Yes, any active detection method = the sub knows. Passives are soon lost in the background noise. Propellers can be internalized, like the purported Magneto Hydrodynamic Drive (MHD) that drove water through a hollow channel inside the sub via magnetic forces on the conductive salt water.
http://en.wikipedia.org/wiki/M... It was popularized in http://en.wikipedia.org/wiki/T...
It is not efficient, but it might be made very quiet. I am not sure if they will be able to make a small neutrino detector to spot the ones made by a nuclear sub. Very hard to detect and orient on - might never be doable.
Any modern torpedo can be tuned into a drone, by replacing the warhead with a smaller one and adding a power source with greater endurance and a stealth prop (modern torps whine a lot = not stealthy).
They can be made oil filled, and adjust depth via a compression chamber that changes the height they hover at with zero head way. With headway, they can be vane steered to assorted depths. Communications = a problem for high data rate. For low data rate the VLF EM wave can have some use for outgoing, and can quietly receive and act. I am not sure how deep it works.
They might find high intensity light communication possible - depends on depth and turbidity. Once known a few gallons of whit latex paint will interfere with some transmissions - buy it can take a lot of pain to cover a large area, so it might be useful in some cases.
A mother sub can lie doggo and have 100 drones in all directions, all also doggo, and engage in swarm repeated communications as long as rand and turbidity allow.
So the use of subs is far from over.
The big problem is ossification at the high levels of the Navy where the guys on top resist advances that change the nature of their war. Yiu recall Nazi Germany invented the Blitzkrieg = high speed mechanized warfare that could advance at 20-50 miles per hour, surround and isolate enemy armies with follow on infantry eliminating them. We are lucky thay Germany had under 10% of it's forces capable. The rest were horse drawn and marched on foot.
Churchill was a visionary, but was unable to budge the old farts in the British army with warnings against German armament.
Similar fossils occupy top spot in the US Navy and US Army and US Air Force and are blocking most advances - but some get through - more are needed.
The MIL-SPEC System needs an overhaul. It makes systems cost 10 times what they should cost with all that extra lard going to the defense contractors = the troops are starved of weaponry, and the budget does not go far.
Russia does not have this problem. Ever wonder why they can keep up with the USA?
The problem is, modern parts are made and tested in automated systems, and after 1-2 years a CPU is old and obsolete. In the MIL-SPEC system it can take 5-6 years to get parts approved. By then hopelessly obsolete and the they are then designed into new weps.
Same with small parts. Simple resistors worth $5 per thousand end up being worth $25 each and come with x-ray micrography to show they are good.
Modern well made parts are so good, all this is a waste.
It comes from WW2 when parts had a 3-4% failure rate. enuf...