Inappropriate tripping devices may either not promote transition effectively or cause significant distortion of the initial boundary-layer development. By reexamining data for reflex geosims of Lucy Ashton and a small 0.80-CB tanker it is shown that this distortion can readily occur. To compare the boundary-layer development near the bow of a ship model under different test conditions the relative position of the virtual origin must remain constant. The concept of a virtual origin is considered using an approximate form of the momentum integral equation and it is found that locating the virtual origin is a highly ambiguous process, and therefore it becomes difficult to decide if the virtual origin remains constant or not. To compare and extrapolate total drag results, a correction for the virtual origin position is usually applied. For two widely different ship forms it is shown that the total skin friction coefficient is relatively insensitive to a poor estimate of the virtual origin position and an inappropriate choice of stimulator.

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