About 3 years ago, I posted the following about a DARPA research project involving the transit of unmanned vessels across long ocean distances. I can envision the use of such vessels in a sustainment role for remote forces, as I will discuss later.
A recent CIMSEC thought piece by CDR Todd Greene has some interesting thoughts along these lines THE NIGHTTRAIN: UNMANNED EXPEDITIONARY LOGISTICS FOR SUSTAINING PACIFIC OPERATIONS:
I think the good CDR is correct, but I also think the innovation connectors are being worked on today:
Supplying widely distributed EABs of varying size, composition, and organic capability presents two sets of challenges – long-range transits across thousands of miles of contested open oceans, and last-tactical-mile delivery over an unimproved shoreline and into the hands of stand-in forces. Today’s systems mainly focus on one or the other, but there is nothing that can do both well.
Innovation must be directed at designing connectors that can bridge capability between these two distinct challenges. They must be able to transit oceanic spaces that feature hostile environments stemming from the open ocean environment and adversary capability. After traversing these many miles, the same system must somehow get supplies across a beach and into the hands of the stand-in force. Innovative connectors are necessary to provide the vital link between the stand-in forces and seabases or logistics hubs.
(Begin quote from my 2020 post)
[Back in 2000], the DoD's Defense Advanced Research Projects Agency announced the concept of a "Sea Train":
The Sea Train vessels independently depart a port under their own power to reach a sortie point notionally 15 [nautical miles, or nmi] from the pier. The four independent vessels then begin the Sea Train mission by assembling in Sea Train configuration and completing a notional 6,500 nmi transit through varied sea state conditions that might require re-routing to optimize travel times or vessel seakeeping. The Sea Train then arrives at a disaggregation point, where the four vessels begin independent yet collaborative operations consisting of transits, loiters, and sprints in varied sea state conditions. The vessels then arrive at a sortie point to begin the aggregation process and conduct a Sea Train sprint from the operational area. The Sea Train then returns to normal transit speed for the remainder of the transit in varied sea state conditions, disaggregates outside of port and the vessels self-navigate to a pier.There has been research in this area as set out here (pdf) in a paper by Igor Mizine and Gabor Karafiath:
A sea train is an arrangement of multiple hulls connected together to form a longer assembly of vessels. The sea train configuration takes advantage of fundamental hydrodynamic principles to reduce the drag of the assembled train below that of the individual components proceeding separately. In some circumstances the sea train arrangement can also offer operational advantages.
DARPA offers up contract bid info here:
The Defense Advanced Research Projects Agency (DARPA) Tactical Technology Office (TTO) seeks to enable extended transoceanic transit and long range naval operations by exploiting the efficiencies of a system of connected vessels (Sea Train). The Sea Train will demonstrate long range deployment capabilities for a distributed fleet of tactical Unmanned Surface Vessels (USVs).(End quoted post)
UPDATE 2023: DARPA has continued its Sea Train work. Technology and software is being worked on DARPA’S SEA TRAIN TO MAKE WAVES WITH TEXTRON SYSTEMS AUTONOMY ENGINE AT THE HELM
DARPA: “NOMARS is not a barge; it is a MUSV [medium-sized unmanned surface vessel]. However, to the other part of your question, we are attempting to develop a next-generation MUSV class that has significantly higher reliability and availability, and carry significant payload for its size. Leveraging any existing design would defeat the purpose of the DARPA program.”
Ultimately what payload the US Navy would have future generations of USVs carry is part of their decision space, but there is nothing unique to the NOMARS philosophy that makes it more compatible with warfighting payloads vs. logistics payloads.”
As set out in Expeditionary Autonomous Transports, the Navy is experimenting with developoing an automous Expeditionary Fast Transport. Though the EFT is not "Sea Train," the technology is not all that different.
There is no reason not to develop "sea trains" composed of various sized units which can contain feature that would meet the two challenges CDR Greene discusses above - including an asset to cover the proverbial "last mile"delivery. If the need is there, a tool can be found to make it work.