Like I said its all about the specific job, like log towing or barge towing, need a different prop for each. Not that complicated. For a hard slog like logs, you need all the turns you can get at low speed, whereas towing barges where speed is increased, a wheel is called for that gets turns at the high end, more pitch. If you try to tow logs with a scow wheel, the engine will be lugged down and inefficient, but the log wheel wont pull as hard dragging a big ol scow around behind it. Its pitch. The term under-propped seems really out of place when referring to a tugboat that as I said typically will have a very large prop for its size. I have not worked on tugs for over 10 years but when last I did a variable pitch prop was not something usually encountered on a tug, maybe things have changed. As far as harbor work goes, I would expect it to be propped more like a log tug than a scow tug, unless it also does a lot of other tasks. If you want all your turns and don't want the engine lugging along bellowing black smoke, you gotta have a slippery prop, less pitch.