Problems firing the Trebuchet in the Order of the Whispers quest line?

On the quest where you fight the Orr/undead on Claw Island, at one point you need to fire a trebuchet at a ship in the distance which can be quite difficult.  Rotate the trebuchet so that it's pointed at the ship, then hold down to about 85% power and then let go.  A single hit will sink it- no need for multiple shots.  In order to tell if you're firing too little or too much, look at the shot as it falls.  If you can see the shot in FRONT of the ship, then you need more power.  If the shot falls BEHIND the ship, you need less power.