OK, I’m no sub expert, but here’s what I think I’ve heard:
When any submarine is above water, it’s less hydrodynamic than underwater.
But for the old diesel boats, if they were completely submerged, and not using a snorkel, they had to use the electric motors. Were they able to go as fast–or faster–using the electric motors?
[qoute]** When any submarine is above water, it’s less hydrodynamic than underwater.**
[/quote]
Older models of submarines were faster on the surface than when submerged. WWII era boats were designed for good speed on the surface and underwater speed was of secondary importance. Post-war designs were the first to place underwater speed as the primary criterion. Hence the teardrop shape of the Skipjack and others of that ilk.
Not to be cruel, but you do prove you’re no sub expert with your basic premise: that a submarine is less hydrodynamic when on the surface. I believe you meant to say that a submarine is experiencing less drag from the water when submerged. (People commonly misuse the term “aerodynamic” the same way when referring to something with a low-drag shape that operates in air)
But this is manifestly false. When submerged, the entire frontal surface of a submarine is experiencing drag. When surfaced, only the part of the hull that is in contact with the water is doing so. The rest of the superstructure is still experiencing drag from air, but since air is only about 1/100 as thick as water, the drag is correspondingly less.
For a submarine to go faster underwater than on the surface, basic laws of physics demand that the power outpust of a sub’s engines be great enough to overcome the increased drag it experiences underwater by a significant margin. Other properties of physics dictate that when power is transferred from one medium to another, there is a certain loss, usually to friction.
The end result of all this is that the electric motors used in those old subs could not possibly deliver the same horsepower as the diesel engines used to charge them. Therefore, the electric motors could not possibly generate enough power to overcome the drag of being underwater by a large enough margin (or any margin) to propel the sub faster than on the surface.
The situation is a bit different on modern nuclear submarines (my brother-in-law happens to serve on one). These subs have a limitless supply of electricity that doesn’t require being on the surface to renew. They typically never surface during an entire three-month tour of duty, except when entering or leaving port. And their only form of propulsion are electric motors, run by the nuclear power plant.
With these subs, the difference between the speed on the surface and the speed underwater is not as great, since they are using the same motors in both cases, unlike the diesel subs. However, they still must cope with greater drag underwater, and the slows them down an appreciable amount.
So the end result is that the only way a sub can go faster underwater is for it to have 1- TWO power sources, one a lot more powerful than the other, 2- the ability to use the more powerful source only underwater.
With current technology, these combination of circumstances doesn’t exist. And for practical reasons, really isn’t necessary, anyway.
IIRC, max bell on a 688 class fast attack boat on the surface was 17.5 knots, full speed. Underwater, max bell is flank, with a top speed of greater than 30+ knots (as carrier escort they were designed to go this fast). Diesel boats max speed on the surface was somewhere around 18-20 knots, and submerged was significantly slower, like around 6-10 knots on the battery (and not for very long).
BF, ex-688 sailor
Modern submarine shapes are less efficient when running surfaced. Most of this is due to energy loss from wake formation. There’s a discussion going on related to this under the heading of Big Ships in GQ, q.v.
True, but you’re discounting surface wave losses. Total net drag from modern submarine hull shapes is less while submerged than running on the surface. Think of the submarine running surfaced: water running up over the bows, bow and stern wakes both dissipating energy, and lots of churning in the wake. While submerged, the submarine has to deal with form drag and parasite (frictional) drag. These are both, as you point out, greater than while running surfaced. However, freed from the limitations of hull speed and wake generation the total drag is less.
I’m curious what his rating is. Absolutely no disrespect is intended in that question.
Yow… “not to be cruel?”
Old diesel-electric submarines were faster on the surface (using their diesel engines) than they were completely submerged using electric power. The reasons for the electric power when submerged was 1) no air was needed for combustion and 2) electric power is quieter than engines.
According to the following site, your typical WW2-era US sub had a surface speed of up to 20 knots, and a top submerged speed of 9 knots.
http://americanhistory.si.edu/subs/history/subsbeforenuc/ww2/
Yes, but he asked about old diesel submarines, which are not the “modern” ships you refer to. Look at old pictures of WWI and WWII diesel sub. They had handrails, deck guns, etc. mounted topside. These items would’ve generated tremendous drag when submerged. However, when surfaced, only the smooth hull of the boat would be touching the water.
As for my brother in law, I believe he is an E-6. He mostly worked with nuclear missles. I did not intend to imply that his working on a sub somehow made ME smarter.
Cool, Ravenman glad my memory has not completely rotted.
I should add Monkey Mensch that you bring up a point I had not considered, that of overall hull design. Subs before about the 1950s could fairly be described as torpedo boats designed to submerge. But with nuclear power, subs actually began to spend more time underwater than on it, with corresponding changes in hull design.
