Monday, July 16, 2018

Ship Stealth and Visby

Naval warship stealth is one of the most difficult things to find any data on.  The best information in the public domain consists of nothing more than vague statements like, “smaller radar cross section than a fishing boat”.

Ship stealth consists of more than just radar signature.  Stealth encompasses various aspects, including:

  • Radar
  • Infrared
  • Magnetic
  • Acoustic
  • Wake
  • Visible Light
  • Electromagnetic
One of the few vessels that claims to be stealthy and has even a bit of descriptive information is the Saab/Kockums Visby class corvette.

The Visby class corvette is claimed to be the first vessel in the world to have fully developed stealth technology.

“The outstanding stealth properties fundamentally change the ship's survivability and improve its mission effectiveness.” (1)

“Flat surfaces and concealed equipment reduce radar signature to a minimum. The hull is designed on stealth principles with large flat angled surfaces. Every feature that need not necessarily be located outside the hull has been built in or concealed under specially designed hatches.” (1)

The concealed nature of much of the ship’s equipment is a noteworthy aspect.  It has been used on other ships but seldom to the degree that the Visby does.

Naval Technology website quantifies the radar detection range of the Visby.

“A stealth corvette of the [Visby] design has a detection range of 13km in rough seas and 22km in calm sea without jamming. In a jammed environment, the Visby would be detected at a range of 8km in rough sea and 11km in calm sea.” (2)

Saab suggests that the vessels small radar cross section (RCS) is due, in part, to the material of construction.

“The vessel is built of sandwich-construction carbon fiber reinforced plastic (CFRP). The material provides high strength and rigidity, low weight, good shock resistance, low radar signature and low magnetic signature. The material dramatically reduces the structural weight (typically 50% of a conventional steel hull). This results in a higher payload carrying capability, higher speed or longer range.” (1)

The carbon fiber material has the added benefit that it conducts electricity and functions to shield radio signals from the ship. (3)

It remains to be seen how the material behaves in combat damage situations.  Does it fracture, does it produce toxic fumes when burned, does it propagate fire, can it be repaired at sea, etc.?

Visby Class Corvette


Other, seemingly minor, aspects of the ship’s radar signature have been minimized.

“Many less noticeable aspects include the coating of the bridge windows with indium tin oxide and gold which prevents radar returns from objects inside. Hatches and vents are also adapted with things such as a honey comb grid which also prevents returns, while radar absorbent materials have also been added in selected areas. The antennas are retractable into the hull as are the floodlight and fog horn, and even the navigation lights have been adapted to not stand out. They are fitted with quadruple redundant LED lights to avoid giving off a heat signature which brings us to the next point.” (3)

“The paint of the ship’s hull has also been specifically developed to reduce the IR signature.” (3)

Radar cross section is not the only stealthy aspect of the ship’s design.  The turbine exhausts have been relocated from the traditional upper superstructure location to the stern and use water injection to lower the exit temperature.

“The gas turbine exhausts have been concealed in hidden outlets close to the water surface at the stern of the vessel.” (1)

“… the exhaust is water injected which brings down the temperature to just above room temperature before the exhaust exits the hull.” (3)

One can’t help but wonder if the location of the exhausts to the stern is a wise idea.  If a missile does manage to home in on the exhaust, it will hit the stern and cause almost certain loss of propulsion and probably fatal flooding.  On the other hand, if the location combined with the water injection are sufficiently effective then a major heat source has been nearly eliminated.  Unfortunately, I have been unable to find any actual data regarding the thermal reduction effectiveness.

Magnetic signature has also been carefully addressed via an active, dynamically adjusted degaussing system.

“…  the ship has been fitted with an extensive degaussing equipment which renders it without a magnetic signature detectable by a mine. This is achieved by fitting degaussing coils throughout the ship which compensates for the magnetic signature of the on-board equipment. These are laid out in an X, Y and Z grid – the equipment also measures the earth’s magnetic field and compensates for this by adjusting the power to the degaussing system. The larger coils can however only manage the ship as a whole. There are specific steel objects that needs separate degaussing setups fitted. Engines, generators and gearboxes among other things.” (3)

The salient point to all of this discussion is the attention paid to every aspect of the ship’s stealth and the degree to which stealth was pursued.  The American Burke or LCS classes, by comparison, have only rudimentary stealth measures applied, mainly in the form of structural shaping (sloped sides).

