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In the past bow bulbs were supposed to be unsuitable for slow, fullform ships. However, the present study reveals that they can cause a considerable reduction of resistance in the ballast condition. This property has been confirmed by model tests and full-scale operation in practice. Parameters for effective bulb design are indicated. The sensitivity to weather conditions and the economic significance of bulbous bow ships are investigated. The second part of the paper describes a series of special experiments conducted with a tanker model for clarifying the physical mechanism by which the bow bulb reduces the resistance of slow, full-form ships. The tests comprised measurements of resistance, viscous wake and wave pattern as well as flow observations by means of film and underwater television. It was found that contrary to previous hypotheses the bulb produced its effect not so much by wave cancellation or improved flow around the bilge as by elimination of the breaking bow wave.