And, as I said before, the whole design is optimized and losses minimized for travel submerged because that is where the sub spends its time. Any choices made between surface and submerged design are made to favor the submerged case.
And, last but not least, there is this site. Scroll down to #14 on the list.
I searched Yahoo! Search for “nuclear submarines+surface speed” without the quotes and got lots of info all which added up to what has been said here. The submarine is optimised for submerged travel and there is a lot of loss in making waves and wakes when on the surface.
As someone who has sailed a boat relatively close astern of a nuclear sub travelling on the surface at speed, I can tell you that they throw a HELL of a huge wake. Much larger than a normal surface ship of similar size.
They’re also rather difficult to discern. For several miles I thought I was converging with a large, black fishing trawler with a smaller one following close behind. It was only when I drew closer that I realized that I was looking at the conning tower and tail of a nuke. It had only a few feet of hull surrounding the conning tower out of the water.
It also made me chuckle that I could see this major warship change course to avoid my little sailboat, which technically had right of way. (When I realized what it was, I made a major correction to avoid it – no way in hell I was getting near that thing – but that was after it had made a small but noticable turn away from me.)
In any event, it was an impressive machine.
Rrrrrrright… Sounds to me like we’re both asserting the same thing. However, I was also qualifying my statement, because it isn’t always true that a submerged vessel will have less drag than a surface vessel, which is what the quoted part seems to suggest. There are too many other factors at play for a catch-all answer.
I don’t have much to add here, other than to state unequivocally that a given power output, modern nuclear submarines with a teardrop-shaped hull travel much faster submerged than when surfaced.
Surfaced submarines produce massive bow waves that rob energy, as does the cavitation of the screw (propeller) on the surface.
In fact, in practice, maximum allowable power is limited while on the surface, as increasing power beyond the limit results in a negligible increase in surface speed while needlessly wasting the nuclear fuel.
-robby (former U.S. submarine officer)
It´s quite simple to see, look at the second picture on Rick´s post, all that troubled water was stirred by the sub, the energy to do so came from it´s powerplant, so that energy goes to splash a heck of a lot of water instead of propelling the boat faster. Once the ship is underwater the waving and splashing stops, so it doesn´t rob the sub from its energy.
Besides, I think that in surface cruising the propeller must be really close to the surface, not just cavitation there, it may actually suck some air too. Correct?
Oh sure. I thought from previous post that it was clear that the WWII type of hull was not optimised for submerged operation and might very well have been slower there than on the surface given the same power capability in both cases.
Robby, I have a quick question for you. How is the term “submariner” pronounced? subMARiner, or SUBmaRINer? Or is it not used at all?
First of all, whether they are optimized for underwater travel does not determine whether they are faster underwater than on the surface. By way of obvious analogy, trucks are optimized for carrying heavy loads but they still can go faster when they are empty. Optimizing them for underwater travel makes them go faster underwater than they otherwise would have, perhaps at the expense of surface speed That’s not the same thing as making the submerged speed greater than the surface speed.
So, I can see a sensible argument that says if it makes surface waves when running on the surface, it must be having an easier time of it - the air wouldn’t yield to the water if it weren’t easier for the air to yield, right? This seems to confirm the less drag argument, sort of - we don’t know if it’s friction per se, or other things like inertial forces. Trouble is, I’m not sure this argument is valid. I think a similar argument could be made that if a car displaces sand while driving on the beach, then beaches must be easier to drive on than pavement.
Maybe the sites covered it all properly. All the same it’s a pleasant curiosity.
I think robby answered it pretty unequivocally. When in doubt, ask the guys who do it / did it for a living.
Yes. Optimising for the submerged case doesn’t guarantee that the sub will be faster underwater than on the surface. However, it turns out that the one optimization that counts is the shape of the hull. The blunt nose greatly increases the drag when on the surface because of the large wave while the drag when submerged is small. If the available power is the same the smaller drag case will have a higher speed. Doesn’t that make sense? And if, as Robby says, there is a restriction on the surface power settings permitted, that difference in speed is further accentuated.
