Siege Charge is the largest, heaviest grenade - it has no trouble blowing up buildings and forts, let alone a pirate's enemies. The siege charge is very heavy, and slows down the pirate carrying it. It can, however, fly just as far as any other grenade. It requires some skill to manage, but when it goes off - Katie bar the door! But be aware that this grenade's Area of Effect (AoE) is VERY big, possibly bigger than even the Stink Pot AoE, and if it lands too close to you, can cause damage to yourself!
To use a Siege Charge, you must have your grenades out and then press 5 or click the Siege Charge skill.