Saab presents a fascinating series of photos related to warship stealth which illustrate their claims about ship stealth.

Here is a photo of a U.S. F-117 stealth fighter.  Note the flat, angular panels and lack of protuberances.


Now, here's a photo of the Visby.  Again, note the flat, angular panels and lack of protuberances.  The similarity to the F-117 is striking.



Finally, here is a photo of a U.S. LCS (Freedom class).  The Navy claims that the ship is stealthy but note the many protuberances, round domes, bits of deck equipment, exposed electrical junction boxes, antennae, etc.





Compare the number of projecting bits of equipment in the bow-on photo of the LCS versus the bow shot of the Visby.  It’s clear which one was serious about signature reduction!

Now, the key question is whether the extreme stealth measures of the Visby are worth it.  For example, if the Burke and LCS can’t be detected until the searching platform is within visual range then any further signature reduction – ala the Visby – serves no practical purpose.  Common sense would suggest that the Visby’s extra stealth measures are worthwhile but without data there is just no way to know.

The related question is what impact the Visby’s stealth measures have on the various ship’s system performances.  If the price of some extra stealth is poorer sensor performance, reduced weapon effectiveness, greater difficulty shiphandling, more mechanical problems associated with hidden doors and such, less efficient propulsion performance, etc. then these negative impacts would have to be weighed against the benefits of some incremental improvement in stealth.  Again, without data we just can’t make any judgments.

Regardless, it’s fascinating to see what a maximum, operational, stealth ship looks like and how it’s designed.



____________________________________

(1)Saab website,

(2)Naval Technology website,

(3)Task Force 72 website, “The Swedish Visby Class Corvette”, Craig Taylor, 18-Nov-2017,




47 comments:

  1. Whatever the flaws of the Zumwalt may be, it definitely takes radar and acoustic stealth seriously to the the level of the Visby, if not beyond. Granted, it is a far larger hull, and there have been major trade-offs late in the development process regarding cost and RCS, but hopefully it delivers a high level of capability when they get the gun sorted out.

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    1. Do you have any evidence, whatsoever, that the Zumwalt has any significant radar or acoustic signature reduction beyond the vague "small as a fishing boat" type statements?

      I have serious doubts about the stealth aspect. The giant, sloped, flat panels make enormous radar reflectors if the radar sensor is at an angle perpendicular to the panels. If the panels are, say, 20 degrees sloped from the vertical, then a radar at 70 degrees relative to the panel would have a perpendicular return. An airborne (aircraft or missile seeker) radar would fit that profile.

      The point is that aside from vague claims by the Navy, I'm unaware of any test data to support the claims. The Navy's record regarding the veracity of their claims is pretty poor, as you know. I'm pretty sure they've never done full ship testing. I would hope that they did some scale testing.

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    2. It's also interesting to note that the Navy abandoned the "stealthy" composite superstructure on the last ship in favor of a steel one. The Navy acknowledged it would increase the radar cross section but, obviously, deemed it worthwhile. I don't know if that is telling us that the choice of material doesn't contribute that much to stealth or if the overall RCS isn't affected that much by the superstructure or if the Navy simply deems stealth not to be worth the added cost?

      Aside from basic structural shaping, the Navy seems pretty ambivalent about ship stealth.

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    3. The AESD Sea Jet program which superficially resembles the Zumwalt, although far smaller in scale, should have been a testing platform for the stealth feature.

      There are constant references in navy literature regarding acoustic stealth comparable to a Virginia Class SSN, which should be the gold standard, if the actual hardware is performing to spec. Presumably real data from trials is classified.

      Clearly the Navy has such faith in its stealth that they omitted Phalanx CIWS and SEARAM entirely. They only put in a couple of 30mm Bushmaster chainguns, which don't seem that useful against a modern AShM. I don't see why they couldn't have put a few SEARAM or Phalanx modules behind sliding doors in the stealthy deckhouse, but the entire acquisition process for the Zumwalts has been hard to comprehend.

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    4. "constant references in navy literature regarding acoustic stealth comparable to a Virginia Class SSN"

      That may be true. On the other hand, there were, and still are, constant references from Navy officials about the LCS dominating the littorals and, well ... none of that has panned out so I don't really have much faith in vague, official statements. They might be true but history strongly suggests not.

