FK 1 was 128m (420feet) long at the waterline and 130m (430feet) long overall. The design had a beam of 11.6m (38.1feet); its forward draft was 4.9m (16.1feet) and the aft draft was 4.1m (13.5feet). The ship would have had a double bottom of approximately 52 percent of the length of the hull, and fifteen watertight compartments. The hull was constructed with longitudinal steel frames. As designed, the ship would have had a displacement of 3000MT, with a full load combat displacement of 3800MT. The modified FK 1a was slightly larger, at 131m (430feet) long at the waterline and 136m (446feet) long overall. The modified design had a beam of 12.4m (40.7feet) and a draft of 4.6m (15.1feet). The ship's hull would have been constructed as in FK 1, with the same number of watertight compartments and extent of double bottom. As designed, the ship would have had a displacement of 4025MT, with a full load combat displacement of 4850MT.
Both designs would have been powered by two sets of Marine-type steam turbines that each drove a three-bladed screw 3.5m (11.5feet) wide in diameter. FK 1 would have been equipped with five Marine-type oil-fired watertube boilers, while FK 1a would have had improved double-ended models. The propulsion system of FK 1 was rated at a maximum of 48000shp for a top speed of 32kn. The improved engines of FK 1a were rated at 52000shp and a maximum speed of 33kn. The designs carried up to 1000MT and 1150MT of fuel oil, respectively, which permitted a cruising radius of 2800nmi at a speed of 17kn. Both designs were equipped with three diesel generators that produced at 220 volts. Steering was controlled by a single rudder.
The armament of both designs consisted of five 15 cm SK L/45 guns in single mounts, one forward, two abreast of the conning tower, and two in a superfiring pair aft of the rear superstructure. The 15 cm gun fired a 45kg (99lb) shells at a muzzle velocity of 835m/s. FK 1 and FK 1a were supplied with 500 and 650 shells for their main batteries, respectively. The guns had a range of 17600m (57,700feet). Both designs were equipped with a pair of 8.8 cm SK L/45 anti-aircraft guns, mounted on the centerline amidships. The guns were equipped with 100 rounds of ammunition each. These guns fired a 10kg (20lb) shells. Both ships would have carried four 60cm (20inches) torpedo tubes mounted on the deck in swivel launchers. FK 1a was also equipped with 100 mines. Both designs called for a crew of 15 officers and 342 enlisted men.
Over the course of the design process that continued through 1916, the size of the projected cruisers increased as the navy added new design requirements. This resulted in the FK 2 design. The length increased to 144m (472feet) overall and 139m (456feet) at the waterline. Their beam increased to 13m (43feet), as did their draft, to 5.5m (18feet). Their displacement correspondingly rose to 4500MT at normal load and 5350MT at combat load, significantly greater than the original design. The FK 2 design was armed with five 15 cm SK L/45 guns and two 8.8 cm SK L/45 anti-aircraft guns. It was to carry the same 60 cm torpedo tubes in twin mounts as the earlier designs. The design retained the same propulsion system as the earlier designs, but with an improved engine type and an additional boiler, which produced an estimated 60000shp for a top speed of 32 knots. Range was to have been 2,800 miles at 17 knots, as in the original design.
The next iteration, FK 3, brought even more increases. Displacement rose to 6000MT normal and 6900MT full load, double that of the original FK design. Length was 155m (509feet) at the waterline and 159m (522feet) overall, and the beam was 14.2m (46.6feet). The armament was also augmented by an additional two 15 cm and 1 8.8 cm gun. Since the size of the ship had increased, a more powerful propulsion system was necessary; a third set of turbines was added, and the number of boilers was increased to thirteen. This produced 70000shp, for the same speed and range figures as in the previous designs.
The final design, FK 4, was larger still. The standard displacement was 7500MT, and at combat load, this rose to 8650MT. The projected cruiser would have been 170m (560feet) long at the waterline, with a beam of 15.4m (50.5feet) and a draft of 6m (20feet). The armament was increased again, with an additional 15 cm gun. The ship's propulsion system would have included six coal-fired boilers and nine oil-fired models.
Ultimately, none of these designs were ever built, much like other late-war German warship designs, such as the L 20e α-type battleships and the s. The German shipbuilding effort largely abandoned surface warship construction and instead focused on U-boat construction in the final years of the war.