Terminology
Yep. That’s my meaning, and I only use the term in that case because I had heard others use it in the past, and it seems there ought to be a distinction between ferrying that’s driven by primarily by momentum as compared to pure paddle power.
Incidentally, and I know I don’t need to clarify this for you, but it might help for others, and that is that the “forward” part of “forward momentum” as you use the term can actually be provided by the boat initially being stationary in terms of moving upriver or down, once you move out into a zone of stronger flow. Even when stationary relative to the river bottom, it’s the boat’s momentum that is the driving force that creates forward motion relative to the water that’s now rushing past the hull.
And by “forward”, I assume you mean …
. . . that there is an upstream (“into the current”) vector component to the boat’s motion relative to the current. Or, stated differently, that the boat has some upstream angle as it moves forward.
Here is the part of your first post that causes me some confusion, because it seems to posit an impossible situation – namely, ferrying while simply drifting:
“You ONLY have complications involving sideways forces on your boat (and the resultant “lift”) for as long as your boat’s momentum causes it to resist drifting with the current. Once that momentum has been lost, paddling and steering are unaffected by current, in terms of actual boat control, and you only set your angle so that you get the right combination of travel direction with current drift so that you end up where you want to be, not because your boat is getting blasted by current from one side.”
While I agree with your relative motion examples, I don’t see how it is possible to have lateral ferry motion at all if an upstream-angled canoe has lost all its forward momentum and is just drifting at current speed.
An angled boat ferries laterally when there is a higher water pressure on the upstream side of the hull than on the downstream side. If the canoe is angled upstream but is just drifting and not being propelled forward, it will never ferry or move laterally at all because the water pressure is the same (zero) on both sides of the hull. A static draw or a static pry/rudder will have no propulsive or steering effect. The canoe will just drift straight downstream.
However, once the paddler applies ANY forward stroke force to this upstream-angled canoe, no matter how gently, the water pressure will become greater on the upstream side of the canoe and the canoe will begin to ferry laterally. Hence an upstream-angled canoe, if paddled forward, will ALWAYS be in jet ferry hydrodynamics as you use the term. This is not like paddling forward in non-moving water, wherein it’s impossible to create a lateral ferrying force with any angle of the canoe.
In sum, it’s not possible to ferry (at any angle) while simply drifting at current speed. Conversely, it’s not possible not to ferry an upstream-angled canoe once the paddler applies any forward stroke force.
Have I confused something (other than the entire forum)?
I agree
I’m as confused about the part you cite from GBG’s first post.
If I’m just drifting at an angle at current speed, I’m not moving laterally across the river. The sticks in the water will always be by my side, but the distance to them won’t change.
Confusion
Yes, you are confused by the same thing that confuses most people who've never gotten a gasp on relative motion. First, the drifting-boat example, whereby one points their boat at another boat that is drifting and paddles straight to it, was not an example of ferrying by means of building pressure on one side of the boat or lift of any kind. It was an example to show how ferrying across a broad expanse of current, whereby the boat has becomes equilibrated with the current, does NOT involve unbalanced forces. And I did NOT say that anything happens while drifting other than drifting, but I DID say that while paddling, the effect of drift is there (Oh, and this is exactly the same principle that makes you go faster paddling downstream than upstream, and by vector analysis, you can apply that principle to going in any other direction. Too bad most people don't see that connection right off).
I started trying to explain this another way, but I quit before I got far because it's just not worth the effort it takes in a communication environment like a message board. If I could illustrate "on the blackboard" how this works, at least to anyone who already understands vectors, I have a great little routine that I learned years ago from a truly masterful physics teacher that illustrates how a person in a boat on moving water, when in the absence of fixed landmarks, cannot discern current speed or direction, because no matter how strong the current, you perceive the water as being still (and the reason for this becomes dope-slab obvious when doing the vector analysis). Don't envision rapids, as in that case you can see the water flying over rocks, etc. Envision being on a wide river enshrouded in fog.
