Its endurance of around three months will depend on its energy source and energy expended particularly its speed (including steady cruising or acceleration, deceleration and turns).
SeaWeb ASW sensors summarised as "eyes in the sky" and "ears in the sea" are becoming more sensitive perhaps making ever larger submarines all the more detectable.
These are all putting traditional manned submarine operations at greater risk particularly slower moving conventional diesel-electric submarines (SSKs).
An SSK moving at its most efficient LIB or AIP speed of 4 knots may not realise that a very large LDUUV like the Echo Voyager has tailed it (in its "baffles") for two weeks. A 51 foot Echo Voyager is much smaller in length (hence stealthier) than a submarine. Even a WW2 Japanese 2 man submarine was 78 feet long.
An Echo Voyager may look large by UUV standards but it may be very difficult to detect if it is dull gray/black, remains at about 600 meters or deeper, has an anechoic coating and is its discrete propulsion.
Different Propulsion Solutions
Propulsion may come in at least three modes, depending on use and level of quietness (discretion) required:
1(a). Maximum discretion might be achieved by advanced battery only (including lithium-ion batteries). A mother submarine (eg. USS Jimmy Carter) could recharge Echo Voyager while Voyager's is sitting on "Jimmy's" back behind the sail.
1(b). The Russian submarine equivalent is a modified Delta IV (designated BS-64 Podmoskovye). This Delta IV could recharge the Russian equivalent LDUUV (perhaps Russia's current sized Klavesin-1R or an enlarged version). Note it could have been such a Russian LDUUV that was sighted off Stockholm, Sweden in 2014-15.
Delta IV BS-64 Podmoskovye is likely alternatively based at Russia's Northern Fleet when tapping Atlantic undersea cables and with the Pacific Fleet (after Pacific cables).
2. To tail a submarine or reconnaissance near an opponent's coast considerable discretion but also greater range might be required. A hybrid advanced battery and fuel-cell AIP solution may quietly extend range.
- A diesel-snorkel combination (to recharge batteries) instead of fuel-cell AIP may be appropriate (though noisy at times).
- Reliance on fuel-cell with batteries that are only for backup is another combination.
3. For non-military uses quiet (discrete) operation is far less important. A different hybrid solution may be appropriate. This time an advanced battery together with regular surfacing to run a diesel engine to recharge the batteries may be appropriate.
- An unescorted, $multi-million Echo Voyager, when surfaced to run a diesel, may be rammed by a boat or ship. It could also be snagged (at any depth) by a trawl net. It could intentionally be "kidnapped" by economic opponents. So there are even civilian risks of unescorted operation.