Just wanted to pass along some photographs I ran across recently, to show why the Coast Guard might have a hard time stopping a modern merchant ship with our 25 and 57mm guns.
This is an extreme example, but it is the way the industry is going. The ship is 1302 ft in length, 207 ft of beam, and has a maximum speed of 31 knots. That means it can transit the 200 nautical miles from the edge of the EEZ to the coast in less than 6.5 hours.
Thanks to James Udan for bringing this to my attention.
Chuck Hill's CG Blog 
Keep in mind that the only time the larger guns would be used is if there was an encounter on the open seas with a larger cutter – close in you have the 87’s or the RBM at 45 ft and I think the largest guns they have is I think a couple of 50 cal guns – – so the baddies just get a few well armed people on board and they take over the ship a few miles from the berth – put the throttle to max speed and aim – takes a few miles to wind up — but straight into the waterfront like the ship breakers do and almost instant mess – and if a tanker some sort of large spill
A crew of 13 is an act of total greed. Virtually any problem and help will have to come from outside.
The base assumption behind the design is that peace and order will reign forever.
Even scarier than being large, is being large with a massively explosive cargo. The attached video is interesting not just because it is the 5 biggest LNG tankers, but also because two of them are ice-classified. But more germane to the topic here is that the largest LNG tanker (starts at 4:00 minute mark on the video) is aircraft carrier-sized at 1130’ Length and 165’ Beam, carrying over 100,000 cubic meters of LNG. (Yikes!!)
A container ships could be carrying a weapon of mass destruction as well.
MK. 48s modified for surface vessel use?
Mk 48 would require no modification, but they are terribly expensive and over built for use against surface vessels, and there are not that many of them in the Navy’s inventory. The Mk48 (and the Mk54) have to be strong enough to resist pressures more than a 1000 feet below the surface.
We need to either use the existing light weight torpedo (if it will do the job of disabling a ship, allowing us to keep our target away from its objective and provide time to bring additional DOD assets to finish it off) or we need a dedicated anti-surface torpedo that will do at least that.
We could probably do fine with a much cheaper, simpler torpedo, cheaper even than the Mk54. Electric propulsion has matured to a point that we can get speeds and ranges better than the best WWII torpedoes using tech from the electric auto industry. The Mk54 torpedo uses electronics and software from the Mk48 and commercial off the shelf processors. Since we would not need to develop new hardware and software, rather just limit the options on existing software, that part should be easy.
To keep it simple we could start with the form of the Mk54, limit its crush depth strength requirement to a depth enough to provide a margin over both, how much it might sink on launch, and how deep it must go to detonate at the most advantageous depth. Give it battery electric propulsion, assuming that is the cheapest capable of at least 40 knots and 8,000 yards. Then provide the largest warhead that would fit within the weight constrains.
If we needed something bigger, we might look at evolving the WWII Mk13 torpedo that was dropped from aircraft or rolled over the side of torpedo boats in the last two years of the war. I weighed about a ton, was 13 ft, 9 in (4.191 m) long, and 22.4″ (56.9 cm) in diameter.http://www.navweaps.com/Weapons/WTUS_WWII.php
Another medium sized torpedo that was used against both surface and subsurface targets was the 19″ Mk37 torpedo, 1,690 lbs. ( kg) less than half the wight of a Mk48 (various mods about 3,500 lbs) and 13 ft 5 in (4.089 m) long (five and a half feet shorter than a Mk48 which is 19 feet in length).
http://www.navweaps.com/Weapons/WTUS_PostWWII.php
The problem is that the Navy does not seem interested, and the Coast Guard seems to have no interest in telling the Navy we need anything that is not already in their inventory.
You would think that the navy would want a half or one third length anti-surface torpedo that would allow them to load more weapons and engage less demanding surface targets.
This post is relevant. https://navy-matters.blogspot.com/2017/06/surface-ship-torpedoes.html
I would like to see us do some testing to determine what we need to do our job.
I wonder what is the goal? Is it to disable the target or to sink the target?
If the goal is to sink a large vessel than a torpedo under the keel and cracking the ship in half is probably the correct and only answer to stop it in a hurry.
If the goal is to disable a large ship, can a missile (NSM as an example) be programmed to go after the stern of the ship to destroy the rudder or propeller. Can a target catalog be created for the missile to have it pop up and go into the engine room as opposed to hit center of mass. Maybe an attack from the stern at the waterline?
Another thought is what is the damage caused if an SM6 is comes down at hypersonic speeds into the engine room of a large container vessel. Enough to disable the ship or is the inertia of ship still going to let it do its damage.
20.000 teu is the norm today.
https://en.wikipedia.org/wiki/List_of_largest_container_ships
This one is unusually large. I think the 15000 containers may mean 15,000 40 foot containers. That would be 30,000 20 foot container equivalent units.
I think this is a Maersk “E Class” containership with a nominal capacity of about 15,000 Twenty foot Equivalent Units (TEU) or 7,500 40 ft. containers. There are now some larger ships in service close to about 20,000 TEU.
Another article on largest container ships here:
https://learnsailor.com/marine-marvels/worlds-largest-container-ships/
They all seem to be about 400 meters in length, or 1312 ft, and there are several of them.
I think the best approach here, if the USCG isn’t willing to change their mindset from a “All gunboat” Cutter fleet, is to field and utilize armed helicopter gunships and RHIBs and USVs armed with APKWS as part of a Force Add-on package. The laser-guided APKWS can come in pods mounting 13 or more guided rockets for precision salvos out to 5 KM.
Helicopter gunships with DAP wing pylons can carry Hellfire, Penguin ASM, Griffin ATGM, SPIKE ATGM, Stinger SHORADs, and heavier guided ordnance. The USCG would need some guided missiles if it intends to carry out its defensive role.
Armed UAVs such as Reapers and Predators can provide some Anti-ship defense.
Small VLSs such as the MML and “Iron Dome” can provide quick VLAs without drilling into the hull, and provide covering guided missile fire against air and surface targets.
However, all these heavily armed assets need to be on USCG Base and Station and guarded more stringently due to the explosive ordnance. It doesn’t help if the Cutter is in one city and the armed helicopter gunship is at another Coast Guard base 880 miles away and needs to fly to marry with the Cutter. All assets need to be at the same spot for utilization.
Finally, the Coast Guard’s Top Brass’s mentality has to change. The USCG might say that they don’t discuss Fleet Tactics or Counter-Threat tactics, but is that really just a “brush-off” to avoid the question? Brainstorming on how to REALLY stop large ships such as this article isn’t a joke that one can just flip off.