What must vessels do if both are sailing with the wind on the same side?

When both vessels are sailing with the wind on the same side, the navigational rule that applies dictates that the vessel to windward shall keep out of the way of the vessel to leeward. This is primarily because the vessel to leeward has the right of way, as it is more maneuverable and has a clearer path.

The rationale behind this rule is based on the dynamics of sailing. The vessel to windward is in a position where it can easily alter course or reduce speed to avoid a collision. In contrast, the vessel to leeward is already in a position to proceed on its course without any immediate need to change, making it less capable of reacting quickly if both vessels were to maintain their courses.

Understanding this rule is vital for safe navigation at sea, as it helps prevent collisions by ensuring that those in the more favorable position (the leeward vessel) can continue their course unimpeded while allowing the windward vessel to take the necessary actions to avoid a collision.