The Steamship Authority’s original budget to convert its three newest vessels into freight boats has been blown out of the water by sharply rising shipyard costs, boat line officials said this week.

The lowest bid to do the work is about $20 million for each boat, general manager Robert Davis told the port council Tuesday morning.

“We’d been looking at something in the $9 million range,” Mr. Davis said.

Supply chain costs and material prices, particularly for steel, have been going up significantly, director of marine operations Mark Amundsen said.

About 30 shipyards, from Rhode Island to as far away as Florida, Louisiana and Texas, requested the bid package, Mr. Davis told the council.

But only two companies actually bid on the job, he said.

“The yards that had previously taken out [bid] packages had indicated that they were fairly busy,” Mr. Davis said.

To reduce costs for the work, Mr. Davis suggested the authority extend its schedule for converting and recommissioning the three former oil-rig service vessels, at least two of which had been intended to go into service this fall.

“[This] is a very aggressive timeline, in terms of getting the work done, and so it appears the [bidding] yards built in a considerable amount of overtime in order to accomplish that,” Mr. Davis said.

He felt there was no reason the authority couldn’t extend the delivery time for the vessels, cutting on overtime costs.

“As long as we have them by the spring of 2024, we should be in good shape, so I think that will make a big difference in terms of some of the cost structure,” Mr. Davis said.

Nat Lowell, who represents Nantucket on the port council, endorsed Mr. Davis’s recommendation, saying the three identical ferries are crucial for both islands and will be worth the wait.

“The opportunity is so good to do this that I don’t care if it takes two more years,” he said.

Mr. Davis said the boat line will work with the low bidder to bring down the current estimate, and re-bid the project if necessary.

The Steamship Authority bought the first two vessels, renamed M/V Aquinnah and M/V Monomoy, last fall, paying about $10 million for both, and the third, M/V Barnstable, in December for $5.7 million.

All three saw brief service in the Gulf of Mexico before being mothballed more than a decade ago. They’ve since passed extensive inspections before changing hands.

An option to buy a fourth sister ship from the same seller, Hornbeck Offshore Services, was due to expire this month.