Ship Superstructures
I’ve noted, over the years, the Navy’s trend toward larger and larger superstructures. Much of this is due to stealth considerations but not all. Let’s take a look at the trend and then we'll discuss the implications.
WWII early war Gato submarines had fairly large superstructures. As the war progressed, these were steadily cut down until only the barest structure remained. This was done to reduce silhouettes and the concomitant chance of visual detection. Look at the Gato profiles below and note that the superstructure was reduced by around 50% which significantly lowered the profile and reduced the total visible superstructure bulk.
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| Gato - Early War |
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| Gato - Late War |
WWII ships in general and the Fletcher class destroyer, specifically, had narrow, small superstructures. In the drawings below, note the relatively wide deck areas on both sides of the superstructure, stretching the length of the ship. The superstructure was around 50% of the hull width for much of the length of the superstructure, widening out to around 80% at the forward end. Also note that the height of the superstructure was fairly short. Combined with a low lying hull, the overall profile was quite short.
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| Fletcher DD |
Note the very small superstructure on the Baltimore class cruiser shown below. On a relative basis, there is more deck space than on the Fletchers. Note that the available horizontal deck space allows the placement of large numbers of weapons. There is also a large section amidships that has no superstructure!
Now, let's take a look at a modern destroyer, the Burke class DDG. In contrast to the WWII designs, note that the superstructure is massively large and in most areas spans the width of the ship. Excluding the flight deck which is not usable space, the only available large deck space is the bow area or top of the hangar which is only usable for equipment if there is no deck penetration.
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| Baltimore Class CA |
Now, let's take a look at a modern destroyer, the Burke class DDG. In contrast to the WWII designs, note that the superstructure is massively large and in most areas spans the width of the ship. Excluding the flight deck which is not usable space, the only available large deck space is the bow area or top of the hangar which is only usable for equipment if there is no deck penetration.
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| Burke Class DDG |
Look at the LCS. Note that the stealthy, slanted superstructure spans the entire width of the hull and covers the deck from just behind the forward gun, all the way aft to the hangar. There is no horizontal deck space in the area of the superstructure. Also, note the relative height of the superstructure compared to the overall height of the ship from the bottom of the hull to the top of the superstructure. Finally, note the width of the superstruture even at the very top. It's still quite wide at around 80% of the width of the hull. That's a lot of weight to carry quite high on the ship. The bulky superstructure also makes the ship quite visible.
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| Freedom Class LCS |
Even with stealth shaping, size still equates to radar signature. The smaller the superstructure, the smaller the radar signature for a given shape. Just as WWII sailors understood that a smaller superstructure translated to a smaller visual signature, so too does a smaller superstructure, today, translate to a smaller radar signature.
Another issue with large superstructures is top weight and stability. A large superstructure means a higher proportion of weight higher up which negatively impacts stability margins.(distance the vertical center of gravity can shift in response to weight growth before stability is compromised). Typical stability margins range from 0.3 m for amphibious ships to 0.8 m for carriers. The LCS, for example, with its overly large superstructure, has a stability margin of 0.15 m - a very low value compared to other classes (1).
Another aspect of superstructure size is its impact on deck space and deck working space. If the superstructure gets too large, the available horizontal deck space for mounting guns, boat cranes and storage, underway replenishment equipment, etc. becomes very limited. Similarly, limited deck space impacts the working space for the crew, be it line handling, weapons operation, resupply, boat handling, etc. Take a close look at the Freedom LCS. It has very little usable deck space relative to its size.
WWII ships had plenty of horizontal deck space and mounted large numbers of weapons and equipment. With modern slanted superstructures, deck space is at a premium and negatively impacts the number of weapons a ship can carry. Often, it is necessary to carve out platforms higher up or on top of the superstructure which, again, impacts stability.
Take a look at the LCS, for example. The weapon pits, 30 mm gun mounts, etc. are at the top of the superstructure. The problem with this is that it places the weight high up which makes the ship top heavy. People talk about adding weapons to the LCS to make it more useful but such discussions overlook the fact that adding such weapons would be difficult due to the lack of deck space. Mounting the weapons on top of the superstructure worsens the already marginal stability.
