A Trimaran. It looks like a large trireme is the center hull and viking longboats on both sides as outriggers: making one very large ship.
The Ship is black, it looks old, and covered in tarnished silver and green copper.
Two large Outriggers sit in the water on each side of the ship to stabilize the massive ship, and prevent it from capsizing.
Make the ship look meaner, and more like a pirate ship. Darker. More foreboding.
Even more black. And define the outriggers more.
Make it taller.