed circular section hull can
resist any pressure)

This is not true. It may maintain shape, but an absurdly great
pressure would squeeze it crushing the crew inside.

RW

Mario

2013-01-20 19:30:47 UTC

This is not true. It may maintain shape, but an absurdly great
pressure would squeeze it crushing the crew inside.
RW

Hm, right.
I was erroneously thinking to a sphere with no cavity inside.

In theory a circular shaped shell (sphere or cylynder) can
withstand a great external (outside) pressure, til it keeps
perfectly that shape.

Only when the INNER pressure (in the metal, not the inside
pressure in the cavity) exceeds it capacity, then the structure
collapses.

If P is the external pressure and D the circular diameter and S
the thickness of the shell, the metal inner pressure is
IP=(PxD)/(4xS)

Roughly, if steel can resist a compression inner pressure
100x10^6 N/m^2 (10 kgf/mm^2) (probably much more than that)
and if S=10mm and D=4000mm then
P = IPx4xS/D = 10x4x10/4000 = 0.1kgf/mm^2 = 10 bar = 100 m water
depth.

Then there is the slimness problem that amplifies any asymmetry
or imperfection or alteration of the shape, and a safety factor
of 10 or more must be used.

I would like somebody could provide the correct figures for this
physical and engineering problem.

--
H

e***@yahoo.com.au

2013-02-26 18:49:39 UTC

Post by Rich Rostrom
Assume that the sub's crush depth is 250 meters,
and that a depth charge would be fatal at
10 meters range and 10 meters depth. Then the
kill radius would be 15 meters at 140 meters
deep, 20 meters at 190 meters deep, 25 meters
at 210 meters deep, and 35 meters at 230 meters
deep.
Of course, my assumption about the depth charge
effect may be complete nonsense.

Depth charges were actually much LESS effective at depth. The
pressure presumably preventing the expansion of explosive gasses.

Post by Rich Rostrom
Subs ran deep to avoid detection and also to
evade sinking DCs. (The deeper the DC has to
sink, the more time the sub has to move away.)
--
The real Velvet Revolution - and the would-be hijacker.
http://originalvelvetrevolution.com

Rich Rostrom

2013-02-26 19:12:27 UTC

Post by e***@yahoo.com.au
Depth charges were actually much LESS effective at depth. The
pressure presumably preventing the expansion of explosive gasses.

Complete misunderstanding of how depth charges
work and of the relevant physics.

The explosive in the DC detonates, releasing a
lot of energy in a very short time. This energy
converts to force applied to the water around
the DC.

Water is effectively incompressible, so the force
of the explosion cannot displace the adjacent water.

Instead there is a very sharp pressure wave moving out.
Its force is inversely proportional to the distance
from the explosion.

The wave eventually disperses into heating of
the water, and movement of water at the surface.

But if the pressure wave reaches the hull of
a submarine - that's something that _can_ be
displaced. The interior of the hull is full of
air that can be compressed, and which is not
at the pressure of the surrounding water.

The hull resists the pressure of the water by
its mechanical strength. But the pressure wave
from the explosion adds to the load, and may
cause failure.

The pressure wave from the explosion is proportional
to the energy released, and is the same regardless
of depth.

High external pressure _might_ suppress the effect
of a shrapnel bomb, but it has no effect on a
concussion bomb, such as a depth charge.

--
The real Velvet Revolution - and the would-be hijacker.

http://originalvelvetrevolution.com

e***@yahoo.com.au

2013-02-27 20:59:54 UTC

Post by e***@yahoo.com.au
Depth charges were actually much LESS effective at depth. The
pressure presumably preventing the expansion of explosive gasses.

Complete misunderstanding of how depth charges
work and of the relevant physics.

Your theoretical ramblings have not relevance to reality
unfortunately. I've seen tables showing that a depth charge that was
effective to 15m at 20m deteriorate to 5m at 100m.

Post by Rich Rostrom
The explosive in the DC detonates, releasing a
lot of energy in a very short time. This energy
converts to force applied to the water around
the DC.
Water is effectively incompressible, so the force
of the explosion cannot displace the adjacent water.
Instead there is a very sharp pressure wave moving out.
Its force is inversely proportional to the distance
from the explosion.

Snip

Post by Rich Rostrom
The pressure wave from the explosion is proportional
to the energy released, and is the same regardless
of depth.

