This is a screen capture at Cal Maritime of a Transas simulation of a Damen 2411 Z-Drive tug escorting a tanker. The tug controls are in the lower right and the red oval indicates the towline force generated by the tug. Ship's speed is about 6 knots.
The pilot orders the tug to "Transverse Arrest, Clutched In" because he/she wants to reduce the forward speed of the vessel.
The red oval indicates the towline force, the green oval is the tugs speed and course over ground. The tug controls indicate direction and power order to the z-drive units. The line force is relatively low because the order is "Cluchted In" not "full power". When the power is incresed one can see that the line pull increases even though the thrust is directed inboard and the prop wash is directed outboard. This is similair to dragging to large oars in the water to create drag. The force will normally be equal to the static bollard pull of the tug at about 8 knots and full power. This technique is used when the speed through the water is so high that there is a risk of overloading the tugs' engines.