Assume that you are the steerer of a 60' steel narrowboat built out of 10/6/4 plate. The boat will weigh in the region of 20 tonnes and, since it floats, this means it will push aside 20 tonnes of water when it is first lowered into the water. Now let's move this boat one length forward. When everything has reached equilibrium again you will fine no groove in the water where the boat used to be (!) therefore 20 tonnes of water will have moved from in front of the boat to behind the boat. In an ideal world the water would have been able to flow around each side of, and underneath, the boat to achieve this. But, of course, it is far from an ideal world!
Now let us assume the boat is travelling through the water at close to the canal speed limit 0f 4 m.p.h. (ca. 6 ft/sec). 2 tonnes of water have just 1 second to move back by 6 feet! We are all familiar with the effect of this dramatic movement - a ridge of water in front of the fore end (the bow wave), a lowering of the water's edge at the canal sides ahead and beside the boat, and a ridge in the water behind as the propellor provides the power to move all this water.
The canal was built 40' wide and
4'6" deep at the centreline with a curved bottom profile (invert). On the
northern Oxford this profile terminates at the edge at normal water level.
On many other canals it terminated at the edge somewhat below the
In the last 200 years mud from bank erosion and other sources has formed a sediment where the passage of boats has been infrequent, leaving an open 'channel' where the boats normally run. The offside edge is frequently silted to water level and the towpath edge can be similar except at the popular mooring spots. (Incidentally, the inside of bends silt up much more rapidly than the outside; any "current" in the canal caused by boat movement and lockage will tend to flow faster at the outside of a bend.) The center of the invert has also silted so that channel depth is more like 2'6" - 3'0" rather than the 4'6", or deeper, of the original build depth.
The objective when travelling past a moored boat is to do so courteously without causing undue disturbance to that boat. It matters not at all whether the boat is occupied! Too often an approaching boat is heard to marginally reduce its engine r.p.m. (from 3000 to 2800, for example!) and to do so far too late merely paying 'lip service' to the idea of 'slowing down past moored boats'. This passing boat has no opportunity to lose any of its momentum and therefore causes the maximum amount of disturbance. There is, of course, the occasional cretin who doesn't give a tinker's damn and acts like he's towing a water skier. If you yell at him you'll also find he's deaf and blind too! And presumably dumb!
So what technique do I suggest you adopt when presented with a boat ahead tied to the bank? Before I deal with that may I suggest that you firstly find out what going dead slow is actually like. Stop your boat in the middle of the channel by putting it into reverse gear. (Use low engine r.p.m. rather than a massive burst of reverse power, all too common these days, which leaves water swirling in any and all directions.) Try to stop the boat in a straight line. Whilst the predominant motion of water past the rudder is away from the direction of travel the boat should steer normally. Only as it comes almost to rest is the effect of the rudder reversed. Now engage forward gear with the engine merely idling. Wait until forward motion has stabilized and test whether you can steer properly. If not, increase r.p.m. slightly until you gain steerage way. Now note how fast you are travelling through the water relative to the bank and what water ripples are produced; probably virtually none at all.
When you next pass a moored boat throttle the engine down almost to minimum steerage way at least 100ft (1.5 boat lengths) before you reach the other boat. If you find your boat has still not lost sufficient speed (look at your ripples) then adjust the distance at which you throttle back accordingly. Proceed past the moored craft and then about a boat length further before smoothly and steadily increasing power to normal cruising speed. That's it! No shouts about water skiers, motorways or such like. No ornaments thrown off shelves on the moored boat nor dinner gravy, or even the dinner itself, spilt in laps. Instead you'll be respected as a competent, considerate boater.