      "Clearly the Navy has such faith in its stealth that they omitted Phalanx CIWS and SEARAM entirely. ... I don't see why they couldn't have put a few SEARAM or Phalanx modules behind sliding doors ..."

      I have never heard a good explanation for those omissions. No ship is invisible to radar, as you know. At some point, the Zumwalt will be detected and then a point defense weapon would seem mandatory. There are also optical and IR guided missiles for which RCS reduction is meaningless and would, again, suggest the need for point defenses.

      Truly baffling.

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    5. The more alarming possibility is that the Navy knows that our point defense is almost hopeless against saturation attacks of modern anti-ship missiles and can only expect to knock maybe one or two bogies per salvo out of the sky. In a peer-state A2AD zone, it means certain death once detected and launched against, unless you're covered by several Aegis ships operating together at full capability. Since the Zumwalt's stealth implies that it would be operating away from the fleet to some degree, if it gets detected and targeted, it would be done for anyways. Perhaps if CG(X) had been funded as a stealthy fleet air defense ship, the Zumwalt might have worked well together with the CG(X) as complementary pairs, but that's only a dream now.

      The problem with Navy officials is that some of them are competent warfighters who know what they're doing, and others aren't. At this point we agree that everyone who defends the LCS is desperate and wrong, but officers attached to the Virginia SSNs seem accurate and cogent. Hopefully the team responsible for acoustic stealth for the Zumwalt was drawn from the submarine community.

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    6. The Zumwalt's VLS cells have bigger volume than those currently in use right? So why don't they start invent new missiles for them .

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    7. "why don't they start invent new missiles for them ."

      That has been a mystery from day one. The Mk57 peripheral VLS cells are only slightly larger than the standard Mk41. The gain was minimal. The cells never made any sense other than the ability to space them around the periphery which is also a dubious benefit since the arrangement ensures that any hull hit will hit at least some of the cells.

      There are many aspects of the Zumwalt design that make little sense.

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    8. The biggest thing I like about the Zumwalt is the hybrid electric drive. I liked it in the Standards that had it and I think it has benefits now. But it has to be worked out. So, let's use the Zumwalts as test ships.

      But... at their cost....

      I'm grasping for straws at finding something positive. The gun was a disaster. The stealth I don't know much about, other than that they put radar reflectors on it at one point during testing to obscure how stealthy it was. The lack of anti air is a mess. The Radar is a compromise. The price is a disaster....

      The only thing I can think to do with them is to test all of our whacky crazy ideas on them. Maybe having them as floating test ships with plenty of power will allow the Navy to test things in real life first before designing a class of ships around experimental technology (Like the Zumwalt. And the Ford. And to a certain extent the LCS...)

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  2. USN experimented with stealth 30 years ago - Sea Shadow.

    But why they did not produce a operational stealth vessel of any size until now nobody knows.

    https://www.lockheedmartin.com/en-us/news/features/history/sea-shadow.html

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    1. "why they did not produce a operational stealth vessel of any size until now nobody knows."

      One reasonable explanation is that they determined that it wasn't possible to provide sufficient stealth for a large size vessel to warrant the effort and functional tradeoffs. I have no idea whether that's the case but it's a reasonable inference.

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    2. One trade off with a stealth ship is that active radars need to be turned off/kept to a minimum. While a home-waters defense corvette like the Visby ban live off of offboard sensors/setting ambushes out of a fjord, an expeditionary, multi-role deep water combatant needs to be able to see. Spending all of the money/opportunity cost to make a ship stealthy and then putting an AMDR on it that can be seen from Mars when its turned on isn't a good value proposition.

      In a more diverse fleet with dedicated surface warfare/strike combatants that didn't have to do duty on ABM or fleet air defense missions, it might start to make more sense. Then some sort of dedicated low probability of intercept surface radar could be used to help the ship avoid threats, penetrate defended space, and strike whatever needed striking (ships or fixed targets).

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    3. In the 90s, there wasn't really a credible threat worth being stealthy about. A resurgent Russia, an increasingly belligerent Iran, and an alarmingly well-equipped Commie China are distinctly 2010s-era threats.

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    4. I would argue that the Russian and Chinese threats were evident before then but that's a side issue. Given that we completely recognize the threats today, one can't help but wonder why the new, not yet built frigate won't be much stealthier than it apparently will be.