As an example of this same principle, consider that the inability to detect ocean current while on ships at sea was one of the great obstacles to real-time navigation in the early days, because even on the ocean there can be very strong currents (often much faster than most rivers) but without modern tools like inertial navigation and GPS, the boat's drift could only be detected with the help of celestial measurements, which is not a real-time measurement but an after-the-fact determination (and the problem was even worse in the days before accurate portable clocks). The bottom line is that nothing could be done to detect current at any given moment, and that wouldn't be true if unbalanced forces were at work in such a case.
In any case, if you try the exercises I described, it would be virtually impossible for you to not clearly see that no unbalanced forces are at work when crossing currents at any angle you might choose, as long as no outside input of force is at work (like really strong wind, the boat's momentum during those first few seconds that a new current first acts on it, or gravity in the case of surfing a wave, or that silly example of having a steel cable stretched tightly across the river). As I've mentioned, there are lots of examples of analyzing such situations online, and all you need to understand them is intro-level trigonometry, so I'd encourage you to go that route if you're dying for a good explanation.
And all of this is why I put so much stress on whether momentum is involved in the ferry situation or not, because for understanding, it really matters. On the other hand, a person can ferry perfectly well without any accurate understanding, so this is just for the satisfaction of "knowing stuff".
People are confused because you neglect
to take into account the forces on the boat while it’s moving relative to the water. More specifically, the boat designs that enables boats to move efficiently. Boats are not symmetrical from bow to stern.
Most boats are designed to create a bow wave to “pin” the bow, while the stern is designed to be “loose”. So trying to correct ferry angle at the bow is noticeably harder than accomplishing the same by moving the stern.
When making turns, one is typically tilting the boat so the “rail” (or chine if you’re talking about sea kayaks) on the hull will act as the new kneel which helps with the turn. But when ferrying, one wishes to go straight at a set angle. So the rail/chine typically not help.
In the middle of current with no obstacle, why would anyone bother to move the hard to move bow instead of the easy to move stern?
Not, that’s not it at all
People are confused because they don't understand relative motion, to the point that they can't even relate perfect everyday examples of relative motion to what goes on with their boats.
I mentioned a type of example problem online. Here's another: Check out articles explaining how sea-kayakers (or any other kind of boater) can determine their true direction of travel when strong currents are present. It's exactly the same thing I tried to explain already, so it involves vector analysis, but once you do the math you'll see what I mean. All the boat needs to be able to do is go forward *through the water that supports it*. Nothing more mysterious than that is involved. Check it out.
Oh, and the same principle applies to airplanes in crosswinds, or as I tried to point out already in regard to river currents, even headwinds or tailwinds (and again, the relationship between winds/currrents of all these various directions relative to your heading is SO, SO CLEAR if you just do the fricking math!! Sorry, but when you don't even know what you don't know, there is no shortcut to telling someone what the answer is. And it's not even that hard - It's just an unfamiliar way of isolating motion).
I see that you didn't see the relevance of the old-time ocean-navigation problem I mentioned. Work on acquiring an understanding of that situation, and then if you can refute it (good luck with that), you'll be on your way to being able to refute everything else I said. Anyway, that's a perfect place to start.
Of course the effect of current drift…
. . . is always there, and it may not be possible to visually detect current without any fixed landmarks. And I agree with your paddling to the drifting paddler example.
But I'm failing to see how there's not a water pressure differential on the hull of an angled canoe under forward propulsion.
Make the angle to the current 180 degrees -- bow straight upstream. Anchor the canoe, like a rock. Is the water pressure greater on the bow than the stern? Now paddle forward straight upstream. Is the water pressure greater at the bow than at the stern -- even greater than when anchored? If the answer is yes, then it should remain yes if the upstream angle is anywhere between 180 degrees and 90 degrees.
The "outside input of force" causing the water pressure differential is the paddle making a forward stroke in opposition to the current. The forward stroke is like the wind or the cable or gravity or a pole push off the bottom.
