Kill Kroozer
The most infamous of all Ork spacecraft, and the backbone of any self-respecting Ork fleet worth the name. Usually hammered together by Meks from the looted hull of another race's ship when they can get it, and festooned in guns of all sizes and types, lone Kill Kroozers on the prowl're a common risk to merchant traffic at the best of times, but during wars whole armadas of these ramshackle craft can be assembled from all the wrecked vessels the Orks've managed to salvage.
Notable Kill Kroozers
Butcher
A Kill Kroozer active during the Gothic War, though this wasn't it's actual name, as Naval intelligence never learnt its native Ork designation, and it just sort of came to be known by this moniker due to its actions.
This vessel was single-handedly responsible for the loss of fifteen transport ships, seven Escorts of various classes, and even a Cruiser during six successive attacks in the Quinrox Sound, as well as the looting of Bralutha Station. It was never caught.
Slaughta King
Another ship from the Gothic War, this one took advantage of the Imperial forces being distracted by Chaos fleets, to lead an Ork flotilla out of the Cyclops Cluster and into the Moab System. This fleet was engaged by the Fourth Cruiser Battlegroup, containing Four Mars-Class Battlecruisers with Nova Cannons; Imperious, Hammer of Justice, Minotaur, and Lord Daros. Lord Daros was boarded and lost with all hands, the other three were all heavily-damaged and forced to withdraw. leaving the Civilised Worlds of Moab wide open for the Orks.
In Battlefleet Gothic
In the original tabletop game, Kill Kroozers were as tough as Grand Cruisers, fitting in between Battleships and Battlecruisers in terms of how many Hits they could take, being Cruiser/10 to their Battleship/12 and Cruiser/8 respectively. Along with Terror Ships, they were the biggest vessels you could take without entering Rok or Space Hulk territory.
They were also very heavily armed for their size with a total of SIX batteries, three Gunz and three Heavy Gunz, with one of each covering its port, starboard, and prow facings. They weren't too strong compared to Imperial capital ship armaments however, at 4 Firepower for the Gunz and 1d6 for the Heavy Gunz, with the prow pair being 6 and 1d6+2. They were also very short range, with Gunz only reaching 30cm, and Heavy Gunz half that at 15cm, compared to the 45cm and even 60cm ranges their Imperial equivalents could hit from. The prow Heavy Gunz Battery could also be swapped out for a Torpedo Launcha for free, with a Speed of 30cm and a Strength of 1d6+2, which could then be loaded with Boarding Torpedoes, reducing the Speed down to 20cm.