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    5. In order for a much stealthier frigate to be designed, the Navy would have to use its least favorite 4-letter-word: compromise.

      They want it all in a small package, and that doesnt compute with all aspect stealth, something I am sure a large percentage of the fossils currently calling the shots dont believe is needed.

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    6. Stealth seems to make more sense for surface combatants that are designed to a CONOPs that has them stationed independently of carrier battle groups. There's no way you can feasibly make a Nimitz or Ford stealthy, and so you suffer diminishing returns on investment if you build a stealthy ship that's designed to escort a non-stealthy one.

      The Visby, and to a lesser extent the Zumwalt and LCS fit that role, but an American frigate designed to provide ASW for carrier groups doesn't need that as much.

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    7. That's a really good point. A carrier group is always going to be using aircraft for forward warning and fire control and will be located by those same aircraft. Stealth would only make sense for a ship operating solo with an AB or doing something specops.

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    8. "Stealth seems to make more sense for surface combatants"

      You should reread the "Stealth For Dummies" post in the list of top posts at the top of the page. It explains why stealth is good for any ship, not just those operating independently.

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    9. Remember everyone
      Stealth isnt a cloaking field
      It alters the probability and range of detection.
      An enemy MRSA might ping a Tico at 200km and get a good guess on range, speed, heading and class.
      It might ping a "stealth cruiser" at 200km and see nothing but ocean, or indeterminate, that is, theres something somewhere but we don't know what, where, where its heading or how fast.

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  3. BTW the French La Fayette class frigate could be considered first generation "stealth" ship.
    But i've also did not find any particular data on the LO characteristics of that vessel.

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  4. And another note - maybe LO or VLO surface vessels cannot fit in existing USN doctrine. They fit very good in pure defensive doctrines such as those of Sweden and Taiwan for example.
    Nevertheless all new corvette and frigate designs have some kind of LO features build in to them.

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  5. By now we should be able to design a ship that varies the reflecting angles of the side panels to some degree based on the location of the searching aircraft/vessel.

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    1. Now that's a truly interesting thought!

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    2. It's called "changing course".

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    3. one thing, just from the eyeball view, is that it seems that seaborn stealth might be different from what's happening in the air. For all of it's failings the F-35 is supposed to have good radar stealth. I believe superior to the F-117. But it doesn't look anything like the F-117. Neither does the F-22.

      IIRC they have moved beyond the 'hopeless diamond' shape in the air.

      I'll try to do some more research.

      Maybe it's good enough for the sea, especially with surrounding wave clutter, and easier to do.

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  6. We spend a lot of this type development. IMO, unwisely w/respect to what we can expect with corresponding radar/EO-IR developments. The 3rd world and their older sensors are NOT what we should be considering when developing this stuff... It seems we can't stay ahead except by sacrificing "performance"- speed, maneuverability, payload, range, lethality, etc.

    The only PROVEN WARSHIP with stealth is the submarine..after it dives...Why not more of those?

    b2

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    1. Because they are expensive, poorly armed, mostly blind, and are hard, not impossible to find

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    2. This comment has been removed by the author.

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  7. While I have no technical information to contribute, I can provide an actual example of carbon made ship burning.

    In late 2012, the indonesian corvette Klewang Class ship burnt down.

    http://www.navyrecognition.com/index.php/news/defence-news/year-2012-news/september/631-fire-destroys-brand-new-north-sea-boats-63m-stealth-fast-missile-patrol-vessel.html

    As it has been a few years since I read this, I have this nagging feeling a contributing factor to the ship completely being destroyed was some failure with the fire suppression system onboard, but I can't find the link, so don't quote me.

    Andrew

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  8. Zumwalt is an example of what happens when you optimise the ship for LO characteristics. That tumblehome hullform is there purely for that reason and one of the consequences of that is that you end up with a much larger ship to recover the stability performance, which is detrimentally affected by the tumblehome.

    It's also worth remembering that LO does not necessarily equate to "invisible". As someone noted above surface warships have several functions / characteristics that make them detectable across sensor spectra. However, what you can do, is increase the contrast between your ship signature and any decoys / CM you're carrying. That's where marginal reductions in signature can have real beneficial effects. But - as ever - it's an ongoing battle twixt sensor and signature/decoy.

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    1. As best I understand it, the ship's size was determined largely by the AGS gun system and, more specifically, the fully automated ammo handling.