An explosion consists of extremely hot expanding gases. If the gases
can not expand their thermal energy is transferred to the water as
thermal energy rather than kinetic.

Post by Rich Rostrom
High external pressure _might_ suppress the effect
of a shrapnel bomb, but it has no effect on a
concussion bomb, such as a depth charge.

Nope

Geoffrey Sinclair

2013-02-28 14:16:56 UTC

Post by e***@yahoo.com.au
Depth charges were actually much LESS effective at depth. The
pressure presumably preventing the expansion of explosive gasses.

Complete misunderstanding of how depth charges
work and of the relevant physics.

Your theoretical ramblings have not relevance to reality
unfortunately. I've seen tables showing that a depth charge that was
effective to 15m at 20m deteriorate to 5m at 100m.

So provide a link to the tables. There are such tables in relation
to air attacks, which have everything to do with the selection of
explosion depth and its effect on kill chances, the shallower
settings resulted in more kills. I would like to see the tables as
all the standard allied depth charges had lethal distances of under
15 metres.

Also lethal distance is quite variable in real life, thanks to everything
being imperfect, from weak points in the submarine hull to non
spherical explosions. An explosion below the hull tends to be
more dangerous.

Snip

Post by Rich Rostrom
The pressure wave from the explosion is proportional
to the energy released, and is the same regardless
of depth.

An explosion consists of extremely hot expanding gases. If the gases
can not expand their thermal energy is transferred to the water as
thermal energy rather than kinetic.

The explosion creates a shock wave by displacing water with the
gasses created by the explosion. The energy can be considered
constant for a given amount of explosives.

As the explosives people know the closer to the surface
the more of the energy escapes into the atmosphere.
..
The size to the bubble created by the explosion decreases with depth
because of the water pressure, as the gasses will expand until their
pressure equals that of the surrounds. Then the air space will collapse
due to the water pressure, creating a second shock wave.

The deeper the submarine the less pressure needs to be applied
to crack the hull, as essentially the depth charge is trying to add
enough to the existing pressure to cause a rupture.

The result is when you go looking there is usually only one value
for lethal radius, no variation with depth.

Post by Rich Rostrom
High external pressure _might_ suppress the effect
of a shrapnel bomb, but it has no effect on a
concussion bomb, such as a depth charge.

Nope

The external pressure has an effect on both weapon and target.

Geoffrey Sinclair
Remove the nb for email.

Rich Rostrom

2013-03-01 05:36:17 UTC

Post by Geoffrey Sinclair

Post by e***@yahoo.com.au
Your theoretical ramblings have not relevance to reality
unfortunately. I've seen tables showing that a depth charge that was
effective to 15m at 20m deteriorate to 5m at 100m.

Indeed. There is a great deal of difference
between the percentage effectiveness of a
depth charge attack against subs at various
depths and the lethality of s depth charge
at various depths.

The former is reduced by the greater time
for the submarine to move away, putting
distance between it and the explosion.

As to the reported claim above...

The pressure at 60 meters is 85 psi.

The pressure at 100 meters is 142 psi.

The crush depth of a Type VII U-boat was less than
300 meters; the pressure there is 426 psi.

The kill is the result of hull failure under
ambient pressure plus the overpressure of the
shock wave from the explosion.

The overpressure wave expands spherically,
so its force is inversely proportional to
the square of the distance from the explosion.

If the kill radius at 60 meters is 15 meters,
then the overpressure at that distance would
be about 340 psi.

By _my_ thinklng, the kill radius at 100 meters
would then be 16 meters, where the overpressure
would be about 280 psi - 60 psi less, but made
up by the additional 60 psi of ambient pressure.

But according to these tables you mention, the
the explosion would be somehow reduced, so the
overpressure would be less than 35 psi at 15 meters,
and reach 300 psi (the kill level) only out t0
5 meters.

I can't imagine how ambient pressure could somehow
suppress an explosion. All the detonation effects
that I know of happen without regard to ambient
pressure. For instance, when the explosive "lenses"
around the plutonium core of a fission bomb
detonate - they are in contact with the plutonium,
and deform it immediately. if the bomb was 1,000
under water, it would still work.