Perhaps you're saying that even if the water pressure is greater on the upstream side of an angled canoe, that's not contributing to the lateral movement across the river. But rather that the lateral movement is solely caused by the act of forward paddling simpliciter, just as in flat water. Hmmm . . . that I might accept. Not sure as of this moment. The reaction ferry with cable seems to argue against it.
But if I do accept that water pressure differential does not contribute to the ferry, I still don't see a difference between the canoe's initial forward momentum caused by the first few paddle strokes into "regular" current (i.e., no wave face) and the continued forward momentum caused by the next X paddle strokes. Absent a "jetting wave" (in my terminology), all those strokes are equally necessary to cause the canoe to begin and continue its traverse across the entire river, along with the angle adjustment maneuvers that I think we all generally agree upon.
Sorry if I'm asking you to repeat things that frustrate you, and I'm not disagreeing with anything you've said about paddling technique, but I don't quite understand the point you are trying to make with your "relative motion" examples. On a river, we are always aiming at, or aiming to avoid, fixed things like rocks, eddies, waves, holes and sweepers . . . okay, not counting the drifting dumped boat and swimming paddler after failed badly failed ferry.
Chine
With a round bottom Solstice, chine shifting sitting weight adds in after setting the rudder.
On crossing the Flamingo channel. It's uh very experiential in that the rudder is doing the job but a feeling arises that we're not getting it done n across, no efficiency
.
A chine "turned' rudder plus propulsion into a more effective device eg Williams'
cambering suspension.
The hull becomes rudder, less turn turn turn friction dragging every stroke.
.
Rather than getting frustrated with people not “able” to understand your explanation, have you thought about the possibility of your explanation not being clear? As any professional teacher will tell you, there’re no stupid students, just inapt teachers.
That is, assuming your understanding is correct in the first place. I argue it’s not. Why? It’s not JUST relative motion! You completely neglected relative acceleration, which is quite a significant factor.
The typical high school example of relative motion is a person walking around the train while the train is cruising in a straight stretch. That’s what you’ve been trying to apply to boats in rivers. The principle of relative frame of reference.
But as soon as you take your physics 101 in collage, you realize the whole thing changes when the train speeds around the bend. Worse when the train is pulling in or out of station. Basically, when one of the frame is accelerating or decelerating, the “vector calculation” no longer apply. There ARE forces apply to the body in the train. That’s why the hanging lamps swing to the outside of the curve, and swing back as the train is pulling out of a station.
Does it apply to the boat in a river? It does because the boat is NOT in a constant motion nor a straight line even if the paddler is TRYING to go at constant speed on a constant heading. The canoe/kayak motion is in a constant micro-acceleration/deceleration.
The effect is of second order. But depending on hull design, paddler skill and actual current, the effect is often easily noticeable.
You were right in considering jet ferry. But a ferry even “in current” is still a lessor version of jet ferry. Depending on how uniform of the current and how smooth the paddling action, that “micro-jet-ferry” can be quite significant.
easier to adjust the angle
by moving the downstream end of the boat. Move the stern for an upstream ferry, easier to move the bow for a downstream ferry.
Okay
It is a little frustrating and I realize that it’s more of a communication thing at this moment than anything else.
As far as little deviations in paddling speed and current velocity, I’ve already said that’s easy to cope with and is not the heart of the matter here, though if you don’t see it that way, I’ve failed in a second way to get my point across. The general trend is what I’m trying to explain.
I have another post in the works, which I hope is more helpful.
Quick Reply
Hi again.
I'll address what I think is your main point, before going back to work on another post, starting somewhat from scratch.
You said:
"But I'm failing to see how there's not a water pressure differential on the hull of an angled canoe under forward propulsion."
"Under forward propulsion" is *key* here. I think I mentioned already that what's going on is actually explained in exactly the same way as why your speed relative to the ground is faster when paddling downstream than when paddling upstream. And if you can understand that, then in the same manner you can (eventually, I promise) understand that ANY upstream angle results in a slower speed relative to the ground than ANY downstream angle, and that while paddling forward, the boat goes *only forward* relative to the water you are paddling through.
That's just food for thought, for now. As I said, I'm working on another post. I'm not sure if I'll work straight through and put it up tonight, or if it will be tomorrow.
