The Royal Canadian Navy has lost the use of another one of its air defence destroyers after rust was found in the hull, leaving the fleet further diminished as more than a dozen other vessels undergo regular maintenance, modernization and repairs.

HMCS Iroquois was tied up in Halifax sometime in mid-April after corrosion was detected in a machinery space in the warship that has also been plagued by structural cracks.

Cmdr. Jay Harwood says the vessel is undergoing an assessment to determine if it needs repairs, what that might cost and whether fixing the 42-year-old ship might prove too expensive before it is due to be decommissioned in the next few years.

"We recognize the need to assess what we're seeing here and make a well-founded engineering judgment," Harwood, who oversees the fleet's engineering state, said Wednesday in an interview.

"There were some areas of concern identified with respect to her structure and right now we're just assessing the overall state of her structure to confirm that she's safe to continue operations at sea."

Harwood would not specify where the corrosion was found or reveal how extensive it is, saying only that it is in the interior and that a navy dive team had inspected the vessel's underside to make sure it hadn't permeated the hull.

This latest setback removes a vital asset from the fleet and reduces certain critical capabilities, says defence analyst Martin Shadwick.

The destroyers serve as command and control vessels, but are also the only naval ships that have long-range air defence missile systems, he said.

One destroyer ready

With HMCS Iroquois indefinitely out of commission and its sister ship, HMCS Algonquin, undergoing repairs from an accident in February, the navy has only one destroyer at the ready.

It is also without many of its Halifax-class frigates, which are undergoing a lengthy modernization program to add radar and command and control systems, while upgrading radar and missile capabilities.

Shadwick said that could make it difficult to find ships to do fisheries patrols or participate in missions aimed at countering piracy, smuggling or drug trafficking, as well as any unexpected missions.

"At the exact time that your destroyer numbers are slipping, maybe permanently, you're also missing a lot of the frigates," he said.

"So the bottom line is, you're short of hulls and have fewer to send anywhere. ... Whatever you're using them for, you just don't have the ships."

That leaves the navy down eight vessels on each coast, with five in modernization and 11 undergoing repairs or maintenance. Out of 33 ships and submarines, only 17 are in service or "employable," according to the navy.

This latest problem comes after fatigue cracks were found on the Iroquois in February when the ship was in Boston. An engineering team travelled to the U.S. to inspect it and deemed it safe to return to its home port in Halifax.

Inspections stepped up

Harwood said they stepped up inspections of the ship after that, which led to the discovery of the corrosion.

He acknowledged that the rust problem could lead to the early decommissioning of the ship, which is due to be retired in the next few years and before any successor ships are in place.

He said the navy has had to juggle some of its operations and pull HMCS Iroquois out of some exercises, including one last week in Norfolk, Va.

"As we conduct our assessment and come up with a plan, that will determine whether any future requirements will have to be juggled," he said.

"In the coming weeks we'll have a good appreciation if we're done with the assessment or if a further assessment is required."

Shadwick said he expects the military would also have to look at her two sister ships to determine if they have similar rust problems. But navy spokesman Capt. Peter Ryan said the focus now is only on HMCS Iroquois.

The Iroquois-class destroyers were built in the early 1970s and extensively modernized in the early 1990s. They were designed for hunting subs, but were redesigned as command-and-control and air defence ships.