The issue isn't just displacement. The speed of the rotating prop drawing water from the canal ahead of the boat is also a factor, as is the cross section of that area of canal, the draft of the moving boat etc. etc. The effect can be observed with a simple experiment (which I usually end up having to repeat most times we moor up. It goes like this... You find a nice quiet spot to stop for the night, bring the boat alongside where you want to moor and crew member jumps onto tow path with centre line and holds boat while 'proper' mooring is done using fore and aft lines. Another boat approaches. Then, at this point one of two things can happen.
Scenario A. They see crew member holding boat on centre line and slow right down well before they get to us. Our boat moves back and forth a little bit (two feet or so), but not to the extent that holding it on a centre line is problematic.
Scenario B. They approach, and pass without slowing down… our boat is drawn towards them when they are 100 yards away, and as they get closer we now need both of us holding on to the centre line for dear life (boat can easily move 8 - 10 feet even with us trying to stop it). They pass obliviously and everything settles down after a few mins and we try to complete the mooring.
I know this is a bit off topic ? but this all too often experience gives me empathy when I see others mooring and I slow right down in advance, knock it in to neutral when I get closer, and glide past imposing just displacement on the other boater... and they hardly move.
Mile after mile of moored boats though? Yes, they should be moored properly because we should be able to go by at a courteous but not ultra slow speed!
Cheers
Chris