I’m a landlubber, so I’m depending on folks who know way more than I do. But 2 thoughts: If I have hostile intent, would I approach at flank speed, knowing that might tip off my target to my intent? Especially since my mass matters more than my speed. Second, ram implies, to my mind anyway, an intent to t-bone, which would be impossible to do by coming up from behind the target. All that’s needed is a collusion.
Collision. Though I wouldn’t rule out collusion, especially if this was a hostile act.