In September 2016, Donald Trump, then a candidate for US president, visited a Philadelphia shipyard and made a big promise. Elect him president, Trump said, and his administration would "build a Navy of 350 surface ships and submarines."

Presumably many of those shipyard workers voted for Trump, as the former reality television star ultimately won Pennsylvania and the presidency. But if those workers chose Trump in hopes of huge government contracts for new warships, they're probably going to be disappointed as experts and even top Navy officials now agree that the 350-ship Navy could be dead in the water.

The scheme is "more than unrealistic,” one Congressional shipbuilding expert told me on condition of anonymity, as they were not authorized to speak to the press. “It would be impossible."

There simply isn't enough money. And the Navy itself, along with at least one key US senator, has rejected every inexpensive idea for quickly growing the fleet.

The Navy possesses 279 large ships today and, in recent years, has built only slightly more vessels than it retires every year. Under President Barack Obama, the fleet shrank to a modern low of just 273 warships before very slowly growing again at a rate of just two or three ships per year.

Adding 71 new vessels during a hypothetical, two-term Trump presidency would require the administration, the Navy, and Congress to craft a radical plan for funding and building as many as 15 new ships every year—six to replace older ships in the process of decommissioning, plus another nine to actually expand the fleet.

Shipbuilders at the Fincantieri Marinette Marine shipyard in Wisconsin laid the keel for the future littoral combat ship USS St. Louis, May 2017. Image: US Navy

Manufacturing and maintaining those extra ships over a period of 30 years would cost as much as $23 billion a year, according to a May report from the Congressional Research Service. Acquiring the same fleet over a much shorter period of time would, of course, cost much more than $23 billion on an annual basis. On average, building a single new ship costs US taxpayers $2 billion.

The Navy has endorsed a parallel plan for 355 ships that, of course, would be even more expensive than the 350-vessel scheme.

Facing such huge bills, Trump's own Navy secretary backed away from his president's pledge. "355 is a good number for people to focus on," Richard V. Spencer said during his July confirmation hearing. But the Navy shouldn't take the number literally, Spencer added: "What I will tell you is that whether it's a 355-ship [fleet] or not, what we also want to get our head around is, can we have a capacity number but have a capability that's even greater than that, [to] have the capability of a 355- that might be a 300-ship Navy."

In other words, if the fleet feels like it has 355 ships, it doesn't need 355 actual ships.

Consistent with this new interpretation of his fleet promise, Trump proposed to buy just eight new ships in 2018, one fewer than Obama bought, on average, during the final years of his presidency.

Eight new ships would have put the Navy on track to grow to just 295 vessels after eight years. Obama himself had pursued a plan that would have expanded the Navy to 308 frontline ships by the mid-2020s.

Congress rejected Trump's shipbuilding plan, and instead fronted its own proposal to buy 13 warships in 2018. Trump signed that bill into law this month.

Acquiring that many new vessels every year for eight years could get the Navy close to its 350- or 355-ship goal. But there's just one problem: it's illegal. During high-stakes budget negotiations in 2011, Congress passed, and Obama signed, a law capping federal spending. Congress's defense authorization for 2018 exceeds the legal limit by $85 billion.

To actually appropriate the extra $85 billion, lawmakers will have to rewrite the 2011 Budget Control Act. That requires 60 votes in the Senate, where Republicans will have just 51 votes once newly-elected Alabama Senator Doug Jones joins the chamber in late December or early January.

Congress's efforts to revive Trump's fleet scheme could sink in the Senate. Democrats are unlikely to rubber-stamp tens of billions of dollars in extra defense spending for an administration that has signalled its intention to deeply cut taxes on the rich while massively scaling back popular social programs.

There were ways for the Navy to grow the fleet without buying lots of expensive new warships. The sailing branch could have kept existing ships longer and reactivated some older vessels it had recently decommissioned.

In 2015, the Navy laid up the last of its 1980s-vintage _Perry_-class frigates, tough but lightly-armed vessels that spent their later years patrolling the Caribbean for drug smugglers or supporting Special Operations Forces off the coast of Africa. Seven of the frigates are available for recommissioning at a cost of as little as $35,000 per ship.

But in December the Navy rejected the idea of bringing back older ships, preferring instead to invest in new ships like the controversial Littoral Combat Ship, which costs around $500 million a pop. Critics say the LCS is fragile and unreliable.

Meanwhile, Senator Marco Rubio, a Florida Republican, torpedoed a Navy initiative to extend the lives of older _Ticonderoga_-class cruisers, several of which are based in Mayport near Jacksonville. After years of negotiations with Congress, the Navy had devised a plan to keep 11 of the cruisers into the 2030s—years later than the government previously projected.

But keeping the ships longer requires extra maintenance. The Navy had been removing a few cruisers from the fleet every year for rework. Rubio objected to the maintenance effort, as it meant short-term losses for Mayport. Noting that each cruiser supports 330 military jobs in Florida, the senator inserted language into the defense authorization barring the Navy from removing any of the ships from service, even temporarily.

Unable to repair the ships for long-term service, the Navy anticipates it will have no choice but to begin decommissioning them in 2020. Rubio saved slightly more than a thousand jobs in his state for a few years. But those jobs might have cost the Navy its bigger fleet, and Trump a key campaign pledge.