You can probably make a fairly stealthy snorkel, but the wake is generally more visible and much harder to do anything about. In the tropics you can even encounter bioluminescent conditions that will change it at night into a big arrow pointed at you. Even without there are supposed to be radar systems that look for the characteristic shape.

 

You are also operating at a shallow depth, and probably have the periscope up as well so the early warning receivers can be used. You will be picking up radar and communications from ships and planes, the question is whether they are getting a good enough return or visual signal from your snorkel, or more likely the wake, to get their attention. This is where the captain and EW officer will really earn their pay … and their ulcers.

 

The 2x sterling engines on a Gottland class are 75kw each (100hp). The 2x diesel engines are 1300hp each (970kw). The electric drive motor is rated at 1800 HP. I think you will see that there is a slight mismatch.

 

The Gottland can cruise at 5 knots underwater on the AIP alone. That’s the limit. No idea of what it would be like with the snorkel up, but it cannot help but be slower, possibly a lot slower, say 3 knots.

 

So all running on the AIP on snorkel does is make it a quieter than running on the diesels, but slower, and without any power left to charge the batteries.

Depends on whether or not there is a safe place to set them down on the hull. In calm seas with the sub not moving or dead slow it would be possible. But if the sub is rolling or the bow wave coming over the deck, it is not.

 

Really, the best way to replenish a small sub at sea is to tie it up alongside an anchored sub tender. Or develop a tender that can take the sub fully on board while underway.

Most of the problem for doing UNREPs with a modern submarines are the result of their design, which is a cylindrical shape that is unstable in roll, lacks freeboard, and is not really designed for surface operations. Running parallel with a replenishment vessel while winching a fuel line over to the top of the conning tower will almost certainly exceed the control authority of any practical fins arrangement, and in any case depending on a dynamic system for stability is asking for a disaster (Yes, modern aircraft get away with it, but they only spend a couple hours in relatively smooth air and then even more hours in maintenance making sure all the parts work. A sub does not have that option.) And this all assumes that the sub has the necessary equipment and connections already, which does not likely.

 

The second problem is that the submarine is not designed for loading cargo while running on the surface. Submarines lack the necessary freeboard, there is too much water over the deck in anything other than the calmest seas to use the deck hatches, so everything has to come down through the conning tower hatch. This will include the refueling hoses, because they are not designed to be dragged through the water, and the conning tower is the only place that might be high enough.

Depends on what they use for fuel and oxidizer. The fuel in most cases is a hydrocarbon that is liquid at ambient temperatures, so there are no special problems. But the oxidizer is generally liquid oxygen which for several reason, but primarily because it is a cryogenic fluid, comes under the heading of NO WAY. Other possible oxidizers include concentrated hydrogen peroxide (of submarine disaster fame), compressed air (very inefficient, but can be produced on the sub itself), and some exotics that are likely to cause the safety engineers to race each other over the horizon.

Any real submarine (I am not sure about the ‘drug subs’, but wouldn’t bet against it) with a snorkel can recharge its batteries using the surface engines and snorkeled air, that is the whole purpose behind developing the snorkel in the first place.

 

Sterling cycle engines are a ‘closed cycle’ engine with a working gas (usually helium or argon). You supply heat from an external source to one end and cool the other, usually with a heat exchanger or radiator. The advantages are that
1.) Almost any source of heat can be used because it is generated outside of the sterling engine. The downside is that materials limitations restrict the maximum temperature at the hot end, thereby limiting the maximum efficiency.
2.) Sterling engines produce much less vibration, and have none of the detonation noise, compared to internal combustion engines.
3.) Because the combustion is outside the sterling engine, the engine operation is not affected by backpressure on the exhaust produced by the depth of the submarine, the way running a diesel AIP is.

 

So it all comes down to how the burners on the Sterling engine are designed. I suspect that the answer is a extremely qualified ‘yes’, but that it is something they would really prefer not to do because of the complexity of the system for using pure O2 and discharging the gasses at depth. At the very least it would result in a loss of power because of the lower concentration of O2 in air and the resulting greater volume of exhaust gases to be discharged.