Blue Bird K4

Blue Bird K4 was a powerboat commissioned in 1939 by Sir Malcolm Campbell, to rival the Americans' efforts in the fight for the world water speed record.

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 Bluebird K7.

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