While we've examined the history of battleship propulsion, from the dawn of steam through turbines and oil and the introduction of gearing, it's now time to examine the pinnacle of the art, the plant built for the Iowa class. Going into (possibly excessive) depth on this system will make it a lot easier to understand the nuts and bolts of steam propulsion, as well as giving me a chance to showcase a part of Iowa very few visitors get to see.

Jim Pobog explaining the boiler control system¹

To propel the 53,900 tons of battleship at 32.5 kts required 212,000 HP, produced by 4,444 tons of machinery.² Iowa's machinery is arranged in four boiler rooms and four engine rooms, alternating in the space between Turrets II and III. Each boiler room contains two Babcock & Wilcox M-type water-tube boilers producing steam at 600 psi and 850 F. It can be divided into waterside and fireside, and we'll look at waterside first. The feedwater enters the boiler and first passes through the economizer, which is a heat exchanger in the boiler exhaust, to get as much heat out of the exhaust gasses as possible. It then goes into the steam drum at the top of the boiler. From there, the downcomers route it into the water drums at the bottom, where it enters the steam tubes that take it back to the steam drum. It is in these tubes that most of the steam is generated. As it leaves the steam tubes, the mix of steam and water is at about 485°F, and moisture separators return any remaining liquid water to the steam drum. The steam goes into the superheater, where it is heated to the final temperature of 850°F and sent to the turbines. The water level in the plant is controlled manually. A boiler technician, universally known as a BT, was stationed near the steam drum. His job was to make sure that the water didn't get too high and flow over into the turbines, or too low, which would make the boiler melt.

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The following is from Jim Pobog, the tour department lead on the Iowa. He was a boiler technician on the oiler Mispillion off Vietnam, and has kindly given me permission to post some of his sea stories here. Here's one in honor of our recent discussions of engineering. Some of the terminology here is best understood after reading Propulsion Part 4.

USS Mispillion (AO-105) off Vietnam

One of the interesting effects of being at sea for long periods of time was that it was easy to lose track of what day it was. The repeated routine day after day led you to doing things on sort of an autopilot, never expecting variation.

At one of these times I was on watch in the fire room in the middle of the night. We were just station keeping, sailing in those big circles, back and forth up and down the coast of North Vietnam.

Quite suddenly the engine-order telegraph rang. The engine-order telegraph is the thing you see in movies, the large dial that indicates what speed the Officer of The Deck wants the ship to go. We were cruising along at Ahead 1/3, then… RIIIING!!!! RIIIIING!!!, and it shows All Back Emergency.

Now that is a thrill, having those bells come out of nowhere in the middle of the night.

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The main tasks of the naval engineer once oil and turbines had been adopted were making the existing plants more powerful and more efficient. The most obvious way to resolve the mismatch between the desire of turbines to turn quickly and the desire of the screws to turn slowly was to use gears.³ The problem is that transmitting tens of thousands of horsepower through gears is not easy.

HMS Hood, the first battleship designed with geared turbines

The geared turbine first went to sea in large ships in 1916 with the three ships of the Courageous class, although they weren't technically battleships. This was necessary to allow them to make 32 kts, by far the fastest sort-of capital ships of the day. They also introduced new small-tube boilers, which were about 30% lighter for the power than the large-tube boilers in use previously. The first proper battleship followed a year later, when the USS North Dakota was refitted from direct-drive to geared turbines. But since there are very few details on that plant available,⁴ we'll look at the first battleship designed with a geared turbine plant, HMS Hood.⁵ She had separate high-pressure and low-pressure turbines on each shaft, along with astern blades in the low-pressure turbine casing. The outer shafts also had cruising turbines which could be clutched in at low speeds. At full power, the gearing reduced the 1500 rpm of the HP turbine and the 1100 rpm of the LP turbine down to 210 rpm at the shaft. Total designed power on each shaft was 36,000 hp, making Hood the most powerful ship afloat, although in practice turbines could be safely overloaded, and she made 32 kts using 151,600 shp on trials at a displacement of 42,200 tons. 24 small-tube Yarrow boilers produced steam at 235 psi, and 1,200 tons of oil could propel Hood 6,400 nm at 12 kts.

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From the dawn of marine steam propulsion, all ships had been powered by coal, and used their steam in piston engines with increasing numbers of cylinders. The beginning of the 20th century saw coal give way to oil, and the steam turbine replace the reciprocating engine.

Nevada (left) and Florida, old and new propulsion

The first battleship to burn oil was the Russian Rotislav, for purely economic reasons. Oil was readily available from Baku in the Black Sea, while coal had to be imported, so half of her boilers burned coal, while the other half burned oil. This was not a success, and the Russians soon returned to the use of coal. In the early days, coal firing had been developed to a great degree of sophistication, while the heavy fuel oil used then was extremely difficult to burn efficiently. The British introduced oil on the King Edward VII class, spraying it into coal-fired boilers in an attempt to improve acceleration and reduce the load on the stokers at high speed. This mixed firing was common on battleships worldwide until coal was totally replaced by oil in the 1920s.