Edit: I accidentally mixed up the speed issue at first. It's fixed now.
Another Attempt
Okay, I knew I'd be opening a can of worms. In the past when I've tried this, I ended up giving up, but there was ONE time a few years ago that somebody else joined in on "my side". Anyway, this time I joined a physics discussion board to see if they had any special hints or online demonstrations. Since some of you think I'm too long-winded, I'll provide the easiest explanation first, and I have divided my post into separate sections, and the sections are not dependent on each other.
Section #1:
Here's a video. I've described this exact thing before as an analogy before, but the teacher here does a better job than I ever did, and he also relates other everyday examples to the process.
http://tinyurl.com/znsdka3
Though the "river" is very narrow in this case, you can clearly see what's happening, and there's no side-slipping of any kind going on here. However, you can see why it would easily appear that there is a side-slipping motion if you did not have evidence that it's not possible (in this case it's not possible because the wheels roll only in a straight line). Also, it's worth pointing out that you could make the treadmill a mile wide if you wanted to, and the car would stay on track for as long as the batteries held up, meaning there's no need for momentum as an external driving force here (you could do a momentum-driven jet ferry with a non powered car in this case, since the distance to be covered is so short).
Anyway, here was my reply: "Wow, this is perfect. I've actually tried to describe that situation - a car on a conveyor belt - to these folks in previous attempts at explaining the situation (a year or two ago), but was met by claims that the example is not applicable. Well, of course it is applicable, and I think actually seeing it in action might make that more clear."
Does it make the situation more clear? I hope so.
Oh, note that at the end of the first example you can see the car get thrown off it's heading when it "hits the eddy line" (when it hits the "non-moving water" represented by the treadmill's frame).
Also note the slightly confusing terminology in the third example, where he says that the speed across the water is slower. What he means to say is that the speed toward the other side is slower (the speed through the water is the same in every case).
Section #2:
I mentioned two things last time that are inextricably connected to the concept I was trying to explain. One was that in the absence of any steering action, the boat always goes straight through the water that supports it (please don't bring up the issue of current variations again. We can deal with that as it happens. It's the main concept I'm talking about here). Directly related to that is the fact that a boat on a large expanse of moving water (like an ocean current or a river like the Hudson when it's socked-in by fog), a boater can NOT detect the current. If the current "hit the boat" when traveling at certain angles, the boat operator or paddler could see it happening and thus figure out the direction the current was coming from, but it's completely impossible, and I already provided examples in navigation that would not be the case if this were not true.
To prove this concept, I suggested paddling among some free-floating markers, which would show that relative to the water, whether it's moving or not, even moving in any direction, the boat travels a straight line *through the water itself*. Lo and behold, here's what some of the physicists wrote back to me. See if any of this sounds familiar ;)
"If these guys have ever paddled in the open sea they could perhaps imagine dropping breadcrumbs regularly behind them as they go. When you are on open water (no wind, of course) and the tide is flowing, you have no idea of your progress over the ground. Your breadcrumbs would lay in a straight line, equally spaced behind you and they would all be drifting with you, in the direction that the tide is running, at the same rate. They would be in a straight line behind you (dead astern) - just where you left them (as far as you, in your floating frame of reference can tell) so that indicates that you are travelling in a straight line through the water (relative to the water). The river situation is the same - even when the flow is very fast - only you can see the bank close enough to be aware of relative speed (and to interfere with people's intuition). This could all be described with the help of a simple static diagram."
In this post, the statement about there being no way to avoid being aware of speed and direction relative to fixed objects with the result being "and to interfere with people's intuition" is one of the key points.
Here's another:
"Instead of an animation, I'd suggest a 'mental experiment'. Tell those skeptical canoe paddlers to imagine that the river is a solid moving surface, like a conveyor belt, and that their canoes are small cars. After a little reflection, sure all of them shall agree that crossing that 'river' is a simple matter of direction and velocity, the path of the car along the solid moving surface being a straight line, with the cars moving smoothly, with no 'hull-hitting forces' at all..."
