General:

Warhammer is a longship of the Iron Fleet. She is a big ship, large and heavy and slow.[1] She is named after the warhammer, a type of weapon.

Recent events:

The Iron Fleet head to Meereen. When they leave the Stepstones, Lord Captain Victarion Greyjoy divides the fleet into three, placing one of the squadrons under Ralf the Limper, captain of the Lord Quellon. Warhammer goes with Ralf, sailing to Lys to sell slaves taken from the Shield Islands and load provisions for the long slow middle age east. At the rendezvous point on the Isle of Cedars, the Lord Quellon crawls in with only thirteen of the ships it lead (among which are Warhammer, White Widow, the Lamentation, Woe, Leviathan, Iron Lady, and Reaper's Wind), with two of the ships storm-wracked and under tow. Ralf explains that they were hit by three big storms, and foul winds between, driving them towards the blighted shore of Old Valyria.[1]

Sources:

1- A Dance with Dragons: Chapter 56 (The Iron Suitor)