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Diesel Electric Submarine Speed Of Advance

D/E submarine

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#1 Josh

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Posted 28 November 2012 - 1601 PM

I've taken this discussion to Warhips1 before, but those who know don't tell and those who were dong the most talking clearly didn't have any clue. I don't expect much information to come of this but thought I'd kick it out there.

Anyone know what is a typical speed that a modern D/E boat can make snorting all the way, assuming they are 'floating the load' as one individual once put it to me? Basically I'm looking for absolute upper transit speed, disregarding 'indiscretion'. Has some bearing on what countries could afford what kind of boat to deploy over what range--for instance the Soryu and Collins seem like much longer range animals than their PLAN or European cousins.

WWII Fleet boats had four diesels and a hull optimized for surface running; they could do 20 knots indefinitely. Simple back of the envelope calculations on modern European and Kilo type boats seem to indicate they couldn't do anything close to this with their power plants; a dozen knots seems as the upper limit...further more I've been told a more typical transit speed is ~6 knots, but I assume that is with occasional snorting, not constant. Without knowing more math and having values like the efficiency and hotel load there's little conclusion an amateur can deduce, though I assume anyone who operates a Kilo/212/Gotland/Song/etc has a very good idea of the limitations. They seem to be roughly in the same tonnage and power range and the non-linear relationship of power to speed (cube rate?) for a boat should make anything but a major difference in power yield a similar speed.

Another thing, does anyone know if/what is the upper limit a snorkel or other mast type could actually withstand the pressure of being raised during snort? I assume all boats have some upper limit on what masts can take what speeds without incurring damage; I can't see a nuke fan tailing water off its search scope at 30 knots even if anyone thought it was a good idea.

I know the Collins class boats were built with several diesels and a large battery bank but no AIP on the assumption (so I've heard) that AIP didn't generate the power density to really aid in transit times compared to more engines running during quicker snorts. I assume the trade off is a larger snorkel and less fuel efficiency combined with increase mechanical complexity and cost.

Edited by Josh, 28 November 2012 - 1604 PM.

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#2 RETAC21

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Posted 28 November 2012 - 1610 PM

Some time ago I found a Chilean article discussing this in the framework of their Scorpenes and the advantages provided by AIP plants. Typically the answer is "depends". If the exercise just involves going from one point to another as fast as possible, there's no need to snort (the Soviets found out is was safer to be on the surface than snorting, since on the surface SOSUS could confuse them with fishermen, but submerged there was no doubt it was a sub). Snorting implies a desire not to be found, so 12 knots sounds about right, but AIP boats are going to be much slower (IIRC around 8 knots).

In an AIP sub, the patrol would be divided among transit time (which would use the snorkel) and patrol time (which would alternate AIP with batteries). Also you need to remember that European subs don't have long transit times* to their stations but Japan and Australia are way off from possible opponents.

* but even so, the Dutch were able to perform missions in the Med during the Cold war.
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#3 Josh

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Posted 28 November 2012 - 1707 PM

Fair point that snorting might be less convenient than just running surfaced, but at least if you snort you're not a radar target for any civilian vessel, plane, or coastal installation. I think anyone with a satellite phone would tweet that pretty quickly in this day and age, with pictures to go with it on facebook. For a modern teardrop hull, form would limit a surface speed, even if available power didn't, and like I said most the power figures I've seen (admittedly, hard to tell how accurate any of them are) don't hash out to a 'snortable' flank speed unless the diesels are nearly 100% efficient and all the power is dumped into the engine.

I've heard the Soviet surface story before...I know a submerge boat would transmit more sound, but if its running on the surface I can't see how an old school passive hydrophone could differentiate any submerged quality of the sound. But its not my field of expertise.

AIP is (for most designs I've seen power figures for) 1/10th the power of the main diesel--that's got to hurt, even though lower speeds require exponentially less power, because you're still providing the same hotel load from that 10% (I assume). 8knt seems exceedingly generous.

Yes, range to your patrol area would drive this requirement. I would assume if you were in what you considered to be a non-combat zone you'd save your AIP fuel/oxdizer, whatever it is, and there after drift slowly and snort only if required because of a high speed run away from or to something. That said, I'm curious what a practical transit speed would be running at just the power level where there was no battery draw so the boat arrives 'fresh'. This seems to be the highest practical transit speed (unless surfaced--though like I said, that would only take *more* power with a modern spindle hull; the Tangos and Whiskey's probably could do better on the surface).

