That explains two ships crossing, but in a busy place like the Straights, maneuvering and changing speed and direction can cause a chain reaction affecting other ships. Just one of many examples-- assume the McCain was moving at top speed crossing in front of the tanker, but another ship accelerated into the McCain's path beyond the freighter. You need to make quick calculations because as soon as the tanker passes behind The McCain going at top speed, it will need to slow quickly and maneuver to follow rules of the road and avoid a collision with another ship X that imay also be changing course or speed. Iif X is a large ship, it may mean the McCain had to slow and if it did so too soon it could be hit by the tanker. Othe maneuvers were possible for the McCain-- it could have turned to Starboard then accelerated  in the same direction as the tanker. But what if there was another ship Y in front slower than the McCain?