Speed is of the essences in my units operations, it’s the other way we stay alive. Even the resupply ferries are fast planning designs (which means that we prioritize speed so much, that we actively sacrifice loading capacity for speed in a vessel that built for hauling load, though it might be added that they are a bit slower than the CB90) That being said 40 knots is not a magical number, it can easily be 35 knots, but lower than 30 and we are approaching a deal breaker.
Some other examples of the need for speed in archipelago warfare:
The in use submersible SEAL carrier also does close to 40 on the surface.
And when we upgraded the CB90 we made them even faster (which required some thinking because of the extra weight of the added ballistic protection)
There is one other country with an equivalent archipelago to ours, and that is Finland, they have their own fast transport the Jurmo
(Norway have sort of similar terain with the fjords and they use the CB90)
Airlifting is not an option when we fight in the archipelago, because we don't rule the sky, we contest it with SAMs. We don’t “control” the archipelago in a strict sense we only make it a damn dangerous area to run ships or aircraft trough. Imagine it more as pre prepared, well equipped guerrilla warfare with the goal of dissuading sea access to our capital. It was a part of the invasion defense against the Soviet Union, and that entire defense plan was based on the fact that we could never stop the Soviet Union if they really wanted to stomp us, but we could be a target that is worth less than it costs, especially since we were “neutral”. (Imagine the defense plan as a modern version of the finish winter war) That why we don't use helicopters, they would be shot out of the sky (sure I trained with helicopters quite a bit, but that is based on how we operate abroad)
90% of the time when moving these vessels will be operating on the surface and then moving slow is not an option, it has to be fast to even keep up with the rest of the battalion but mainly to be able to out maneuver, or out run pursuers, and to be able to quickly leave an island which has been used to launch SAM’s, Hellfire’s or mortar shells. (Or if a forward OP is discovered)
The most useful features thanks to the submersible design would actually be the ability to hide the boat in a good way when we are hiding on an island (which, is almost all the time so it should be a really useful feature), and I can also see semi-submerged sneaking being used quite a bit (only the snorkel and the bridge over the waterline) fully submerged movement is a good added capability that certainly will be used those last 10% but it will not be how we regularly move around.
On to some engineering and capabil
Would it be realistically possible to “hinge” the two floating/ballast hulls so they can be raised to the sides? The goal of this would be to reduce “draft” when underwater, semi submerged and when making beach landings.
What would be the downsides to using larger electric engines and using them not only when submerged but also as propulsion when surfaced? The boat would of course have diesel engines to give juice to the engines when air is available (The electric engines would probably drain the batteries in 30 min’s if they were used full burn without help of the diesels). The goal with this sort of arrangement would be to easier facilitate “hinged” hulls.
I am also thinking about possible other versions (with the 10 man troop compartment removed).
Mine laying version (we use smart mines quite a bit).
Weapon carrying variants not sure which but some among these SAM, mortar, light anti ship missile and auto cannon.
Medical version, that’s one funky ambulance.
Possibly one with a small pressure chamber for use by combat divers and underwater saboteurs.
How good would a AWACS be at detecting a small surface vessel in an archipelago environment? Should the vessel be shaped to reduce detection risk?
I envision this boat being used by a island nation surrounded on all sides by a massive archipelago (or by Sweden that suddenly decides to burn allot of money on the amphibious forces)