The Burke has a VLS cluster mounted on top of the hangar due to the lack of main deck space and that elevated mounting negatively impacts the stability.
The modern trend of larger superstructures also results in more functions being placed above the main deck, in less armored (to the extent that anything is armored on modern ships) and less protected spaces. Remember, unlike torpedoes, anti-ship missiles tend to hit the superstructure. We are going to lose combat functionality by having more of it "exposed" in the superstructure rather than buried in the hull where armor, tanks, and void spaces help provide a measure of protection.
The odd part about the entire trend towards slanted, stealthy superstructures that span the width of the ship is that I've seen no evidence that narrower superstructures with more exposed deck space are any less stealthy. I've read that vertical sides and bulkheads generate a significant radar return but I've never read a word suggesting that horizontal deck space generates a significant return. If the sending/receiving radar were positioned directly overhead then the deck would constitute a perpendicular surface and would generate a large return but any other angle will just scatter the radar wave away from the sending/receiving unit and if the unit is directly overhead then the ship has already been spotted!
Admittedly, my understanding of ship radar stealth is rudimentary, at best. However, until someone can demonstrate why a horizontal deck surface is bad, I've got believe that the benefits of additional deck space far outweight any supposed gain in stealth and the additional drawbacks to a slanted superstructure, such as top heaviness and lack of weapons mounting space, further reinforce that belief. We need to re-examine the entire ship stealth concept as it relates to equipment, weapons, and sensor mounting.
Another aspect of superstructure size is its impact on deck space and deck working space. If the superstructure gets too large, the available horizontal deck space for mounting guns, boat cranes and storage, underway replenishment equipment, etc. becomes very limited. Similarly, limited deck space impacts the working space for the crew, be it line handling, weapons operation, resupply, boat handling, etc. Take a close look at the Freedom LCS. It has very little usable deck space relative to its size.
WWII ships had plenty of horizontal deck space and mounted large numbers of weapons and equipment. With modern slanted superstructures, deck space is at a premium and negatively impacts the number of weapons a ship can carry. Often, it is necessary to carve out platforms higher up or on top of the superstructure which, again, impacts stability.
Take a look at the LCS, for example. The weapon pits, 30 mm gun mounts, etc. are at the top of the superstructure. The problem with this is that it places the weight high up which makes the ship top heavy. People talk about adding weapons to the LCS to make it more useful but such discussions overlook the fact that adding such weapons would be difficult due to the lack of deck space. Mounting the weapons on top of the superstructure worsens the already marginal stability.
The Burke has a VLS cluster mounted on top of the hangar due to the lack of main deck space and that elevated mounting negatively impacts the stability.
The modern trend of larger superstructures also results in more functions being placed above the main deck, in less armored (to the extent that anything is armored on modern ships) and less protected spaces. Remember, unlike torpedoes, anti-ship missiles tend to hit the superstructure. We are going to lose combat functionality by having more of it "exposed" in the superstructure rather than buried in the hull where armor, tanks, and void spaces help provide a measure of protection.
The odd part about the entire trend towards slanted, stealthy superstructures that span the width of the ship is that I've seen no evidence that narrower superstructures with more exposed deck space are any less stealthy. I've read that vertical sides and bulkheads generate a significant radar return but I've never read a word suggesting that horizontal deck space generates a significant return. If the sending/receiving radar were positioned directly overhead then the deck would constitute a perpendicular surface and would generate a large return but any other angle will just scatter the radar wave away from the sending/receiving unit and if the unit is directly overhead then the ship has already been spotted!
Admittedly, my understanding of ship radar stealth is rudimentary, at best. However, until someone can demonstrate why a horizontal deck surface is bad, I've got believe that the benefits of additional deck space far outweight any supposed gain in stealth and the additional drawbacks to a slanted superstructure, such as top heaviness and lack of weapons mounting space, further reinforce that belief. We need to re-examine the entire ship stealth concept as it relates to equipment, weapons, and sensor mounting.
(1) Government Accountability Office, "LITTORAL COMBAT SHIP - Additional Testing and Improved Weight Management Needed Prior to Further Investments", GAO-14-749, July 2014
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