PORTLAND – The Portland Fire Department should have reported an accident that damaged the City of Portland IV fireboat, and there are indications the vessel was outside a marked channel when it occurred, Coast Guard officials said Thursday.

The fireboat struck something underwater just north of Fort Gorges in Casco Bay shortly before sunset Saturday during what Fire Chief Fred LaMontagne described as a “navigation training” run.

The impact sheared off one of the boat’s two propeller shafts and damaged a propeller and the rudder. LaMontagne said he is investigating, but comments by fire department officials to the Coast Guard suggest the boat may have been outside the channel.

Lt. Nick Barrow, Coast Guard sector command center chief, said he spoke with the department to determine where the accident occurred so other vessels could be warned, if necessary. He said he was told the object posed no hazard to vessels in the channel.

Asked if that suggested the fireboat was outside the channel, Barrow declined to comment. He said the Coast Guard was only concerned with whether to warn other vessels. LaMontagne also declined to comment, saying determining the location is part of his investigation.

The chief has refused to say how many people were aboard and who was at the helm.

Barrow and Lt. Mason Wilcox, chief of investigations for the Coast Guard in South Portland, said the accident should have been reported verbally Saturday night, with a written report filed in five days, which would have been Thursday.

Technically, hitting an underwater object is considered an “unintentional grounding,” even if the vessel is able to continue, as was the case Saturday, Wilcox said. He said the incident should have tripped other reporting triggers because it limited the vessel’s maneuverability and likely caused more than $25,000 in damage.

LaMontagne said Wednesday that he didn’t think the city was required to report the accident, and Barrow agreed at the time. But Wilcox said Thursday the accident did meet the “marine casualty” reporting threshold.

It was the second time that the $3.2 million fireboat has been damaged in Casco Bay. In November 2009, the vessel hit a ledge in Whitehead Passage between Peaks and Cushing islands. The repair bill was $90,000, and the boat was out of commission for three months.

Saturday’s accident did not completely disable the boat, but it has been placed on limited duty — the city’s older fireboat, the Cavallaro, will be used for most calls.

The fireboat’s insurance carries a $25,000 deductible.

Under Coast Guard regulations, the city could be fined up to $5,000 for a first offense of failing to report an accident, but Barrow said that’s unlikely.

The Coast Guard has great latitude on whether to fine or warn over violations, Barrow said, and Portland is “one of our key port partners.” The Coast Guard wants to maintain a cooperative relationship for future joint training, island medical evacuations and bay search-and-rescue operations, he said.

Wilcox also said the fireboat pilots don’t need licenses because the vessel isn’t commercial.

Some of the fireboat pilots carry Coast Guard licenses to operate vessels up to 100 tons gross tonnage, a figure that refers to a boat’s internal capacity or volume. But LaMontagne said the city allows unlicensed firefighters to operate the boat, provided they have trained under a licensed pilot.

LaMontagne said late Thursday that he and other fire officials plan to sit down with the Coast Guard in the next few days to review the accident.

Staff Writer Edward D. Murphy can be contacted at 791-6465 or at: emurphy@pressherald.com