Can submarines be refuelled or resupplied under water?

I know that some kinds of military airplanes can be refuelled while still in the air.

Are there any submarines that can be refuelled and/or resupplied while still under water? I suppose this feature might be important on missions where a submarine needs to remain underwater for a very long period of time. If another submarine could bring it fuel, food, and other supplies, then it wouldn’t need to surface.

If we’re talking about nuclear submarines, they can go decades with out being refueled.

As far as food and supplies go I’m guessing they could if they really wanted to. But I don’t know why they would. Seems very impractical to me

Yeah, nuclear submarines can go a very long time without needing more fuel. Because of that, they can de-salinate water and generate their own oxygen if they need to do so, so that takes care of water and air. That leaves food - with modern preservation methods (including freezers) they can store a whole lot of that in port. A modern nuclear sub can stay down for months no problem, even a year, probably longer.

Regardless of how long they can go between refueling, nuclear submarines definitely can NOT refuel underwater.

Total WAG here: I’d say that they could be designed to do that, but it’s not really needed so they are not. I would also guess that in an emergency they may be able to use the torpedo tube or missile launch ports (or whatever the correct term is), to transfer supplies, though I don’t know if it would be possible to refuel the reactor from inside that way.

I’m not aware of a procedure or system in place for full resupply of a submarine whilst underwater. Maybe there is one, but I’ve never heard about it.

What submarines do have is lockout chambers that allow you to leave and enter a submarine while it is submerged. They function somewhat similarly in principle to airlocks on a space vessel. Basically there’s a door between the chamber and the rest of the sub, that can be closed and made watertight, so people go into the chamber, the door is closed behind them. There is another hatch in the chamber that opens to the outside, that is also water tight. Then the chamber is filled with water and allowed to equalize pressure with the outside, then you can open the outside door and swim out. There are also escape hatches on submarines, and part of submariner training is training for “escape scenarios” where you learn how to potentially escape a submerged submarine and get to the surface if necessary–note that that is only realistically possible at certain depths. If you’re at 800ft deep or something it probably isn’t happening.

The most common use of lockout chambers is for special forces, Navy SEALs can run special operations out of a submerged submarine where they egress the sub while it is underwater and do whatever it is they are there to do. There’s also a sealed “box” near the tower of the submarine that will contain additional special operations gear–it is more convenient for them to store it outside and access it that way than to lug it all through the lockout chamber out the hatch.

So obviously in a very theoretical sense, you could send divers down with supply parcels and have them go back in through these same means, and in theory resupply a ship. However going in/out while submerged isn’t the most convenient or fun thing to do, it’s slow and difficult. The hatch isn’t that big, so you’re going to be moving VERY small payloads in with each transfer. That’s why I say very theoretical sense, I do not think it’d be a standard way of supplying a sub underwater.

The Navy has Sub Tenders, which can resupply a deployed submarine with consumables and spare parts.

USS Emory S. Land - Wikipedia

To the best of my knowledge, no U.S. sub can resupply underwater from a submersible nautical equivalent of a KC-135. I suppose, in an emergency, a DSRV could tote some groceries or parts to a sub, but that’s not what it’s designed for.

Smarter Every Day-Food on board a sub.
Deep Dive Playlist.

Point of note: the USN DSRVs Mystic and Avalon are decommissioned. Current submarine rescue vehicles are remotely operated and while it would be theoretically possible to resupply them with food and swap out crew while submerged using a SRV attached to the escape trunk, it would not be very convenient or fast. There is no way that torpedoes or vertically launched missiles could be loaded (and you certainly would not attempt to do so by opening torpedo or missile tubes while submerged) and the nuclear reactor could not be refueled as this involves removing and replacing the entire packaged core during a mid-life overhaul which requires partial disassembly of the pressure hull; the nuclear reactors in naval vessels are not designed for regular replacement of fuel elements the way that commercial power or research reactors are because there it would not be practical or safe to do so, and would entail a design much too large to fit within a submarine hull.


Fundamentally, submarines aren’t the sort of vessels that need frequent replenishment. Even the diesel fleet boats of WWII had patrol durations in the neighborhood of 60 days, with some stretching that to as long as 90 days.

