The original propeller fitted to the aircraft had been damaged in a landing mishap, and the owner/pilot ordered a replacement from the aircraft manufacturer. The new propeller was of different dimensions and weight to the original, and the owner sought and obtained the manufacturer's assurance that it was suitable for the aircraft type. A number of static and taxy trials were completed without problem. However, on the first take-off, the aircraft had reached a height of about 50 feet when the propeller failed. The aircraft pitched down and struck the ground. A subsequent examination of the propeller revealed that it had failed as a result of the applied centrifugal forces. It had been constructed in four laminations, using two types of wood. One lamination contained a large knot in the hub area, and it was probable that another lamination was affected by brittleheart. This weakness is characterised by minute compression failures in the walls of the fibres, induced by growth stresses in the particular tree. When the propeller failed the aircraft was at a critical stage of flight, and stalled. There was insufficient height available for the pilot to regain control before impact with the ground.