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I’m venturing outside of my usual remit today, and discussing an issue that’s about 90% air warfare, although it obviously applies to naval aviation, too.

F/A-18F Super Hornet loaded for a strike mission

We can broadly divide combat aircraft operation into two categories, which I’ll call strike and responsive. Obviously, there’s a continuum between the two extremes, but this dichotomy will help make clear something not commonly understood outside of the military world.

Strike operations are missions launched against (usually) fixed targets, probably at the direction of high-level commanders. Let’s work an example:

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75 years ago today, the USS Iowa was commissioned for the first time at the New York Navy Yard.

The original bell is also scheduled to arrive today from Des Moines, where it's been on display for the past 25 years. I wish I could be there for the celebration on Saturday.

Steam first went to sea in the early years of the 19th century. At first, it was used by navies primarily to tow ships in and out of harbor, being immune to wind and resistant to tides, both serious problems for sailing ships. Later, steamships were used extensively for minor roles, such as dispatch boats. In the 1830s, the first steam warships were built, such as HMS Gorgon. However, the use of paddle wheels meant that they could only mount a limited broadside armament, so only small ships received them. Larger ships retained sail exclusively until the advent of the screw propeller. Besides clean broadsides, screws also allowed the engines to be entirely below the waterline, where they were protected from enemy fire.

HMS Gorgon

The British and French began modifying existing warships, both frigates and line-of-battle ships, in the mid-1840s. The resulting "blockships" were intended to use their steam only sparingly, as the inefficient steam engines gave them only limited endurance. As the engines improved, so did the importance of steam in ship design, and by the time HMS Warrior was designed, her steam plant was as important as her sails. But before we examine them it more detail, we need to look at the basics of a steam plant.

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One year ago today, I wrote the first post in what is now Naval Gazing. IrishDude asked in one of the OTs about people's hobbies, and I responded by talking about my work as a tour guide on the Iowa. The discussion continued, and thanks to David Friedman's questions about the classic Iowa vs Yamato duel, I ended up writing out a text version of my fire control spiel. That got enough of a reaction that I kept writing, and have continued to do so since.

So I guess I have to say thanks. Thanks to IrishDude for asking the initial question, and to David Friedman for the follow-up. Thanks to everyone else for encouraging me to keep writing, particularly in the times when I wasn't getting a lot of comments and have felt like giving up. (I've mostly learned that I usually don't get a lot of comments when I've done a good job of explaining my point and so nobody has questions, but there are still days when I do wonder where everybody is.) And thanks to those who provided technical comments and pushed me to raise my game. Thanks to Scott Alexander for putting up with me filling his Open Threads with battleships. Thanks to Said Achmiz for hosting me, and dndnrsn for proofreading my columns here.

And thank you for reading. It's been an interesting year, and I'm certainly in a very different place than I was when this started. Naval Gazing has largely filled the hole left by the Iowa when I moved to Oklahoma, and I hope that I've made your lives more interesting over the past year, too. I plan to continue to do so for the next one.

I've referenced HMS Dreadnought many times, as befits one of the most influential warships of the 20th century. However, I've never told the story of her origin in one place, bringing together the many threads of development that went into her design.

HMS Dreadnought

Dreadnought was in many ways the natural result of improvements in the technology of warships, most notably gunnery and propulsion. Gun ranges were increasing thanks to the work of men like Percy Scott, and the torpedo had become a major threat at close range. This greatly reduced the effectiveness of the 6" QF batteries of the pre-dreadnoughts. Longer ranges limited rate of fire to allow for spotting, and the steeply-falling 6" shells had much smaller danger spaces than the 12". At the same time, improved armor meant that it was now possible to provide adequate protection from the 6" across much of the ship's sides. Read more...

I've been asked several times about classes. A class of ships is a group of ships all built to the same set of blueprints, at least in theory. As with most naval matters, things are more complicated. In some cases, members of a class are essentially identical, while in others, they're totally different.

Iowa, New Jersey and Missouri as completed

As you'd probably expect, we'll start with the Iowas. All four ships were built to fairly similar plans. The biggest structural differences are Iowa's three-level conning tower, intended to house an admiral and his staff in battle,⁶ and the heavier armored bulkheads on the ends of the citadel in Wisconsin and Missouri. The most obvious external differences early on were in the bridgework. Iowa commissioned with an open bridge around the front of the conning tower, like the earlier battleships, while New Jersey entered service with a circular enclosed bridge wrapped around the front of the conning tower. Missouri and Wisconsin had a square bridge, which Iowa and New Jersey received during refits.⁷

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