Vessel motion control based on propeller slip

A control system for controlling at least one marine drive unit on a vessel,

• the control system being configured to obtain data indicative of a speed through water, STW, of the vessel,
• the control system being configured to determine a propeller slip of the at least one marine drive unit based on a propeller speed of the drive unit and on the speed through water of the vessel,
• where the control system is configured to obtain a relationship between drive unit thrust and propeller slip, and
• where the control system is configured to obtain data indicative of a desired thrust to be generated by the drive unit, and to determine a target propeller slip corresponding to the desired thrust, based on the relationship between drive unit thrust and propeller slip, the control system being arranged to control the propeller speed of the marine drive unit to generate the target propeller slip and to maintain the propeller slip below a predetermined propeller slip limit,
• where the control system is arranged to control a plurality of marine drive units on the vessel, where the control system is configured to obtain the data indicative of speed through water separately for at least two drive units out of the plurality of drive units.