In the X-wing series of books several Star Destroyers were either destroyed or disabled by Proton torpedoes and concussion missiles fired by fighters; something only slightly better armed than the speeders you're talking about. The primary reason the Acclamator in the Clone Wars turned her lower hull to the enemy was it had no hatches that could be blown open, something the Imperial 1 Star Destroyer can boast. Regardless of how well armored a ship is, any place where there is a seam is a danger. For that matter, armor itself can be a problem. The Air Craft Carrier Shinano, built on a battleship hull, was struck and sunk by only four torpedoes fired by Archerfish. The primary cause according to their own court of inquiry was that the shockwave from the torpedoes drove beams that seperated the inner and outer armed hulls, causing ruptures in the inner hull.
Last edited by machievelli; 01-10-2010 at 10:24 PM.