[Gwypaas]

All depends on how far you go with your maneuvering capabilities, which is simply related to costs. Dynamic positioning [0] has existed for a long while and has now moved down even to small dual engined boats [1], on the other hand it requires much more tech and sideways and rotational thrust capabilities than what regular cargo vessels have.

A single prop with a bow thruster, or especially without, can't move freely in the 2D space which makes it much more complicated because you have to move towards the dock with the correct angle depending on wind, current and what not. Then time your maneuvers while taking the environment and for example prop walk [2] and other effects into consideration.

[0]: https://en.wikipedia.org/wiki/Dynamic_positioning

[1]: https://www.youtube.com/watch?v=491RkaOYfr4 Volvo Penta Assisted Docking

[2]: https://en.wikipedia.org/wiki/Propeller_walk

[yobbo]

But the space is not 2D unless it's a small calm canal. Waves/swells/wakes can make the difference in vertical position of the bow relative to the stern change by meters (or whatever depending on length) but it can be significant relative to human size. In some situations it might be impossible to take on/let off passengers, so the boat would need to plan its motions taking this into account while otherwise not obstructing or causing danger.

The goal then for the robot is not to position itself in 2D space with meter-level precision, the goal is to behave reliably and predictably in the harbour under chaotic circumstances, with sub-meter precision. That requires predicting movements and drift, taking wind into account. Since this is almost impossible, skippers learn to read the water/wind to understand what is appropriate for safety distances, speed, angles, etc.

A road vehicle could just refuse to move if the environment becomes unrecognizable, but a boat keeps on moving, so the robot can not just "give up".

It's true that thrusters increase the manoeuvrability.

[Gwypaas]

In the spaces where waves and wakes make a difference the tolerances are larger. The one exception is for example smaller commuter ferries without a ramp, then you might have to time it as you say.

Wind and current doesn't really matter as long as you have dynamic position which abstracts it away as long as you keep within your power budget. The real hard part here is moving through a shifting environment, for example the current often reverses along banks. Especially in a river based harbor environment where you've created piers and what not disrupting the flow.

In a previous life I worked as a skipper in just those conditions, passenger ferries in the 25-40m sizes with a couple of hundred passengers in river harbour environment. Two props and bowthruster and you have to keep all the considerations you mention in account. Especially since you can't angle the stern without angling the bow (duh) when you're applying reverse thrust to dock. And with some speed forward you move the center of rotation forward making the bow thruster less effective.

It was actually easier with quite a strong wind since then that would overcome the current easily and you would lie on the wind margin side of things. If you come too high just make the docking take a bit longer so you would blow down and land perfectly. In almost calm conditions you would instead have to guess which would win that time, wind or current.

That said, the ones with two pods, one at each end you just balance things out and go straight to the dock.