The name "K4" was derived from its Lloyd's unlimited rating, and was carried in a prominent circular badge on the forward hull.
K4 was built by Vosper & Company as a replacement to the Blue Bird K3, which had set three other water speed records for Malcolm Campbell before the K4 was built. It also used the same Rolls-Royce R engine.
K4 was a three-point hydroplane. Conventional planing powerboats, such as Miss England or Blue Bird K3, have a single keel, with an indent or "step" projecting from the bottom of the hull. At speed, the force on this step is enough to lift the bow upward, reducing the wetted surface area of the hull and thus also the frictional drag. A "three pointer" has a two distinctly separate floats fitted to the front, and a third point at the rear of the hull. When the boat increases in speed, most of the hull lifts out of the water and planes on these three contact points alone. These points being even smaller in area than the planing hull of a monohull hydroplane, have even less drag. Having a broad spacing between the front planing points, the three-pointer is less susceptible to instability caused by small disturbances than is a monohull. However, if the bow lifts beyond its safety margin, the aerodynamic forces (not the hydrodynamic forces of the water) on the broad forward area of the hull will cause it to "kite" upwards, leading to a somersault and crash. This is what happened to both Slo-mo-shun and possibly Bluebird K7.
Jet engine trials
After World War II, Sir Malcolm unsuccessfully re-engined K4 with a de Havilland Goblin turbojet engine but did not gain any records. The new superstructure did gain the nickname 'The Coniston Slipper' .
Donald Campbell (Malcolm's son) had not previously attempted record-breaking, but after Sir Malcolm's death at the end of 1948, the threat of an American challenge to his water speed record spurred him to defend it. Under the terms of his will, Sir Malcolm's possessions, including the record breakers, were auctioned off and Donald was forced to buy them back. K4 was then re-engined with a propellor and one of the previous Rolls-Royce R engines.
Donald tried the boat but deemed it too slow, so after another superstructure rebuild and finally structural failure in 1951 it was replaced by the jet-powered K7, in which Donald set several records, then died during a final record attempt in 1967.
- Harris, Fred (2000). Skimming the Surface. Ainsdale Press.
- Tremayne, David (2005). Donald Campbell: The Man Behind the Mask. Bantam Books. ISBN 0-553-81511-3.