I've seen this too. When a ship is going to pass by I try to make a intersecting course with the enemy trireme. If I do not make a intersecting course my Athenian triremes will tail behind as well, until it bumps into the back of the ship, instead of losing sight of the enemy, but it also doesn't give me a ram bonus either.
I have also realized, sometimes when you pause, and then unpause the game the triremes come to a complete stop, and then continue going. Losing all momentum and sometimes ramming speed. Why is this? It can be the worst thing that can happen during a naval invasion.
Also, Hythlodaeus, not only do triremes fight better in boarding action when they have troops garrisoned in them, they are also slower.
My tactic when I fight with triremes is to space them out. Let's see if i can give you a picture. ^ = triremes
With this formation it allows me to intersect triremes. Allowing one trireme to be rammed by another trireme is also something I'll use, if the enemy trireme is moving slow enough of course. When the enemy rams you, he will get stuck to your trireme, giving you enough time and a opening to ram it full speed and sink it in one hit.
Something about troops on triremes. Sometimes no matter how good the infantry on the triremes are it can still be sunk doing boarding actions. How this happens is when one of the triremes rams the other, depending on the speed (As far as I know), it gives the trireme a ram bonus. The ram bonus depletes the hp of the ship getting rammed. for example, trireme A rams trireme B, going at 3/4 of top speed, equals ramming bonus of 1750. (those aren't exact numbers, just examples) That 1750 is taken out of trireme B's health of 2000, which leaves it at just 250. Which is more than low enough for a scout cavalry brigade to defeat a phalanx brigade on a trireme.
The triremes are very interesting on this game, and one of my main reason of playing, but they do have some bugs to be worked out