On Design Constraints

Monday, November 3rd, 2008

Donald Pittenger shares his thoughts On Design Constraints — which he, as a humanities type, never had to consider:

People whose trade is ideas and words face far fewer and less critical constraints than, say, the designer of a battleship.

So to make matters more concrete, let’s consider some of the many constraining factors for battleship design. The last true battleship was commissioned in 1946 (HMS Vanguard), only 40 years after the completion of the first modern (all big-gun) battleship HMS Dreadnought. That’s a pretty short run, but a well-documented one. My favorite source on battleship design is [Battleship Design and Development 1905-1945] by Norman Friedman.
Friedman suggests that a good rule of thumb is that around 60 percent of the displacement of a battleship can be devoted to propulsion, armor and weapons — the remaining 40 percent is used up by the ship’s structure and all the other bits that go into it.

That weight budget for arms, armor and propulsion makes life interesting for the designer.

(The “designer” at this point isn’t necessarily a naval architect. “He” is a committee of seasoned officers who consider capabilities of proposed ships in light of operating doctrine and conditions of their own navy as well as current and likely future characteristics of potential enemy battleships. For example, during the pre-Great War naval race between Britain and Germany, German designers didn’t need to allow as much space for fuel and crew quarters as did the British because the Germans could safely assume that their ships would be fighting in the North Sea and wouldn’t have to steam very far. The British had a world-wide empire that required ships to range far from home.)

Here are the basic elements of the weight-budgeting game. The faster the ship is supposed to go, the greater the weight for propulsion systems. But that means sacrificing some armor or weaponry. Jacky Fisher, First Sea Lord during an important part of the Anglo-German arms race, very much liked the idea of fast ships. He held that a fast ship (or in practice, 3-5 ship unit) would control the initiative in battle; they could engage the enemy or break contact at will. Since he wanted these ships to be able to destroy enemy ships, armament couldn’t be stinted; what had to go was protection. So part of the British fleet in 1914 was comprised of fast, battleship-gunned but lightly armored vessels called battlecruisers. They were and remain controversial. The British lost three of them at Jutland in 1916. That might have been due to a design or procedural flaw related to ammunition handling. Battlecruisers have their defenders, but even navies with such ships had a doctrine that battlecruisers were to avoid slugging it out with actual battleships if possible.

The American navy opted for protection over speed. The idea was that one or two well-placed shells could drop the speed of a fast ship (or any ship, for that matter) and, at that point, its most important need was survivability in terms of adequate armor protection. The Germans also followed this philosophy.
Battleship designers in those days were seriously concerned about the danger posed by torpedoes launched from small, fast ships, the solution being secondary quick-firing four or five inch guns for defense. Until the war, such guns were placed in casemates strung along the upper part of the ship’s hull. But with the battleship at speed in any but the calmest seas, many of the guns couldn’t be operated dues to splashing water. The obvious solution was to mount the guns higher, on the main deck itself. But this would raise the ship’s center of gravity, making it less stable and seaworthy. After the Great War most navies plated over the casemates and did move the guns higher, but that required some compensating weight-shifting and perhaps making beams wider. (A constraint for U.S. designers was the width of Panama Canal locks. If battleships were to be quickly moved between the oceans, their beams had to be narrow enough to pass through the locks. Sizes of land-based and floating drydocks also constrained ship designers.)

Placement of armor also was constrained. Only so much of it could be carried, so the question became: What parts of the ship should be protected? Up until the very early 1900s, ships’ guns were comparatively short-ranged. This meant that the trajectory of the shell at maximum range was, comparatively again, flat and that the target ship would be struck someplace on the side of its hull or superstructure. So the sides of battleships were armored, but not the decks. By 1910 the range of big guns extended to several miles and shells at such ranges plunged onto decks and upper surfaces of superstructures. This meant that placement of armor had to be reallocated. The ultimate American solution, one that was later accepted by other navies, was “all-or-nothing” protection. Rather than putting armor plating along the entire length of the hull, the U.S. Navy started building battleships with armor concentrated to protect vital parts. Such parts include: magazines, turrets and ammunition-handling systems; smoke uptakes from the engine rooms and the rooms themselves; control stations, the conning tower especially; and enough of the hull to retain buoyancy. On the other hand, bow areas were unprotected.

That last bit about “all or nothing” protection reminds me of one of my favorite operations research anecdotes.

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