PORT TOWNSEND— Fifteen ferry sailings on the Port Townsend-Coupeville route have been canceled due to low tides that impede passage of the large vessels in and out of Keystone Harbor.

Seven additional sailings were added to the route to compensate for the cancellations.

Ferries spokeswoman Susan Harris-Huether said the cancellations are planned on a quarterly basis and are based on tide tables that are forecast in advance.

By using these tables, the ferry system is able to predict which sailings will need to be canceled, she said.

The 6:30 a.m. and 9:30 a.m. sailings from Port Townsend were canceled Thursday and again today, with 5:50 a.m. and 7:20 a.m. sailings added.

According to a posting on the Washington State Ferries website, http://tinyurl.com/4mta2j2, extreme ebb tide currents of 3.5 knots or more cause steering and control problems for ferries entering Keystone Harbor.

Steering is crucial while entering the narrow end of a teardrop-shaped harbor such as Keystone, and extreme currents can increase the danger of going aground.

As the bow of the boat passes the entrance to the harbor, the stern, still in the current, is buffeted and pushed north, making it difficult to maintain course.

The state ferries’ solution is to determine when the current is 3.5 knots or greater and schedule accordingly.

Harris-Huether said the system works and that the traveling public understands the concern for safety and appreciates the opportunity to plan ahead and minimize the inconvenience.

After today, the ferry schedule will be amended on three days.

On Saturday, the 9:30 a.m. sailing from Port Townsend and the 10:15 a.m. sailing from Coupeville will be canceled.

On July 30, the 8 a.m. sailing from Port Townsend and the 8:45 a.m. sailing from Coupeville will be canceled.

On July 31, the 6:30 a.m., 8 a.m. and 9:30 a.m. sailings from Port Townsend and the 7:15 a.m. and 10:15 a.m. sailings from Coupeville will be canceled.

On that date, rescheduled sailings for Port Townsend will leave at 6:10 a.m. and 7:40 a.m. and from Coupeville at 6:55 a.m.

For more schedule information, visit http://tinyurl.com/5trslk7.

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Jefferson County Reporter Charlie Bermant can be reached at 360-385-2335 or at charlie.bermant@peninsuladailynews.com.