And another:
"This exercise is best carried out with a fairly large motor boat that has a tiller (rather than wheel steering) and a wide channel with a reasonably even current flowing. It is not hard to get a grasp of the angles and the lines involved. A good ferry glide is very impressive - even to the guy at the tiller! Paddling and turbulence tend to cloud the issue and the physical memory from having learned what, rather than why really does't help."
That last sentence says it all. And here's my reply back to that post, because I think it might help clarify this aspect even more: "Thank you. Here's another reason the motorboat idea is good. You can look back at your wake and see that it's straight behind you, not trailing off diagonally (mainly by looking at the trail left by the prop itself). I might be more aware of this than most canoers because I also row canoe-like boats, and when rowing you face the rear, so I can constantly see that my wake describes straight-line travel of the boat through the water, even going cross-current (my own breadcrumb trail!)"
And yes, I often row my guide-boat at various angles across some rather swift sections of rivers, and the wake always describes straight-line travel if straight-line rowing is what I'm doing. It's the "moving shoreline" that tends to confuse the issue.
Another illustration of an identical situation:
"I guess another example would be a visit to an airport on a windy day, watching the the landing planes "crab" their approaches (turning the plane into the wind so as to make their landing straight along the landing line but with the orientation of the plane at an angle to the landing line...)... the car on the treadmill is good."
Here's the most recent reply on the physics board:
"If the boat is moving straight across a flowing river, then the boat's velocity with respect to the ground is perpendicular to the bank (assuming bank is perpendicular to the river), and the boats velocity with respect to the river will have an upstream component equal to the river's downstream component relative to the ground. Ignoring the air, the boat only interacts with the water. The boat will not hit any floating objects that are not directly ahead of the boats path with respect to the flowing river. To a ground based observer, the objects will have a downstream component relative to the boat, same as the downstream component of the water."
What he is providing here is a perfect explanation (though perhaps not easy to interpret because the wording is a bit awkward) of what I said about following a straight trail of free-floating markers, or paddling within a pattern of free-floating markers. When paddling along the line of markers, the method of paddling needed to stay on course will be the same whether the water is still or flowing, and if flowing, it won't matter the direction of travel relative to the current (again, don't focus on the fact that there will always be small deviations in the current. None of us are confused by those when going up or downstream, and the same is true at other angles to the flow). And if paddling among a cluster of free-floating markers, none of those markers will strike the boat from one side. They will only strike from the front, the same as when paddling on a lake, and again, this comes back to what I said about currents being undetectable while traveling in a boat, unless you have fixed landmarks as a reference (and again, this does not include situations where an external force, such as momentum (a short-lived force) or propelling the boat with a pole are involved. And by the way, ferrying a canoe when using a pole really DOES involve the kinds of forces you guys imagine, but note that the canoe's source of propulsion is now connected to the stationary river bottom, not the same water that is supporting the boat as is the case when paddling.
Section #3:
Yesterday I paddled on the big ol' Wisconsin River with my girlfriend and mentioned what I was trying to explain here. She asked for more info, and then, just like everyone here, she insisted that there "must" be water hitting the side of the hull during a ferry. I tried numerous ways of illustrating that it's easy to "see" it that way and that it "works" as an explanation if that's what you believe, but that it is wrong. One thing to illustrate the relative-motion source of confusion was to imagine that the water in the river is stationary but the shore is moving. In that case, what do you need to do to hit your intended destination on the opposite side? Answer, you need to "lead" your target, and in terms of how the boat moves through the water itself in comparison with how it moves relative to the shore, all motions and relative motions turn out to be exactly the same as when ferrying across a real current when the shore is stationary! But in the moving-shoreline and stationary-water example, even though everything works out same in terms of relative motion, you can't possibly think of a reason that water would be hitting the hull at any point except the very front of the bow. It was suggested by one poster on the physics board that you could demonstrate this with two sliding transparency sheets. He suggested using it to show motion relative to both the river bottom and the moving water, but you could do the same thing to illustrate moving through still water as the bottom moves by.