      The hull form was partly due to LO requirements but did not need to be a tumblehome design. An outward, instead of inward, hull slant would have worked just as well and provided more seaworthiness. Again, my understanding of the tumblehome design was to facilitate semi-submergence which, I guess, could be considered an aspect of LO.

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    2. The AGS and ammo handling takes up a fair amount of volume, but relatively little weight. That tends to mean you want to maximise the volume of your hull (which a tumblehome doesn't do). What a tumblehome does do is reduce the likelihood of surface ducting returns for inbound ASM. It also reduces the righting moment of the ship in stability terms. The way you fix that is by increasing both the waterplane area (more beam) and hence increasing the metacentric height and possibly the centre of buoyancy. Where that leads you, (inexorably) is a larger ship.

      The semi-sub bit is a red-herring. To get any meaningful submergence (lets say six feet for sake of argument) you have to provide for a lot of water ballast. 100 tons of waterballast gets you about an inch submergence, so you're looking at north of 8000 tons of water to sink 6 feet (less than one deck height). The volume to accommodate that, simply isn't there in that hull, let alone the pumps to remove it.

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    3. "meaningful submergence"

      I've never been able to find any definitive information about the Zumwalt's semi-submergence capability, if it even made it into the final design and build. If you have some actual information, please share it.

      I'd like to know whether the capability exists, what the design submergence is, how it's accomplished, how it was intended to be used tactically, and so on.

      I've seen a few cross sectional construction photos and none seem to show anything that looked like a ballast tank.

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    4. "What a tumblehome does do is reduce the likelihood of surface ducting returns for inbound ASM."

      Radar ducting, which is what I assume you're referring to, results from certain temperature and moisture boundary conditions, the upper bound of which is found around a hundred feet up or so. Assuming I've got that basic concept correct (I'm not a radar or atmospheric expert, by any means), I don't see how a tumblehome hull reduces the likelihood of ducting. Perhaps you'll explain and offer a source reference?

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    5. Zumwalt has no semi-submergence capability. At all. its an impractical idea. That's the entire point of my post above. Semi-submergence was an idea you postulated above.


      You won't find an open source one on ducting anywhere. For good reason.

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    6. "Zumwalt has no semi-submergence capability. At all. ... Semi-submergence was an idea you postulated above."

      From a 2012 DoDBuzz article,

      "As it dominates and attacks, DDG 1000 will do a trick that none of today's surface combatants can do -- ballast down to reduce its profile above the surface, which is why the Navy used to list a "battle displacement" separate from its normal one. Downey cautioned that this was as much to help with stability for its guns as to reduce its radar cross section, but he said the ship's waterline would go up by as much as a meter when it was crouching."

      Reference: Zumwalt

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    7. "You won't find an open source one on ducting anywhere. For good reason."

      So, your knowledge comes from what source?

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    8. "Zumwalt has no semi-submergence capability. At all. ... Semi-submergence was an idea you postulated above."

      Here's another article and quote from a 2013 article,

      "tumblehome designs are notoriously unstable when quickly turning and when firing ordnance. To counter that effect, Zumwalts can release ballast and sink lower into the ocean when the going gets rough."

      Reference: Zumwalt

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    9. "You won't find an open source one on ducting anywhere."

      There are lots of articles on radar ducting. There are none that I know of that relate tumblehome hulls to radar ducting. As best I can tell, there is no relationship. Unless you can offer a source, I've got to evaluate this as incorrect.

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    10. Not entirely sure how "releasing ballast and sinking lower in the ocean" is supposed to be physically possible. Mr Archimedes will probably object, so I'd query the veracity of that argument /article. The other article also suggests that ballast is required to counter the effects of tumblehome (stability in turn, stable gunfire platform), rather than as an enabler to those. Ergo, the tumblehome hull is there for another reason - LO as I originally said.

      As for open sources on ducting - there are lots of papers on the effects of ducting on ship mounted radars. There are few if any (open source) on the effects of ducting on ASM seekers.

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    11. Given that you are on the semi submersible topic, I wonder whether a semisubmersible ship is not a possibilty. Leave a relatively small stealthy section above water with bridge and sensors and submerge the bulk of the ship ie vls etc underwater. The old oberon class submarines would have a hurricane bow and cruise on the surface. All that would appear above water would be a triangular section.

