Engine troubles will force the ferry service between Yarmouth, N.S., and Portland, Maine, to run a little slower and less frequently this month.

One of the Cat's four engines is out of commission, which means the ferry isn't running at full power though it remains in service.

Bay Ferries Ltd., which operates the Cat, said the problem was first detected a week ago and it has made scheduling changes as a result.

"On June 28, Bay Ferries crew identified a failure in the Cat's starboard outer main engine," the company told CBC News in an emailed statement.

The Sunday crossing was cancelled and there will be upcoming disruptions as a modified schedule is introduced. Customers will be contacted directly if a service cancellation is anticipated, the company said on its website.

5 round trips a week instead of 6

"Under this modified schedule, the vessel is completing five round trips per week as opposed to the six trips originally scheduled for the month of July," Bay Ferries said.

The high-speed ferry normally takes 5½ hours to make the crossing between Nova Scotia and Maine but now takes an additional hour.

The company and the manufacturers of the malfunctioning engine, MTU, are investigating the problem.

In March 2016, the Nova Scotia government reached a 10-year, $32-million agreement with Bay Ferries Ltd. to manage and operate the ferry.

The deal included $9 million toward the ferry's retrofit executed last spring at Detyens Shipyard in Charleston, S.C.

The Cat ferry is a former U.S. navy vessel that had previously been used as a fast ferry in Hawaii.