Vashon’s two oldest ferries are now sailing a little slower due to mechanical problems, but so far they’re mostly staying on schedule, Washington State Ferries officials say.

The Klahowya and Tilikum, which serve the north-end triangle route and were built in the 1950s, are both experiencing drive-motor problems that require them to sail at slightly slower speeds. The Klahowya was also out of service for a time for repairs, but is now back on the route.

David Moseley, head of WSF, said in a press release that the ferries are operating about two knots slower than the usual speed of 14 knots, or at about 14 mph compared to the usual 16 mph.

“These two ferries, plus the Evergreen State, are fragile because they are the oldest in the fleet,” Moseley said.

He said that during the first weekend in April, only four ferry departures on the triangle route were more than 10 minutes behind schedule — a regular on-time performance for the boats.

Marta Coursey, a spokesperson for WSF, said that the ferries on the triangle route did get behind schedule for a period of time last weekend because they couldn’t keep up with increased Easter weekend traffic. On Monday they were sailing on schedule again.

“During our peak travel times we’re able to keep the schedule, during heavy traffic and major commute time,” Coursey said. “Where we see record travel on all of our routes, that’s when we’re not able to keep up.”

The three ferries are slated for replacement; the first new 144-car ferry is currently being built and will be operational in 2014.

In the meantime, WSF is waiting to find what repairs to the boats would cost before deciding how to proceed.

“We will continue to monitor the situation closely and be as creative as we can in terms of scheduling,” Moseley said. “But there is simply no way of getting around the fact that these ferries are very old and on borrowed time.”