Public Service Film No. 1: Introductory Propulsion

Well, between funky work schedules and travel-inhibiting snows, it has been longer than I would have hoped since my last visit to SLATER. It has been my intention to withold my current assignment until I have completed it. I'm still wavering at this point, in that an incomplete story is not that interesting, so based on that, I think I'll continue to play my cards close to my vest and instead go a little more in-depth on the ins and outs of propulsion aboard SLATER and all CANNON-Class DEs.

We'll begin at the beginning. While underway, the man in charge of the ship was the Officer of the Deck (OOD). Of course the CO is in overall command, but the details were handled by the OOD, especially when the CO was asleep. Courses, bell orders (speeds), and other such minutae was the OOD's responsibility. So, let's devise a scenario: the OOD wants, for whatever reason, to change the speed of the ship. This is no emergency, so there's no need to alert the CO; this is merely a routine matter. The OOD will give the new order to the lee-helmsman (as opposed to the helmsman, who is actually steering the ship), for example "All ahead two-thirds" or "All ahead flank". The lee-helmsman will acknowledge that order and operate the Engine Order Telegraph (EOT). Anyone who's ever watched the movie Titanic or any old war movie will be able to pick out the EOT in this photo. For the benefit of the cinematically-challenged, I have marked it in the photo of the pilothouse.

Time to dispel a common misconception: the lee-helmsman, when he operates the EOT, has absolutely no control over the engines. None. It's an Engine Order Telegraph; all it does is transmit the order to the enginerooms. The watchstanders in the enginerooms are the only people who can control the speed of the ship. Of course, our watchstanders in the engineering spaces are paying close attention to their EOT and notice when the order comes in; but just in case, there is a bell that rings when the order comes in, hence the term "answering bells" for getting underway; when the order comes in to the enginerooms via the EOT, the watchstanders have a basic panel copy of the EOT and they acknowledge the order and let the lee-helmsman know they are responding to his order.

Complicated enough? It sounds that way, but in reality it's quite simple. So now the enginerooms have a new bell to answer, let's look at how this is accomplished. There are four engineering spaces aboard CANNON-Class DEs. They are commonly called "enginerooms", which is technically incorrect. Two of them, the first and third, are enginerooms, they contain the engines, GM 16-cylinder diesels putting out 1700 horsepower apiece. There are two of these monsters in each of the two enginerooms. The shafts from these diesels actually go through the bulkhead to the second and forth engineering spaces, the two motor rooms. The four diesels turn four large generators, which are wired to four electric motors, two on each main shaft, and each main shaft turns one screw (propeller).

So, why was this ship designed this way? Several reasons. First, they couldn't hook the diesel up to the screw directly: a ship of this size needs a relatively lower screw RPM to be effective, otherwise it would just froth up the water and the ship would go nowhere. During peacetime they would use a large transmission called reduction gears to slow the shaft RPM to a usable speed. The problem is that reduction gears are difficult to produce (and therefore produced slowly). While the DEs were being built at breakneck speed (the record is 23-1/3 days), the builders couldn't wait around for reduction gears to be produced.

Secondly, reduction gears are expesive. Very expensive. So expensive, in fact, that to this day, the Navy does not buy the reduction gears we put in our ships: we lease them. When the ship is being scrapped, the reduction gears are inspected. If they're still good, they're put into a new ship being built.

Thirdly, this diesel-electric design means that if one diesel or generator fails, the other diesel-generator pair can turn that shaft, admittedly at a slower speed, while the other diesel-generator pair is powered off and repaired. So rather than running at a maximum of 1/2 speed while one of the two shafts is out of service, the ship can theoretically reach 3/4 of maximum speed, since one of the four diesel-generator pairs is out of service for repair. Additionally, there is no need to have all four diesels running at all times: while escorting convoys, one diesel-generator pair on each shaft would be more than enough to keep up with the brutally slow convoys, thereby extending fuel economy.

Speaking of fuel economy, that's fourth. Direct reduction gear coupling produced as fuel efficiency of about 70 gallons to the mile. No, not miles per gallon, gallons per mile. The diesel-electric setup cuts that nearly in half, to a mere 35 gallons per mile. In wartime, when resources are stretched to the breaking point, this is a crucial point.

Also in each engineroom is an additional 8-cylinder diesel. These two diesels are connected to another, you guessed it, generator. These two generators supplied all the electrical loading for the ship, lighting, winches, electric ranges, everything. And, just to be safe, there is a 3-cylinder diesel that is an emergency electrical generator, if everything else fails. I can't confirm this, but I have a feeling that, in a pinch, there was a way to cross-connect the generators used for propulsion into the power system, but I'll have to do a little more looking into this theory.

Winter Quarters

Without trying to introduce any drama, let's just say that this winter has been somewhat interesting. Of course, this being my first winter outside of the Southeast, that should not come as a surprise. Having qualified on the second plant at work, I am back at my original plant to re-qualify as a staff instructor. What does this mean to you, my loyal fan base? Simply that I will be in the area for another two years, and should be bringing you more stories and information about SLATER, rather than having to shift the focus on whatever it is that I would be doing at my new command.

