I was watching a sub movie, and the sub was in trouble, and needed to return to port, so it submerged, and the captain told the observer that the sub could go faster submerged.
This doesn’t make any sense. When the sub is submerged, shouldn’t it go slower because it is displacing more water. The engine strength would be constant, so shouldn’t the sub go faster when it is pushing against less water (IE surfaced)?
It was a new sub, the show was NCIS (the one with the sarin canister on the sub) (I called the person observer b/c I didn’t remember the character’s name)
I know that the WWII u-boats ran faster surfaced
The ‘tear-drop’ designs of modern submarines are optimized for underwater sailing, whereas the WW2 designs were generally optimized for sailing on the surface. What era was the movie? After all, it may have been wrong 
WAG alert…
I’d imagine that when on the surface, due to the turbulance, a slower speed would need to be maintained.
In addition as depth increases, the pressure around the propeller increases, and decreases the effects of cavitation (small bubbles formed by the pressure of the prop blades), providing more thrust for a given RPM.
I’m sure that our naval engineers will be along soon enough to give more details, but that should give you a start.
-Butler
(military technolgy nut)
I think the answer is hydro and aerodynamics. Either way the majority of the sub is in water, but the layer break between air and water probably would cause all sorts of turbulence, robbing the sub of speed.
Subs are very carefully designed to travel very fast while submerged. Sticking the conning tower and part of the hull above water is not the configuration for optimal speed. They’ve been designed to have very low drag under water.
For the record, this is simply a WAG, I have no facts to back it up.
Aren’t sub hulls designed to deliberately compress while submerged? Wouldn’t this lower the overall surface area of the sub, thus lowering friction? Or would that be too infinitessimal to have any real impact?
Would that be something like the crumple zones on a car? I would see why they would want that to happen in an emergency (but if a sub got low enough to use its “crumple zone”, I would think that it would be so damaged that nothing would save it:confused: I guess I don’t see), but when you bend steel/metal, it gets weak and breaks, why would you want the hull of a sub compressing (as I read it bending all over) as part of normal operation?
Except for later designs, the dramatic difference in speed between surfaced and submerged submarines was mainly because they had to use batteries underwater, not because of the shape of the boat. They just didn’t have the same power when using the juice. Today’s diesel-electrics have much more efficient electrical power systems.
Well, no, think more like the ribbed surface of the SR-71, which is designed to function best at ultra-high speeds… when the air friction warms up the surface of the plane and causes the material to expand. Or how skyscrapers are designed to sway, quite considerably, under wind or earthquake sress.
As I understand it, they design submarines to “contract” as pressure increases, working with the hardship of deep ocean rather than trying to resist it.
I just wonder if this compression is enough to significantly reduce the sub’s friction.
The comnpression should not be enough to significantly affect drag unless the surface was designed to be hydrodyunamic at depth and therefore had open seams at lower pressures that would increase skin friction drag.
I understand that subs will be faster at depth where you lose cavitation and force laminar flow, but what about at a small depth, say 50 feet or so. Would it still be faster than at the surface?
A major concern when designing displacement (non planing) hull shapes is minimizing the influence of the bow wave. In fact every displacement hull has a “hull speed” where the minimal energy is wasted and the most effective use of fuel is accomplished because the hull traverses one complete wave length at a certain speed. The formula for hull speed is a function of waterline length.
You can imagine the energy wasted when a hull constantly raises a volume of water in front of it. In fact many ship employ the bulbous bow to help keep the bow wave down.
Submarines generally employ very simple designs to minimize friction drag with little or no consideration for bow waves I suspect.
Maybe in your Navy, but the US, Russia, UK, France, and others use nuclear powered subs.
Actually the Soviets were still building diesel subs into the 90’s - the Kilo was produced from 1982 to at least 1990.
The British Upholder class was a diesel that first entered service in 1990, though they were all pulled out of service by 1994.
