What is the effect of being in a seaway on a vessel's GZ curve when a wave crest is midships?

When a vessel is in a seaway and a wave crest is midships, it affects the GZ (righting arm) curve primarily due to the ship's behavior in relation to the water's surface. When the wave crest is directly at midships, the vessel experiences a temporary shift in its metacenter and stability characteristics.

As the vessel rolls due to the wave action, the effective waterplane area can decrease. This reduction occurs because the waves can cause the hull to ride higher than normal on the crest, which, in turn, results in a lower waterplane area. A smaller waterplane area can contribute to a decrease in stability because the vessel relies on sufficient waterplane area to create the righting moment that opposes the heeling due to forces like wind or waves. Consequently, with reduced stability during this condition, the GZ curve is adversely affected, which aligns with the reasoning behind the correct answer.

Understanding this concept is crucial for a Chief Mate, as managing stability in rough seas is vital for vessel safety and performance. The decreased waterplane area translates to a lower righting arm for any given heel angle, making the vessel more susceptible to capsizing if significant forces come into play, such as strong winds or further wave action.