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    12. My understanding is the size of the Zumwalt, now quoted as 15,612 tons FLD, was partly due the requirement of the tumblehome hull as has had to be designed with a very large reserve of buoyancy to so as to accommodate large counter flooding tanks of unknown size with powerful pumps and that increases the ship's tonnage as well as the AGS system, guns and magazines. Navy never disclosed any figures.

      The Zumwalt's design comes with its 'stealth' flat inward sloping sides, tumblehome hull, with the long, wave piercing bow to slice through the sea. The problem is that as a ship pitches and heaves at sea the righting force decreases as the ship heels with no buoyancy from normal flared bow to make the ship come back up and with the waves coming from stern ship pitches down and can lose transverse stability as the stern comes out of the water and capsize.

      To make a tumblehome ship stable it has to be very stiff, with a very low centre of gravity, rolls very short and rapid, need to be damped with stabilizers, though if Zumwalt damaged in action the tumblehome looses stability as it lists, collision, mines, torpedo, ship will roll over and sink very quickly if not for the large counter flooding tanks.

      https://youtu.be/R-s3S3F8Mao

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    13. "Mr Archimedes will probably object, so I'd query the veracity of that argument /article."

      Construction on Zumwalt began in 2008 so basic design was largely completed at that point. The articles were written in 2012+, so 3-4+ years after construction began the semi-submergence feature was being discussed. This suggests that the feature made it into production, however, that is still an open question in my mind.

      The main point is that the article authors didn't just make this up. It was a real ship feature.

      Given that you were totally unaware of the existence of such a feature, and believed I was making it up, I'm far more inclined to doubt you rather than the article's authors.

      Regarding ducting, I've given you the opportunity to cite sources that support your contention that a tumblehome hull somehow decreases radar ducting and you've been unable to provide any. It seems clear that you were engaged in unsupported speculation and that, quite likely, a tumblehome hull does nothing to decrease ducting - it may reduce the degree of reflected energy, as any sloped surface would, but there is no evidence that tumblehome impacts ducting.

      I have no problem with you making speculative remarks. If you have a theory about tumblehome and ducting, feel free to share it but clearly state that it is speculation and do not try to pass it off as fact. This blog is based on fact and logic. If you have such a theory, I'd love to hear it.

      You are zero for two on semi-submergence and ducting. Please do more research for your comments and be sure you have facts to support your statements.

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    14. "I wonder whether a semisubmersible ship is not a possibilty."

      There have been conceptual designs along those lines from time to time though none of any significant size has ever been built that I'm aware of. Russia and NKorea have built small patrol boat size semi-submersibles, I believe.

      The old submarines were, of course, surface ships that could briefly submerge although they were not optimized for surface warfare and surface operations. They do, however, illustrate the challenge in such a concept. If too much is submerged then the surface capabilities are reduced.

      A WWII Gato, for example, was only around 60 ft shorter than a Freedom class LCS but had half the beam and even less deck width. That narrowness significantly reduced the ability to mount deck equipment and weapons. Imagine the LCS with a deck that was only about 15 ft wide!

      Many of the old ironclads were semi-submerged. The Monitor is the classic example with its deck awash and just the gun turret sticking up.

      So, yes, it's been done on patrol boats, conceptualized for larger ships, but never built on a larger ship, as far as I know.

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    15. Aside from being disrespectful, you've offered zero evidence to back up your statements. For these reasons, I've deleted your comment.

      If you'd care to provide sources and data, feel free to repost - politely.

      Delete
    16. I've deleted another of your comments for same previous reasons.

      If you want to make your case, do so with facts. Show some references. Show some calculations.

      You saying that every other author is wrong and that you are right is not exactly proof and given that you didn't even know the Zumwalt had the capability (if it does) and your incorrect statements about tumblehome hulls and ducting, your credibiilty is low. I'll believe the articles I've read.

      Argue with data and calculations, not insults. Failing that, remain quiet.

      Delete
  9. I agree with all of your points. But the Zumwalt for example is bloody huge! It’s about the length of a Deutschland class battlecruiser! What’s there to stop it from being spotted from enemy fast movers or due to its large size causing torpedos to send to the bottom several billion in USD. And speaking of supposed stealth, what size fishing boat are we talking about? A pleasure craft? A rowboat? Or one of those Deadliest Catch sized fishing vessels.

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