Modern nuclear subs are pretty much self-sufficient in terms of everything but food, which is the usual patrol length limiting factor. The record, AFAIK, is 140 days, by the USS Pennsylvania, an Ohio class ballistic missile submarine. Most patrols are 60-90 days even now (must be something crew-related).

I am not a submariner but I am pretty sure the answer is no. Except for, in an extreme emergency, using the small DSRV-type vehicle to attach to the hatch, but that would be extraordinarily unusual.

The whole point of remaining submerged is to hide the subs presence. Once you start doing a refueling operation its presence is given away. Doesn’t make any sense to plane for this contingency.

How so, if the refuelling were to take place while submerged?

Please note, they cut a hole in the hull to remove the reactor. Talk about your screen door on a submarine.

Even assuming that this were possible, the question becomes why would you want to? What I mean is that you would be sending the replenishing submarine full of fuel, food, and torpedoes out to meet the submarine to be replenished that is short on one or more of the three to transfer them over so that the submarine to be replenished can remain on station. Why not do the much simpler thing at this point and have the newly arriving submarine, which also has a fresher crew, simply relieve the original submarine on station?

The only time submarines were used to resupply other submarines were the milch cows used by Germany in World War II. These were modified Type IX long range U-boats that had their armament removed and carried additional fuel allowing them to transfer fuel while on the surface to Type VII U-Boats to allow the Type VII’s to operate off the US East Coast, something the shorter range Type VII’s didn’t have the range to do on their own.

First of all my logic assumes we do not already have an underwater refilling vessel, that one would need to be developed. Presently ships that run on fuel are all refueled by a tanker vessel coming next to them and connecting hoses.

Second, even we we had an underwater fueling vessel the very act of the two ships coming together under water and passing fuel from one to the other would most likely be very noisy and draw attention to the secret sub. So what’s the point? Just doing it to do it? Maybe if we had an underwater station beneath the Artic ice cap it would be nice to be able to resupply them, but we don’t.

Because a nuclear sub only needs to replenish food and the odd piece of equipment, it’s theoretically possible to do it while submerged. It would be crazy slow and possibly dangerous, but it could be done.

I can imagine a sub laden with food packed in waterproof cases approaching another, then divers going out from one to the other carrying the goods, or some sort of rigging between the two to transfer material.

This would be a crazy thing to do except in extremis - for example, if you had a sub that was not known to exist and you simply couldn’t risk surfacing it.

Nuclear subs only have 135-160 people on board, typically. If we are sending over emergency rations, it wouldn’t be be all that much food. About a half-pound per day per person. Call it 75 pounds of food per day for the ship. Properly neutrally-bouyant, a single diver could carry that much over. Ten divers making ten trips each could provide food for another 100 days. Whether this was ever done or even contemplated, I don’t know. But it’s feasible.

The other thing to consider is that a submarine’s endurance isn’t just limited by its food supply. There’s also the mental factor as well. You don’t want the crew to remain cooped up in an enclosed space for too long.

Former submariner here. The question has mostly been answered. For modern nuclear submarines, the reactor cores are designed to last for decades. More specifically, for the class of submarine I served on (improved Los Angeles-class) the sub had a reactor core that was designed to last for 15 years. Since the sub itself was designed to last 30 years, only one refueling was anticipated over the life of the sub. Note that refueling the sub basically means cutting the pressure hull open, which is a major operation that takes over a year as part of a major overhaul. My understanding is that the newest subs being built now have cores designed to last the life of the sub, so no refueling is ever needed.

Nuclear subs make their own potable water (via desalinization) and oxygen (via electrolysis of the desalinated water). Carbon dioxide is also scrubbed out.

The only other consumables are food and weapons. There’s no practical means of transferring either of these to a submarine while submerged.

With respect to food, though, a submarine can hold a lot of food if you put your mind to it. We routinely went on deployments with several months of food. You’d likely need to convert all of the reefers (refrigerators) to freezers, though, so there would be limited fresh food from the start.

Could the reactors have a longer lifespan just by having a bigger compartment to hold fuel? (I’m sure there must be more to it than that - if it involves classified info, then no need to divulge)