VICTORIA -- The Royal Canadian Navy is set to deploy a submarine for the first time since 2018, and says it will have most of its aging sub fleet back in the water by the end of the year.

HMCS Victoria, one of four submarines in the Canadian fleet, began dive trials off Vancouver Island last week, testing the integrity of the boat's hull after six years of maintenance and upgrades since its last deployment.

All of Canada's second-hand military subs have been grounded and inoperable for the past year following a busy 2017-18 deployment season for two boats, HMCS Chicoutimi and HMCS Windsor.

The Chicoutimi spent a record-breaking 197 days at sea in 2018, patrolling the Asia-Pacific region from its home port in Esquimalt, B.C. The deployment was the longest ever for one of Canada's Victoria-class submarines and also marked the first Canadian sub visit to Japan in 50 years.

The Windsor, meanwhile, embarked on a near-simultaneous 130-day outing to the Mediterranean Sea for a pair of NATO training and counter-terrorism missions.

"Submarine operational cycles and deployments are cyclical and, on completion of these two historic deployments, we entered into a reconstitution phase for the submarine force," said Capt. Jean Stéphane Ouellet, commander of the Canadian submarine fleet.

"This allowed time for the crews to recover from the high tempo of operations and for the maintenance facilities to conduct work to prepare for the next period of high-tempo operations," he added.

HMCS Victoria, the first sub to return to sea following the year-long deployment gap, will feature a new sonar system and battery when it departs CFB Esquimalt in the coming months.

Its deployment will be followed shortly by HMCS Windsor, which returns to sea this summer after an extensive drydock work period. The Windsor will be tasked with test firing a new heavyweight torpedo system that’s slated for rollout across the fleet.

With both boats deployed internationally by summertime, the Canadian sub force will turn its focus to getting HMCS Corner Brook ready for its own sea trials later this year.

The Corner Brook has undergone extensive repairs and upgrades since June 2011, when it struck the seafloor off B.C., causing severe damage to its nose cone and sonar system.

Maritime Forces Pacific spokesperson Capt. Jenn Jackson said having three subs simultaneously operational – one off the East Coast and two off the West Coast – "is a historical milestone we are aiming to achieve this year."

Canada bought its four used subs from Britain in 1998 at a cost of $750 million, and has since put billions into maintaining and upgrading them.