As noted, the JMSDF and ozzies (don't know their acronym off the top) have much larger displacement designs who's sensors and range seem much more adjusted to long range and deeper water. They seem in a completely different class than RoW D/E's.
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#4 shep854

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Posted 28 November 2012 - 1741 PM

Being optimized for submerged running, a modern sub will not only be slow on the surface, it will be a wallowing, rolling, miserable pig of a ride. Up through WWII, subs were actually surface ships that could dip underwater for short periods.

Edited by shep854, 28 November 2012 - 1742 PM.

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#5 Van Owen

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Posted 28 November 2012 - 1957 PM

Ozzies=RAN, Royal Australian Navy.
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#6 Marcello

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Posted 29 November 2012 - 0342 AM

Postwar submarine could snort at speeds of over 10 knots if I understand correctly, so the mast ability to withstand flow is not likely to be the major show stopper at any speed reasonable for modern D/E boats. However high speed snorting is bound to be a pretty unpleasant experience in anything but perfect weather, the engines are going to draw air from the interior pretty fast everytime the waves overtake the snorkel. Automatic variable height installations might help, but probably only to a point.
AIP is very useful to increase underwater endurance, but typically it means adding a third engine (in addition to diesel and electric) which takes up mass and space and becomes useless once its specialized fuel/oxydizer are exhausted. I can understand that some designers might prefer extra batteries instead.

Edited by Marcello, 29 November 2012 - 0511 AM.

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#7 RETAC21

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Posted 29 November 2012 - 0533 AM

Well, AIP is not an alternative plant in the sense that it replaces the diesel 1:1 but rather a mean to increase the discretion of the SSKs by allowing them to remain submerged and not having to make noise snorkeling while on the patrol area, but transits to and from the patrol area are performed "conventionally" (i.e. snorkeling)

Here I found the article: http://www.revistama...0/2/navarro.pdf

Googletranslating the important part:

"Considering that a typical underwater mission of a medium sumarine of about 1400 tons* of displacement is about 50 days, of which 20 days are transit (7-8 knots) with a indiscretion coefficient of 20% and another 20-25 days of patrol (4-5 knots) with an IQ 6-8%, the time a submarine is total indiscreet be set in about 135 hours (96 hours for the traffic and 39 hours for the patrol."

IQ=Indiscretion coefficient is the ratio between the time spent snorkeling over the whole time spent at sea.

* Evidently a Type 209
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#8 Josh

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Posted 29 November 2012 - 0905 AM

Being optimized for submerged running, a modern sub will not only be slow on the surface, it will be a wallowing, rolling, miserable pig of a ride. Up through WWII, subs were actually surface ships that could dip underwater for short periods.


It would be fair to say that about every sub up until the three Albacores, and most navies didn't make that switch until probably the 70's.

EDIT: some designs like the type XXI could be considered hybrids, depending on your definition, but not completely optimized for under water running.

Edited by Josh, 29 November 2012 - 0908 AM.

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#9 Archie Pellagio

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Posted 30 November 2012 - 0448 AM

While snorting on diesels or running on batteries? Snorting speed is limited due to damaging the equipment.
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#10 Josh

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Posted 30 November 2012 - 1055 AM

Snorting on diesels specifically. And yes, I assumed top speed would be somewhat limited by how much pressure you could put on the snort before you bent the pipe.
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#11 Josh

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Posted 30 November 2012 - 1454 PM

Actually now that I think about it--you wouldn't want to transit that way for two reasons--not only would the snorkel be speed limited, but your recharge rate is at its lowest when you charge it at full. So it would be much more efficient to run the batteries as low as you dared, then snort and charge up to maybe 80-90%, then repeat until you got close to a combat zone and discharge less and charge up more until you cut over to AIP. So I guess that average speed of advance is more what I'd be curious about. A post up top mentions 7-8 knots...pretty much explains why the USN never wanted D/Es ever again right there.
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#12 Heirophant

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Posted 30 November 2012 - 1551 PM

Actually now that I think about it--you wouldn't want to transit that way for two reasons--not only would the snorkel be speed limited, but your recharge rate is at its lowest when you charge it at full. So it would be much more efficient to run the batteries as low as you dared, then snort and charge up to maybe 80-90%, then repeat until you got close to a combat zone and discharge less and charge up more until you cut over to AIP. So I guess that average speed of advance is more what I'd be curious about. A post up top mentions 7-8 knots...pretty much explains why the USN never wanted D/Es ever again right there.