Diesels have two big advantages over nukes - they’re cheaper to build and operate, and they’re much quieter when running on battery, which they do most of the time. For coastal defence, they’re ideal. US Naval strategy is to keep enemy vessels as far away from the coast as possible, so we’ve never gone in for a coastal defence submarine class.
It isn’t that they’re designed to do it, but that it can’t be avoided. Any metal has a positive bulk modulus, and will shrink under uniform pressure. All the stuff mounted inside the hull has to be able to take hull compression. You might have seen the scene in Das Boot (I think) where a veteran sailor stretches a taut string across the torpedo room, and watches the rookie’s face as he sees it go slack during a dive.
Subs have circular cross-sections so that the pressure differential across the hull will be balanced, keeping structural loads in compression only and not in bending, where the imposed stresses would be much higher - that’s the same reason airplane fuselages are circular, even though the higher pressure is internal for them.
Only by a few percent at most, and no, it wouldn’t much matter.
This engineer thinks **butler ** has it - propeller cavitation. Drag is actually lower on the surface because of reduced displacement (you don’t have to push as much water out of the way) and reduced wetted area (friction). Heavy waves could push the sub off level or off course, and power would be needed to restore them, though.
I think Brutus and Rick Jay have the answer as to why older submarines were slow under water while newer ones are faster under water.
WWII subs, for example were surface ships that could go under water for short periods of times However, their propulsion for underwater travel was limited in endurance, and power.
Modern, nuclear subs are designed to be underwater at all times and their surface performance is not all that good. Their propulsion systems provides all the power needed via a nuclear reactor that drives closed-cycle steam turbines that drive the propellors.
Nuclear subs spend very little time on the surface. On fishing trips to San Diego we occasionally would see one departing. Pretty impressive. They are the size of WWII light crisers, move fast and generate a large bow wave when on the surface. They went out off shore not very far, submerged and weren’t seen again for several months until they resurfaced just outside San Diego and returned to harbor.
Soooo…
Does anybody think they can definitively answer ‘yes’, they do go faster underwater. Because I’m still not convinced. They most certainly can manuever faster submerged in that they essential ‘fly’ like an aircraft (pitch & bank).
But I still find it hard to believe that, in a straight line, the surface turbulence is even going to come close to matching the reduced drag of a significant portion of the hull not having to push water out of its way.
I speaking about a modern day nuclear sub.
Modern subs are faster underwater than they are on the surface. On the surface energy has to be wasted pushing the bow wave out of the way. Look at the opening scenes of Crimsion Tide as the boat is leaving the harbor. The bow wave is massive. The second limiting factor is the cavation of the prop.
Once the boat is underwater the flow is smooth over the hull, no bow wave like on the surface, and the prop doesn’t cavitate (unless the skipper screws up)
ON WWII subs they were very slow underwater do to the design of the hull (a surface design) so the drag underwater was higher then it was surfaced.In addition the deck gun(s), conning tower, periscope shears, and all the junk topside acted as brakes, and slowed the boat down.
On this page you can see the difference between a WWII sail/deck and a later design. Compare that to the clear deck on a Los Angeles Class boat In this pic you can also see the amount of water displaced when running on the surface.
Yes.
The above taken from Sections 1.4 and 1.5, Chapter 5 of Principles of Naval Architecture, SNAME Publications. Basically, what the excerpt is saying is that surface ships have a added drag component not present in submerged bodies. What Hail Ants is referring to is the effect on reducing so-called “wetted area” in form and frictional drag.
Unfortunately, I can’t give a definitive answer because there are too many variables, like submerged depth, ship’s velocity, and sea condition. Generally though, wave-making drag constitutes a larger percentage of total resistance at higher speeds than form and friction, which can be easily minimized.
Based on observation of how much of a nuclear sub sticks out of the water when one is on the surface and knowing the size of the whole vessel, it is doubtfull that the increased frictional drag because of the increased wetted surface when submerged is anywhere near the losses that your cites speak to when on the surface.
