An expedition ship is defined less by its size than by its independence. It carries the equipment to put passengers ashore anywhere, because the destinations it visits have nothing to land against.
The destinations have no infrastructure
Remote coastlines, polar shores and uninhabited islands have no pier, no dredged channel, no navigational aids and often no accurate chart of the immediate shallows.
A conventional cruise ship depends on all of that existing. Removing it removes the ship's ability to operate, which is why the itineraries are separate categories of travel.
Expedition operators solve the problem by carrying the shore infrastructure with them, in the form of rigid-hulled inflatable boats, a launching arrangement and trained boat drivers.
Inflatables suit an unknown shore
An inflatable boat has a soft tube around a rigid hull, shallow draft and an outboard that can be tilted clear. It can be run onto a gravel beach and pushed off again.
That tolerance for grounding is the point. A hard-hulled tender would be damaged by the same contact, and a landing site cannot be surveyed in advance every time.
The boats also carry a small enough group that a landing can be staged progressively, letting guides assess conditions ashore before the next boatload is dispatched.
Launching is the limiting operation
Boats are launched by crane or from a stern platform, and both methods require the ship to be steady. Swell at the launch point ends the operation regardless of conditions ashore.
Passengers transfer at a low platform, stepping from a moving ship to a moving boat. This is the part of the day that most often gets canceled, and it is why boarding is choreographed and assisted.
Recovery is harder than launching, since the boats must be brought back aboard in whatever conditions have developed. Operators plan landings with a margin for the sea state building.
Guides and ratios are part of the equipment
An expedition ship carries a team of naturalists and boat handlers in a ratio to passengers far higher than a conventional vessel, because every landing needs supervision ashore and afloat.
Many polar and sensitive destinations operate under visitor guidelines that cap how many people may be ashore at one site at once, which sets the group size directly.
Those caps are why expedition ships are small. A vessel carrying thousands could not cycle everyone through a site within a permitted window, so the whole model depends on a modest passenger count.
Why itineraries are written loosely
Expedition itineraries name a region and a set of candidate sites rather than a fixed sequence of days, because ice, wind and swell determine what is reachable on any given morning.
The captain and expedition leader select from the candidate list each day, sometimes changing the plan between breakfast and the first landing.
Travelers who book expecting a conventional port schedule are frequently disappointed by that flexibility, which is not a shortcoming of the operator but the operating method itself.