Diesel-Electric subs aren't about global high-speed deployment, something the USN is tasked with doing.

They are about ultra-stealth in regional scenarios. Just look at the Cold War operators of both Nuclear and Diesel-Electric subs: no, not the US, but the UK, France and the Soviets.

These nations had needs for both types: Nukes for global offensive power, and diesel-electrics for the stealth advantage in NATO vs WarPac scenarios in European, Arctic and Med waters. The D/Es cost less? That was a bonus, not the deciding factor.

The point with D/Es was, and is, that when running on batteries/AIP, they have no machinery nor water-circulation noise. That is their main advantage over nuke boats, and if the point of a sub is stealth, D/Es have that in spades.

You want to go at flank speed underwater, and stay underwater? You better invest in Nuclear subs.
You have time for a drawn-out deployment schedule? Diesel-Electrics, especially the large 3,500-tonne types, are a very viable technology. The Australian subs could operate off the African coast - but they'd need weeks to get on station.
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#13 Josh

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Posted 30 November 2012 - 1723 PM

Didn't say they weren't useful for other countries...just since the US has global commitments and home base generally means crossing an ocean, it makes sense for the USN.

That said, its not like the USN nukes managed to stay out of mischief despite their plant noise. I do wonder with the most modern silencing technologies in platforms like Virginia and Astute whether there is quite a marked difference anymore.
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#14 Marcello

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Posted 01 December 2012 - 0349 AM

Actually now that I think about it--you wouldn't want to transit that way for two reasons--not only would the snorkel be speed limited, but your recharge rate is at its lowest when you charge it at full. So it would be much more efficient to run the batteries as low as you dared, then snort and charge up to maybe 80-90%, then repeat until you got close to a combat zone and discharge less and charge up more until you cut over to AIP. So I guess that average speed of advance is more what I'd be curious about. A post up top mentions 7-8 knots...pretty much explains why the USN never wanted D/Es ever again right there.


Running the snorkel at the 10-12 knots a modern D/E can make at surface is by accounts possible with post WW2 technology.The USS Pickerel ran at an average speed of 10+ knots for three weeks in 1947 snorting all the time and I can't imagine modern masts being weaker. Yes early german snorkel were limited at 6 knots and I have seen pics of at least one of them snapped, but this was overcome. It is just not advisable under most circumstances, as it is going to be an unpleasant ride if anything else.
From what I understand the practice in the patrol zone with a non AIP D/E is to use the diesel for few minutes at once and even charge a bit less than the sub discharge, within certain parameters obviously.

The point with D/Es was, and is, that when running on batteries/AIP, they have no machinery nor water-circulation noise. That is their main advantage over nuke boats, and if the point of a sub is stealth, D/Es have that in spades.


There has been a great deal of progress in terms of plant quietening, nowadays they don't need to run circulation pumps for reactor cooling anymore for example. That said I suppose the smaller size of a D/E boat, if anything else, might still give it an edge. Also not all AIP are equal: fuel cells are probably a bit more quiet than Stirling engines.
That said it must be noted than an high end D/E can be purchased relatively freely, nuclear technology is fairly restricted. D/E also cost less per unit, which might be an advantage in some cases, though not always: certainly the USN is better off with an all nuke fleet but in some cases a larger number of conventional boats might be more useful.

Edited by Marcello, 01 December 2012 - 0709 AM.

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#15 Shortround6

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Posted 01 December 2012 - 1103 AM

A basic problem is fuel consumption. As a very rough rule you need the square of the power to go twice as fast. 12knots needs 4 times the power as 6 knots or 16 knots needs 4 times the power (fuel) as 8knots. Every day at 16 knots is worth 4 days at 8kts. You may get there faster but you can't go as far or stay as long.
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