Bayliner steering

Question
  I have a 1974 Bayliner flybridge with Mercury dual station power steering. The problem I have is that underway the steering bis hard to turn then becomes easy . This happens at both stations. I checked the obvious, belt condition and tension are good. Fluid level and appearance are good as well. With the boat sitting and the engine off the steering feels normal.With the engine idling it appears to be fine as well. Any help would be appreciated as my tech sources leave a lot to be desired here.

Answer
Hi Kevin

Thank you for selecting this site to reply to your question.

I'm not a mechanic, but it sounds like you've explored all aspects from top to bottom quite thoroughly. However, the binding problem exists at both stations and only when underway. About the only thing I can think might be causing the periodic stiffening in the steering is deterioration within the hydraulic system. Maybe its within the pump or cylinder bushings or seals, if all else is in good, sound condition, and well secured. Should all else fail, engage a knowledgeable & reliable marine mechanic.

Safe Boating
Ron