In Part 1, we examined some of the historical examples of the effects of underwater explosions from mines and torpedoes on ships. We found, as we did with our scholarly examination, that the myth about torpedoes breaking the back of ships is just that – a myth, at least for ships the size of large destroyers and above. Further, we found that even significant structural damage – significant in the sense of threatening to sink the vessel – was rare to non-existent. The next obvious question is, why? Where does this torpedo damage resistance come from? What is it about the structure of a ship that provides such resistance?
The answer is both obvious and largely unknown and unrealized, at least outside naval architect circles and possibly even within. The answer is keels. Note that the answer is in the plural – keels. Few people realize that ships have multiple “keels”. Now note the enclosure of the word in quotes, indicating that the word is not to be used literally. Huh? What are we talking about?
Ships have multiple “keels” (I’ll now stop putting the word in quotes, for ease of typing), most of which are unintended as such but are nevertheless present.
Consider … A keel, without getting too technical, is the bottommost, main structural longitudinal member of the ship. It runs the length of the ship and provides the backbone upon which all the other structural elements attach, either directly or indirectly. For this reason, the torpedo bubble crowd believed that if the keel (the ship’s “back”, like the spine of a human) were broken the ship would automatically sink.
What few people realize is that there are other longitudinal structural members in a ship that act as keels.
Armor belts on the side of a ship are complete, solid structures that run a significant length of the ship and are intimately attached to the ship’s structure. Thus, they constitute two additional keels.
Armored (or even simply thick) decks also run the length of the ship and act as longitudinal structural members or keels. There can be one or more, depending on the number of armored decks the ship has.
Some ships have longitudinal bulkheads which also act as keels.
Each of these keels has the strength to hold the ship together by itself. Thus, even in the unlikely event of the failure of one keel, the others are sufficient to protect the ship from breaking in two and sinking.
Noted naval historian Robert Lundgren discussed this phenomenon in a NavWeapons website forum topic (1). Here are some of his comments.
“A ship with a fully developed side protective system is not subject to the type of break-up a lesser vessel is due to under-keel explosions. No capital ship ever in history ever broke in half due to an under-keel explosion even when it was a nuclear explosion.”
“In a battleship, the ship has what they call a soft keel. Any longitude bulkhead that makes up over 50% of her length becomes a strength member of the hull girder. In an
example, her four bulkhead system on each side gives her eight additional
strength members and her third bulkhead is her armor belt which is extremely
difficult to place into sheer. The side protection system is so strong it can
support the weight of the ship even if the flat keel is destroyed. Each layer
of the side protective system acts as an additional keel so in an Iowa she has
8 side keels and her flat keel and she actually has three upper strength decks
with the second deck being an armored deck which is also difficult to bend. In
the roughly 2 seconds an under-keel explosion has to work on the hull the side
hinges that form on lesser ships never form on a battleship or even a fleet
aircraft carrier. Therefore, the upper strength deck or decks are never
placed in stress. What does occur is the under-bottom is either holed or
crushed in and depending on the damage will depend on the amount of flooding
just like a side hit by a torpedo. The ship will whip just like Tirpitz did but
not break up.” Iowa
“The 4,000 lb warheads under Tirpitz were roughly equal to 4 x MK 48 torpedoes or a 1,500 lb warhead detonating 50 feet under her keel. All underwater explosions work the same. So if a MK 48 1,500 lb warhead gives X amount of force at 50 feet this can equal a 4,000 lb warhead at 100 feet and the 28 kiloton nuclear warhead may be the same at 2000 feet and so on. So the distance and the amount of ocean on top of the explosion is important. Even
break up at Arkansas Bikini. She basically was flipped over and
landed upside down on an empty sea bed as all the water had been blown out of
the lagoon. Her hull was crushed when all that water came back down.
Her sides held her together while she was in mid-air and her armor is cracked
in one place near her bow but she is intact.”
There you have it. There’s the explanation (well, one of them) for the resistance of ships to underwater explosions. Additional resistance is also imparted by the numerous other shorter, smaller structural elements, all of which function to spread the stress load throughout the entire ship’s structure rather than having it concentrate in one spot. The spreading or dissipation of the stress helps to prevent structural breakage at the point of impact. We’re wandering into structural engineering, now, and that’s well beyond the scope of a simple post so we’ll leave it at that. Suffice it to say that ships have a greater inherent resistance to underwater explosions than most people realize.
This is not to say that underhull explosions are not powerful and damaging – they are and for smaller, lighter built ships they may well prove fatal. But, as we proved in our examination of the torpedo myth, and in our examination of historical data, they are not the instant death that the torpedo myth crowd believes.
This concludes our examination of the torpedo myth and puts it to rest, once and for all.
(1)NavWeaps website forum, Topic: “Threat: Torpedoes That Go Under The Keels”
31-Mar-2014, username: rlundgren,