Consultants have "on call" contracts with a huge overhead rate on the hours that are worked to cover keeping the staff available (though, usually, staff are working on other things when not working on that contract - still, gotta do the paperwork and management). Lawyers have retainers - money you pay to have the lawyer available, plus whatever costs are incurred when actually working. Plumbers charge a minimum (usually and hour or 2) plus travel time, and if you're like me whatever's going to go wrong always happens during nights and weekends (overtime rates!). Don't railroads have to pay some minimum (1/2 day or the like) just to call somebody in for a job, even if it only takes a couple of hours? So 4 hours to deal with tying up a barge, even if the unloading or loading is done some other time, seems not out of line. Of course, if the company expecting the load (or wanting to load up the barge) times things right perhaps they can get it done in one day (minimize the demurrage...) rather than calling out the dock workers another time. Lots of "minimum charge" things running around.