So, you may be right, that modern subs are more efficient underwater than on it. But I’d like to see some kind of cite that this actually affects the respective top speeds in both conditions though. It just seems a bit farfetched that removing a third of the boat’s surface from the water would be more than counterbalanced by wake formation.
Lizard I read your post as saying that all subs, regardless of era, are slower underwater. If I misunderstood the fault is mine. I was getting a little off topic anyway. My first post was much more to the point. Earlier subs were designed as surface ships capable of submersion.
Hey! My first simul-post!
No, that is what I meant. The speeds of most modern submarines are classified, but I figured that this was obvious, from simply considering physics involved. If it is NOT rue, I’d like to see some more detailed explanation f why it isn’t.
The most successful U-boat design of the war, the Type IXB, had a surface speed of 18.2 kts surfaced and 7.3 kts submerged.
As it happened, 7 knots was too slow to overtake most supply convoys in the Atlantic, even the “SC” designated convoys, which were comprised of the slowest merchant ships (I think that anything under 7 knots was too slow for convoy duty).
However, prior to 1943 the U-boat’s surface speed of 18 knots was actually faster than most of the convoy’s escorts, including the flower-class corvettes. The four-stacker destroyers the U.S. traded the U.K. in exchange for basing rights at Bermuda were slightly faster, but there were usually only one or two escorts in a convoy which could chase down a fleeing U-boat. Moreover, since the speed of even the faster escorts was very close to that of a U-boat, the chase could draw the escorts far away from the convoy, leaving it vulnerable to other boats in the wolf pack.
These speeds defined the tactics used. Before radar came into wide use, the U-boats preferred to attack convoys at night while surfaced, often using their deck guns and simply running away if chased. After radar came into wide use, the U-boats would use their superior speed to get ahead and in the path of the convoy, submerge, then unload a spread of torpedos as the convoy passed. Once the escorts had passed by, the U-boat would surface and work its way around the side of the convoy again.
I remember reading an eerie account of a radar man aboard an escort ship who said that they could clearly see signals of U-boats passing the convoy, too far out of range to be confronted. He knew he’d be seeing them again the following night.
The one advantage diesel/electric boats still have over their nuclear powered big brothers is that running on battery power when submerged makes them virtually silent. A nuclear sub’s reactor must always keep its coolant pumps on, even when at all stop (the reactor is never shut down except in an emergency).
This could be significant in terms of, say, a North Korean or Chinese diesel/electric fast attack sub battling a US boomer.
As someone who designs subs for a living, I would be missing a prime opportunity to be smug if I didn’t say “I could tell you how fast a modern sub goes, but then I’d have to kill you.”
With regards to the noise issue, again, let’s just say we’re making them mighty quiet these days. 'nuff said.
I would have said significant in terms of a diesel/electrick battling a 688 class. To battle a boomer (kind of alliterative, isn’t it?) you have to find it. And finding those girls is a stone bitch. IIRC our own subs have trouble finding them in exercises. They do a great impersonation of a whole in the ocean.
A 688 class would be the one trying to find the diesel/electric. As you very nicely pointed out electrics are dead quiet when running slow and on batteries.
Those boats are refered to as a mobile mine field, they get one or two salvos off ,and then they have created a flaming datum point.
Since the speed of any given Diesel electric is more or less known , a radius of travel can be determined , in which case the crew of that boat , can measure the remaining time on earth , in hours.
Basically there is a reason the States and England have moved on to nukes, given the oppurtunity and cost , I can guess that the australians would probably have wanted a 688 or a virginia class sub squadron , instead of the collins class, but thats a guess.
Declan
Actually, the reactors boil water, which turns to steam, which in turn drives steam turbine engines. More powerful, less wasted space, lighter weight overall.
I believe they do have a small (well, relatively speaking) electric as a last-ditch backup way to turn the screw, but it’s an emergency thing, not regular propulsion.
As I recall, somebody on these boards once said most modern subs have four ways to power the/a screw- nuclear/steam, electric, diesel and something else.
WW2 Era subs were designed to run fast on the surface, and the shape of the hull has a significant impact on submerged and surfaced drag.
The first submarine designed to travel primarily underwater was the Type 22 U-Boat. They had a tear-dropped shape hull and ran diesel engines submerged with a snorkel. Their submerged speed was actually greater than their surfaced speed, and from what I’ve read, this is primarily due to the shape of the hull.
So the hull design and role plays a significant role in submerged and surfaced speeds.
. . . .gasp Must. . .raise. . .head. . . above. . . rising tide of. . . factual errors. . . glug