The practical end of the fact that I've just had to move lies in that, rather than having photographs of my most recent work on the #1 3" gun, my camera is buried in a box. Which box, I do not know, beacuse among the contents of the first box I packed up was my drawer of miscellaneous items that I felt I could do without the longest. The drawer that contained my Sharpie. Not feeling the need to dig it out halfway through packing, all of my boxes are unlabeled. It'll make unpacking feel more like Christmas: "I wonder what's in this one..." As such, I've noticed that there is a group of photos that I've never bothered to write about, so I figure this is the perfect time to use them.

The biggest use of DEs during the war was anti-submarine warfare. DDs took care of most of the work in the Pacific where aircraft were more of a threat. In the Atlantic, the foe was found under the sea. As such, the weapons needed to attack this foe also had to go beneath the waves. Low-tech but powerful, the weapon most used was the depth charge. (Pictured is a cutaway display with the entire fusing mechanism installed. Well, without the explosive charge, of course...)

Depth charges are also known as "ash cans", because that's what they resemble. A depth charge, or "d/c" is a metal barrel 28" long and 18" across across filled with high explosives. The d/c in the photograph is a 300 lb. Mark VI, and was fused hydrostatically. Now, I'm going to hit a brief physics lession here, so for those of you uninterested in figures and formulae, feel free to skip down to "END OF PHYSICS LESSON"

Everyone knows, if for no other reason than watching old submarine movies or SCUBA diving, that the deeper you go, the more water pressure builds up. This is due to the virtual incompressibility of water and gravity. Atmospheric pressure is about 14.7 pounds per square inch. This varies from day to day, and is easily calculated from barometric pressure: 29.92" Hg on the barometer is approximately the 14.7 psi that everyone is familiar with. The fluctuations in the barometer from day to day are due to several factors, one of which is the density of the air. Of course warm air is less dense than cold air. Another factor is height/depth. Also, the actual thickness of the atmosphere at a certain point, measured from the surface to the top of the atmosphere, varies slightly daily.

Okay, so the formula for all this is P=pgz/c, where P=pressure (lbf/in^2), p=density (lbm/in^3) and z=height/depth (feet) and g=gravitational acceleration (32 feet/sec^2) and c=32 ft*lbm/lbf*sec^2, a constant that corrects for lbm (pounds mass) and lbf (pounds force). The two are equal on earth, but a 10 lbm ball on earth will still be a 10 lbm ball on the moon, although it weighs less than 10 lbf. Confused yet?

Okay, so what does this mean for us? Water, pure water has a density of about 63 lmb/ft^3. I say "about", because like air, water will become less dense with higher temperatures and less dense at lower temperatures. But that's pure water. Seawater is more dense due to the salt, fish poop, et cetera dissolved in it, something like 65 lbm/ft^3. But this changes, because the salt and fish poop dissolved in the seawater changes from place to place. But despite all this, we can assume that the density of saltwater will remain constant. Since we are still on earth, gravity will remain constant, and c, our constant, is always constant. So since P=pgz/c, the only thing that changes is z, depth. As such, we can safely assume with reasonable accuracy that at depth "z", we will always experience the same "P". On this principle the fusing of the d/c is based.

END OF PHYSICS LESSON

The fusing mechanism is a pressure switch, basically. Before being launched, the d/c will be set to the depth that the sonar station and Combat Information Center (CIC) believes is closest to where the U-boat is or will be when the d/c gets there. As the d/c sinks deeper, metal bellows (inside the brass sleeve in the left and the black "cup" on the right) will expand, putting more pressure on the firing spring. When the bellows expand enough, they will press on a small rod in the middle of the firing spring. The firing pin is in a sleeve, and in the sides of the pin are three ball bearings that prevent the pin from sliding completely through the sleeve. When the bellows press on this rod, the ball bearings retract inside the pin. With nothing restraining the pin, it shoots through the sleeve, setting off the primer. The primer sets of a 5 lb. booster charge (the grey can in the cutaway picture) which in turn sets of the 300 lb. main charge.

No exact figures are known, but the best calculations suggest that the kill radius on a d/c was a mere 30 feet. If the d/c went off within about 70 feet of the U-boat, damage will be significant, but rarely fatal. Beyond that, and it was only dangerous to the fish. The significance of this to the crews of DEs is that they would have to be outside of this kill radius themselves or risk damage by their own d/c. Of course the deeper the fuse setting the less of a problem this was.

The DEs had two methods of delivery. The first was a simple roller rack on the stern of the ship. Angled astern, a d/c could be rolled off the ship into the water. The second was the K-gun, which were on the sides of the ship. The K-gun could launch a d/c over the sides and cover a wider area than the racks alone. As sonar got better and crews gained more experience, the depth charges became more deadly to the U-boats and their crews. Approximately 40,000 young German men served on U-boats during all of World War II. Just under 30,000 of them never returned to port. This casualty rate, upwards of 70%, was surpassed by only one other community of fighting men on both sides during the entire Second World War: Imperial Japanese kamakazi units.