The shape of the hull for best surface use has been refined over a period of centuries of study and thousands of hours of testing in model tanks. The hull has to have room for cargo, machinery and living space; be stable so that the ship won’t capsize; be strong enough to stand up to the stresses; have a shape that offers as little resistance as possible to motion through the water while on the surface. The first three also apply to a submarine, with the cargo being stores and weapons, and the shape was optimised for submerged travel which resulted in a poor shape for surface use.
The OP is something I have always questioned.
I have been a scuba diver (*PADI) for many years. I have been signed off on virtually every skill level available. I have dived with probably a few hundred people over that time. Most of my dives involved large groups of people. After a dive we would head back to the boat or to land, whatever the case may be. Not to brag but I am a good breather and would always have plenty of air left in my tank. So on the way back I would drop down about 3 to 5 feet and swim back with the group while taking in a view of the bottom. I would consistently be back to the target before all the people on the surface. A friend that I frequently dive with noticed this. We experimented, he under the surface and I on the top and it seemed that the underwater diver was always a bit faster. So one day at the docks in Santa Barbara California we found a PADI instructor and posed the surface verses underwater question. The instructor thought about it and suggested that maybe it was the waters “surface tension” that held the surface diver back.
It does seem strange though. After all, a diver wears a buoyancy compensation device, air tank, regulator, and a face mask. All these items for the most part are out of the water while swimming on the surface. But under water they don’t appear have any appreciable effect, just the opposite.
Jim
- PADI = Professional Association Of Dive Instructors
Remember though, you are using flippers, and I imagine they are breaking the water’s surface and therefore not producing nearly as much thrust as when they remain entirely submerged. Even if they come very close to the surface without breaking it, they will still lose some power.
robby’s already given the answer. An oddly unsatisfying answer, since it doesn’t really tell why increasing power/RPMs doesn’t increase the boat’s speed.
Is the main factor in operationally limiting power because of screw cavitation, as others have mentioned, or something to do with the physics of traveling at the water/air boundary? If it’s cavitation, what would happen if you put a different screw on optimized for surface pressure operation?
It isn’t at all uncommon for drag characteristics to increase abruptly at some critical velocity. If drag coefficient is plotted vs. velocity things will be going along smoothly as velocity increases and then suddenly the coefficient will jump in magnitude. Such curves are usually plotted vs. Reynold’s number which is a parameter that includes velocity and the viscosity of the fluid.
Turbulence is another characteristic that can appear abruptly. Up to some critical Reynolds number the flow is laminar and then the flow will suddenly become turbulent with just a small increase in velocity.
So it isn’t too unusual for increased power to result in a smooth increase in velocity for a while and then a critical point is reached at which time the drag increases rapidly and adding more power merely puts the added power into disturbing the flow with very little increase in velocity.
Sorry but you are comparing apples and Kitchen sinks. How much load a truck can carry is not what we are discussing, we are discussing if a change of the medium in which the sub operates changes the amount of drag/ and top speed.
What you are comparing is will the sub go faster with all of its torpedos loaded, or will it go faster empty. Not a valid comparision.
Here is a chart of Older subs and the speeds they can atain
As I mentioned before, the hulls on these boats were basicaly a surface ship that could go underwater, look at the pictures on this page and you will see what I mean.
As far as speed goes from here
Or http://www.naval-technology.com/projects/la/specs.html
You will note that they do not list a surface speed. This is because 688 class subs are NOT designed to cruise on the surface.
and from here
you will note that this is over 3X the speed available to a Gato class WWI sub (listed above) And
Even though she was not optomized for underwater performance, the Nautilus was faster underwater then on the surface.
Both pronunciations are common.
I did, however, once hear a submarine force flag officer (an Admiral) make a big point at a briefing that the only proper pronunciation was SUBmaRINer, and that no self-respecting submariner would ever use the pronunciation subMARiner, as it implied that submariners were literally sub-mariners, or somehow inferior to other mariners. :dubious:
I didn’t say that surfaced speed does not increase with increased power on the surface; I said that beyond a certain power output, the further increase in speed is negligible and a waste of power. Above a certain power output, it is possible to increase the power output by as much as 100% (doubling the power output) and gain less than a 10% speed increase.
The largest issue is the large surface bow wave created, which would be unaffected by changing the screw. Also, why would you want to substitute a screw optimized for surface operations when a modern nuclear submarine spends more than 95% of its time at sea submerged?