PURPOSE:To control a main engine remote control device and autopilot for protecting safety of a ship by automatically detecting a stormy weather condition during sailing. CONSTITUTION:In a stormy weather navigation assist device connecting an electromagnetic log 1 for detecting a log speed of a ship and a tachometer generator 2 for detecting a rotational speed of a propeller, an arithmetic processing means 10 is provided, and in the arithmetic processing means, based on the input log speed V and porpeller rotational speed N and a set pitch P of the propeller, an arithmetic formula Sl=[1--30.9x{Va/(Na.P)}]X100[%] (where, Va is mean value of log speed, Na is mean value of propeller rotational speed) is used to calculate a virtual slip Sl. Further, after calculating a difference 'Sl-So' between the virtual slip Sl and a virtual slip So set at calm time, when this difference exceeds a specified value, a stormy condition is judged, to obtain a deceleration command, weather adjusting mode switching command and an alarm signal based on judging.
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