--
The real Velvet Revolution - and the would-be hijacker.

http://originalvelvetrevolution.com

Michael Emrys

2013-03-01 06:10:48 UTC

The overpressure wave expands spherically, so its force is inversely
proportional to the square of the distance from the explosion.

Harking back to my solid geometry days, shouldn't that be inversely
proportional to the *cube* of the distance? Just wondering...

Michael

Rich Rostrom

2013-03-01 10:36:59 UTC

Post by Michael Emrys

The overpressure wave expands spherically, so its force is inversely
proportional to the square of the distance from the explosion.

Harking back to my solid geometry days, shouldn't that be inversely
proportional to the *cube* of the distance? Just wondering...

No, the square. The force is distributed
across the surface of the sphere, which
is proportional to the square of the radius,
not through the volume of the sphere, which
is proportional to the cube of the radius.

A = 4 pi R squared

(No, pie are round; cornbread are square!)

V = 4/3 pi R cubed

--
The real Velvet Revolution - and the would-be hijacker.

http://originalvelvetrevolution.com

Michael Emrys

2013-03-01 14:18:11 UTC

The force is distributed across the surface of the sphere, which is
proportional to the square of the radius, not through the volume of
the sphere, which is proportional to the cube of the radius.

Gotcha. Thanks!

Michael

Roman W

2013-03-02 15:25:16 UTC

Post by e***@yahoo.com.au
An explosion consists of extremely hot expanding gases. If the gases
can not expand their thermal energy is transferred to the water as
thermal energy rather than kinetic.

This simply absurd. An expanding gas bubble does not have time to
exchange heat with water.

RW

e***@yahoo.com.au

2013-03-02 16:41:37 UTC

Post by e***@yahoo.com.au
An explosion consists of extremely hot expanding gases. If the

gases

Post by e***@yahoo.com.au
can not expand their thermal energy is transferred to the water as
thermal energy rather than kinetic.

This simply absurd. An expanding gas bubble does not have time to
exchange heat with water.
RW

Yes it does. It is extremely hot so heat transfer is high, it is
extremely dense so the transference is high. The bubble can not
expand as fully as it does near the surface so the energy is retained
as heat an then dissipates into the soroundings via heat transfer or
expansion as it rises. Each 21 ft equals depth equals 1 atmosphere of
pressure so a submarine at 100m is at near 15 atmospheres. This means
it will achieve only 1/15th of it size. The bubble is clearly not
going to get as big as it does at say 30m.

The bubble created expands and contracts as much as 15 times creating
alternate shock waves. As it rises it also creates strong forces. If
near the surface the energy will be expended following the path of
resistance and simply blowing fountains of air into the sky.

Mario

2013-03-02 23:11:26 UTC

Post by e***@yahoo.com.au
An explosion consists of extremely hot expanding gases.
If the

gases

Post by e***@yahoo.com.au
can not expand their thermal energy is transferred to the
water as thermal energy rather than kinetic.

This simply absurd. An expanding gas bubble does not have
time to exchange heat with water.
RW

I'm afraid you are incorrect, the equivalence is
10 meters water (33 feet) per atmosphere (bar).

(One of the advantages of metric system... :-)

Post by e***@yahoo.com.au
This means
it will achieve only 1/15th of it size. The bubble is clearly
not going to get as big as it does at say 30m.

Uhm, I think that at double depth the bubble volume should be
one half... and radius 0.8 times (cubed makes 0.5)

Not sure of that... one should calculate the energy needed to
displace a sphere of water at various depths...

Post by e***@yahoo.com.au
The bubble created expands and contracts as much as 15 times
creating
alternate shock waves. As it rises it also creates strong
forces. If near the surface the energy will be expended
following the path of resistance and simply blowing fountains
of air into the sky.

Is there any (scientific) source on underwater explosion
physics?
It's an interesting (scientific) topic.

Maybe I should ask on a physics NG...

--
H

m***@netMAPSONscape.net

2013-03-03 04:47:13 UTC

Post by Mario

Post by e***@yahoo.com.au
This means
it will achieve only 1/15th of it size. The bubble is clearly
not going to get as big as it does at say 30m.

Uhm, I think that at double depth the bubble volume should be
one half... and radius 0.8 times (cubed makes 0.5)
Not sure of that... one should calculate the energy needed to
displace a sphere of water at various depths...

