A method for determining the cumulative lateral slip of a towboat or combination barge tow/towboat. The method comprises providing two positional sensor instruments mounted at either end of a combination barge tow, each positional sensor capable of determining its geographical location from radio transmissions from, e.g., GPS satellites. The velocities of each positional sensor, both parallel and lateral (perpendicular) to the bearing of the barge tow, are determined. The cumulative sum of the absolute differences of the lateral velocities of the two positional sensors over time is calculated, stored and displayed by a microcomputer.