Roll-coupling effect on ship maneuvering motions is not negligible for high speed vessels, large container ships, passenger ships etc. To perceive the roll-coupling effect on the ship maneuverability, this paper proposes new simplified formulae for course stability criterion and steady turning index in the framework of the linear motion theory. Then, an approximation is employed where the depth wise acting point of lateral force related to roll moment is constant in any oblique motion mode. The course stability criterion including the roll-coupling effect is written as<br />
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where C′ denotes the usual course stability criterion, and the linear hydrodynamic derivatives and l′ the depth wise acting point of the lateral force. The formula is simple and useful for checking the course stability taking the roll-coupling effect into account. To obtain the hydrodynamic derivatives, oblique towing test and circular motion test with various heel angles were conducted for models of a container ship, a pure car carrier and a ferry. In general, course stability deteriorates with increasing Froude number Fn or decreasing of metacentric height GM′, whereas turning performance improves and the time until response reaches steady turning state increases.