As a big part of the Marines' job is to storm enemy beaches, having something that will get them to that beach is pretty important. They're pretty good at looking for new ways and technology to do that, including the common (but no less amazing) LCAC - Landing Craft, Air Cushion, which is exactly what it sounds like. Fuel-thirsty hovercraft like the LCAC may have the raw technological and "gee-whiz" edge going for it, but the Marines are accustomed to discovering that the fanciest tool for the job might not be the best. The "Ultra Heavy-Lift Amphibious Connector" (UHAC)" looks like a promising LCAC replacement, even if it also looks a little backwards in technological progression. Its looks may belie some pretty impressive yet simple technology, but it's not the first gargantuan tracked or truck-like vehicle the Marines have fielded, either.
The granddaddy of all amphibious tractors was the rather creatively named Amphibious Tractor, though officially it was listed as the (somewhat more creative) Landing Vehicle Tracked or LVT. The soldiers who rode in them called them amtracks (of various spellings), gators or buffaloes. The LVT was first prototyped in 1940, a good deal before America entered the war. The military and civilian leadership was smart enough to figure out that such entry was inevitable and when that time came the potential for "island hopping" from Hawaii to the Japanese home islands was very high. The US developed and built a highly refined and capable amphibious doctrine almost overnight spearheaded by innovative and radical ships and vehicles including the Roebling Alligator. The Alligator was little more than a set of caterpillar tracks straddling a pontoon-like floating structure (containing the engine) with the operator's cab stuck on the front and everything else riding flatbed-style.
Image from Olive-Drab.com
The actual production version of the LVT added sides to transform the flatbed-on-tracks into a tub-on-tracks and eliminated the tall cab for a lower profile and armored position buried deep within that tub. The cavernous volume and simplicity of the LVT not only made it reliable and simple, but adaptable. In addition to carrying almost 10,000 lbs of cargo (including 24 fully armed and ready combat troops) additional armor can be carried as well as machine guns to provide better protection and firepower. Variants were also developed that closed off the tub with an armored structure that also formed the base for turret-mounted guns and howitzers, transforming the LVT into a light battle tank or mobile artillery. Just three years after the Army and Marines tested prototypes, LVTs became ubiquitous in nearly every land battle in the Pacific, especially becoming iconic at Iwo Jima and Okinawa. The LVT concept in one form or another has been continuously refined and its latest operational descendant, the Amphibious Assault Vehicle has been in service for decades and will eventually be replaced by wheeled armored vehicles.
Image credit Meers Dekker via Wikipedia
The LVT isn't exactly "a boat that's also a truck," in the sense of a boat being large. Yes, it's large, but not "I can't believe this thing goes on land" large. Fortunately, they've got that covered too! The LARC-V (Lighter, Amphibious Resupply, Cargo, V -ton) is, quite literally, a boat that happens to be a truck in the truest sense. As in, they slapped heavy-duty low-pressure truck wheels on a boat hull and called it a day. They operate like and are used as pretty much what you'd expect a boat that happens to be a truck to be like. Almost a thousand were built for the Vietnam War, but combat attrition and surplus needs have whittled that number down to 200. Not every excess hull has been scrapped - as many as 100 are now privately owned like the above example, giving tours much like the demilitarized WWII-vintage "Ducks" found crawling around D.C.'s and Boston's streets.
Oh, yeah, and the LARC-V is the smallest in its family.
The LARC-XV (XV as in 15, as in 15 tons) puts new meaning to the term land yacht. It makes even the WWII Ducks - which, mind you, are based on 2 1/2 ton three-axle trucks - look like toys in comparison.
And finally, if that's still too small for your tastes, there's the LARC-LX - yes, if you've picked up on the theme naming and are up on your Roman numerals, that's 60 tons of rated cargo load. This is no longer a boat that happens to be a truck - it's a friggin' barge that happens to be a truck. Literally in fact - it was originally designated the BARC, where the "B" stood for just that, barge. Look at how pathetically small that operator's cab/pilot house is! Watch as it makes even heavy construction equipment feel inadequate!
So, what can you carry with a LARC-LX? I'm glad you asked!
Somewhere, Xzibit is extremely pleased.
The LARC-XV was used in the same roles that the LCAC is used for today - in fact the LCAC ended up replacing the LARC-XV. The LARC-XV may have been huge, but it was also slow and unwieldy and consequently less efficient than using boats that decidedly were not also trucks like the LCM (Landing Craft, Mechanized). It also needed to be carried on the top deck of ships rather than in the well deck of amphibious assault vessels, and the entire LARC family was more of an Army thing. The LCAC is supremely cool with its raw power and hovercraft wizardry, but it's lacking a certain quirky awesomeness from not being a boat that also happened to be a truck.
But now even the LCAC has apparently worn out its welcome and the Marines are going back to the drawing board with slower but (hopefully) more versatile tracked vehicles. We'll see how long the UHAC lasts, but it will be following a proven legacy left by the LARC-XV and, if nothing else, will bring its own awesomeness factor by being a boat that also happens to be a tank.