The Marines’ current Amphibious Assault Vehicles, shown here, are 40 years old.

CRYSTAL CITY, Va: For years the Marines have argued they need a new amphibious combat vehicle that can cut through water at high speeds so Marines can get to the beach safely and then fight their way inland. But Marine Commandant James Amos signaled yesterday there just isn’t enough money to buy a “planing” vehicle capable of skimming over the water like a speedboat. Instead, for now, the Marines will buy something much more modest.

“I envision the ACV as kind of a phased approach,” Gen. Amos said when I asked about the program’s status in a Q&A at the RAND Corp. “Phase one being, ‘okay what can we get now that makes sense, that’s affordable in this budget,” he explained, “and then [see] what is it that science and technology can give us in the future for perhaps a phase two, which might be a high-water-speed vehicle.”

Note that “perhaps” and “might be.” Amos’s second phase sounds more like an aspiration than a plan.

“My sense is the S&T [science and technology], the R&D [research and development] is not quite there yet” for high speed movement through the water, Amos said, “but I still have the need for an amphibious vehicle” now.

“Right now,” he said, “we have a 40-plus-year-old amphibious tractor,” the Amphibious Assault Vehicle-7 (AAV-7), aka the LVTP-7, which was first built in the 1970s. “We need the Amphibious Combat Vehicle [soon] to replace these 40-year-old vehicles.”

But what kind of replacement? What kind of ACV is actually going to be in the 2015 budget request — due out in weeks — and in the Pentagon’s accompanying 2016-2019 budget plan, the notorious and powerful POM (Program Objective Memorandum)? It sounds like a new, improved, evolved, but hardly revolutionary version of the existing AAV-7. The “Phase 1” Amphibious Combat Vehicle will plow through the water the old-fashioned way, much like the AAV-7 and indeed its ancestors all the way back to the famous “amtracs” (amphibious tractors) of World War II.

The ACV will not be able to skim over the water like a speedboat (planing), a capability the Corps has pursued at great expense and with great frustration since at least 1988. That’s when the Marines began developing an Expeditionary Fighting Vehicle (EFV), a “water-skiing tank” that became so costly, complex, and mechanically unreliable that then-Secretary of Defense Robert Gates cancelled it in 2011. Ever since then the Marines have wrestled with what their new amphibious troop carrier needs to be.

“We’ve done the homework now. It’s taken three years of a lot of rigorous analysis to figure out the high water speed business,” Amos said at the RAND event. (RAND is closely linked to the Army and the Air Force, and Amos is the first Marine Commandant to visit that he or anyone else I talked to can remember). “You can build a high-water-speed vehicle and you can make it affordable — yes, I’m convinced of it — but the issue is the trade-offs in the capabilities inside that vehicle.”

Within the limits of current technology and budgets, it seems, making an affordable troop transport that can skim across the water at high speeds requires too many compromises to its capabilities as a combat vehicle once on land. And while Marines come from the sea, they fight on the land.

“This vehicle will live predominantly, probably 99 percent of its time, ashore,” Amos said. “In Iraq we had our amphibious tractors ashore; we had hundreds of them ashore,” for years.

So why not just buy regular ground vehicles — say, the Army’s 8-wheel-drive armored Strykers — and move them from ship to shore on some kind of landing craft? The Navy has plenty of those, after all, from the hulking LCU (Landing Craft, Utility), which wouldn’t have looked out of place at D-Day, to the LCAC hovercraft (Landing Craft, Air Cushion).

The answer lies in space and time. Amphibious operations don’t have much of either. You can pack a lot more amphibious tractors carrying a lot more Marines in the space that a hovercraft takes up, a Marine official told me. In fact, the Navy amphibious warship specifically designed to accommodate the maximum number of hovercraft, the LSD-41 Whidbey Island class, can carry either (a) four LCACs, (b) three LCUs, or (c) 64 AAV-7s.

What’s more, while the LCAC can cross open water faster than an AAV, it can only go up on the beach, not beyond — and it’s a big, lightly armored target. By contrast, a force of AAV-7s or future ACVs has “all with the capabilities to swim in the open ocean, swim to shore simultaneously, and mass against an enemy over and beyond the beach,” the official said. Moving the same number of vehicles ashore on LCACs would require multiple trips by the vulnerable hovercraft back and forth between ship and beach. That takes a lot of time, and time is the one thing a commander never has enough of, whether he’s trying to save civilians after a hurricane or kill enemies in a battle.

That’s why the Marines have insisted on having a force of amphibious tractors since they were first invented in 1930s. The problem is the current amtracs, the AAV-7s, were designed in the late sixties and entered service in 1971. Despite decades of upgrades, they are painfully out of date and dangerously under-armored: Powerful roadside bombs have been known to destroy AAV-7s and instantly kill or main all the two dozen Marines inside.

So Amos appears prepared to defer a high-speed amphibious vehicle and push that “Phase 2” ACV into an indefinite limbo of research and development (much as the Army is doing with their Ground Combat Vehicle).  For now, Amos will accept a much more modest ACV, one that is “good enough” for operations both afloat and ashore, one that the Marines can get in service before the 40-year-old AAV-7s before they either get any more Marines killed or simply fall apart from sheer age.

“If I made the decision today that we’re going to pick vehicle X, Y, or Z, it would be probably seven years before the initial operational capability of that [new] vehicle,” Amos said. “That means that the vehicles that we currently have we would be using 50 and 60 and maybe even 70 years later.”

“So,” Amos summed up, “we’ve got to get on with it.”