Is there any (scientific) source on underwater explosion
physics?
It's an interesting (scientific) topic.
Maybe I should ask on a physics NG...

Wikipedia's page on depth charges has a section on "underwater explosions" and
the reference

Jones, Charles R. (January 1978), "Weapons Effects Primer", United States Naval
Institute Proceedings

might be a good place to check.

http://en.wikipedia.org/wiki/Depth_charge

The article
http://en.wikipedia.org/wiki/Underwater_explosions
seems to be more about nuclear explosions.

Mike

Roman W

2013-03-03 01:18:34 UTC

Post by e***@yahoo.com.au
Yes it does. It is extremely hot so heat transfer is high, it is
extremely dense so the transference is high.

Do you have a source for that?

The bubble can not

Post by e***@yahoo.com.au
expand as fully as it does near the surface so the energy is

retained

Post by e***@yahoo.com.au
as heat an then dissipates into the soroundings via heat transfer or
expansion as it rises. Each 21 ft equals depth equals 1 atmosphere of
pressure so a submarine at 100m is at near 15 atmospheres. This means
it will achieve only 1/15th of it size. The bubble is clearly not
going to get as big as it does at say 30m.

But the amount of mechanical energy transferred is not measure by the
increase of bubble size, but by the work done by it on rhe water.

dW = P dV

At greater depth you have smaller dV but higher P (pressure).

RW

Bill Shatzer

2013-03-02 23:12:43 UTC

Post by e***@yahoo.com.au
An explosion consists of extremely hot expanding gases. If the

gases

Post by e***@yahoo.com.au
can not expand their thermal energy is transferred to the water as
thermal energy rather than kinetic.

This simply absurd. An expanding gas bubble does not have time to
exchange heat with water.

Where then does the energy go?

Recall the first law of thermodynamics aka the law of conservation of
energy.

Roman W

2013-03-03 01:19:16 UTC

On Sat, 02 Mar 2013 18:12:43 -0500, Bill Shatzer

Post by Bill Shatzer
Where then does the energy go?

I think it goes into shock waves, I.e. jnto mechanical work.

RW

Bill Shatzer

2013-01-14 20:57:38 UTC

Post by Padraigh ProAmerica
How close to a submarie did a depth charge have to be to inflict severe/
fatal damage?

It rather depends entirely on the depth charge and the submarine.

Obviously a 600 lb depth charge has a larger killing radius than does a
300 lb one and a submarine with heavier hull plates is more resistant to
damage than a thin-hulled one.

The Mark 6 depth charge was generally credited with a lethal radius of
about 6 meters while the larger Mark 7 was rated as having a lethal
radius of 9 to 10 meters. Both numbers were likely optimistic.

David H Thornley

2013-01-15 13:46:45 UTC

Post by Bill Shatzer
Obviously a 600 lb depth charge has a larger killing radius than does a
300 lb one and a submarine with heavier hull plates is more resistant to
damage than a thin-hulled one.

I'd suspect it also had to do with the workmanship. The explosive
force is not going to be focussed on on part of the hull, so
what will matter is how strong the weakest part is. Manufacturing
irregularities (which the late war Germans had plenty of) could
be deadly.

--
David H. Thornley | If you want my opinion, ask.
***@thornley.net | If you don't, flee.
http://www.thornley.net/~thornley/david/ | O-

Jim H.

2013-01-15 15:39:27 UTC

Post by Bill Shatzer

Post by Padraigh ProAmerica
How close to a submarie did a depth charge have to be to inflict severe/
fatal damage?

It rather depends entirely on the depth charge and the submarine.
Obviously a 600 lb depth charge has a larger killing radius than does a
300 lb one and a submarine with heavier hull plates is more resistant to
damage than a thin-hulled one.
The Mark 6 depth charge was generally credited with a lethal radius of
about 6 meters while the larger Mark 7 was rated as having a lethal
radius of 9 to 10 meters. Both numbers were likely optimistic.

I think I also read long ago that the depth of the explosion also
affected how strong it was. Greater depth and water pressure had greater
'muffling' effect on the depth charge. I'm not real clear on the
physics involved, but it sounds reasonable. And I don't know if
the effects of depth would be proportional on both the explosion's
force and the depth stress on the hull.

Jim H.

k***@cix.compulink.co.uk

2013-01-15 15:40:15 UTC