Anyway, eventually a light came on in her head and she suddenly visualized that if you focus on the water, and nothing but the water, you can see that it always approaches the boat (with the boat now being the frame of reference) from dead-ahead, regardless of your paddling direction relative to the current.
That's why I say you guys REALLY need to try fooling around with paddling among free-floating markers (get a bunch of big sticks and go out on a big river, and paddler through that armada of sticks from every direction you can). You'll see.
Now, I've said repeatedly that if you can do vector analysis (and all of you now can after watching the video linked above), this becomes easy. Throw in some basic trigonometry and you can actually calculate the true speed and direction relationships that are involved, but that's not necessary. Instead, it's enough to know that the longer the arrow, the faster the speed it represents. And once you have that under your belt, you can see that calculating a ferry angle involves all of the exact same interactions between boat velocity and current velocity as are going on when simply comparing the difference between going straight upstream and straight downstream.
Section #4:
As another way of explaining how the lack of an externally-based force makes it impossible to create differential forces on opposite sides of the hull, visualize the difference between propelling your boat with a paddle as compared to a pole. With just the paddle, you are attempting to harness the potential driving force of the current in the same way as trying to detect the rotation of the earth by getting a better grip on the ground. It won't work. Planting a pole against the bottom is a whole other situation. Too bad there's not a real-life way of showing what would happen if using some non-earth-based object, you anchored yourself from moving along with the rotating earth, but you can imagine the result. You'd definitely feel it!
Took an extra cup of coffee
…but I think I got it. There is a combination of angle and forward momentum that when matched with the current will move the boat perpendicular to the the current - what we call a ferry. Reduce the angle so you are pointed upstream and adjust the forward momentum and you can stall out in the current - maintain a consistent position. Increase the angle without increasing the forward momentum, and you will begin to be carried downstream.
The video provided a great example, but unlike a treadmill and a battery operated car, the force of the water and canoe aren’t perfectly constant. The current is faster in the middle than that at the shore, and could be much faster when flowing through/around an obstructions. Moving into faster water without either reducing the angle or increasing the speed and you will get pushed downstream - probably spun downstream. There is a point where those easy straight vector lines don’t apply anymore.
So I guess the bottom line is this - ferrying is process of matching the angle and speed of the boat to the current in a way that carries you perpendicular to the current. You do this by paddling on the downstream side of the boat, and by making angle corrections at the downstream end of the boat.
Close, but not all the way there
When you speak of things such as "paddling on the downstream side of the boat", you are clearly thinking of those situations where your momentum becomes an "outside" driving force upon entering a zone of stronger flow, and that's not the situation I'm trying to clarify. Once your ferry has progressed far enough to be away from such sudden variations in current velocity, which side of the boat you paddle on no longer makes any difference, because as is key to this understanding, the boat moves straight through the water and there are no unbalanced forces applied by the current. But of course, as you know, you won't find such a uniform condition on the kinds of rivers you paddle, though if you pay close attention, you'll find that situation in smaller zones, and can learn to recognize it.
I've ferried across the Mississippi many times, and can assure you that for 100 yards at a time or even more, you won't feel the slightest nudge on your boat that needs to be corrected for, yet the current is quite fast and you MUST have the proper heading to get where you are going. On smaller, more turbulent rivers, you can easily adjust your ferrying to account for changes in current velocity while still envisioning the relative motions by the same means I have been trying to clarify. It's just that it's also very easy, maybe easier, to "understand" the what's going on by using incorrect reasoning (one of the replies I got this morning mentioned that this understanding is sure to require thinking that's "outside *their* box", with "their" referring to most paddlers). Still, the process of getting the job done works regardless of your background of understanding.
It's interesting that this morning on the physics board, there are some new replies indicating that the members there face this kind of confusion from private pilots all the time. An offhand remark indicates that the number of questions from private pilots about dealing with cross winds is "never ending." Flying in a cross wind is the same situation as a long-distance ferry (no momentum at work, and therefore no unbalanced forces of the air upon the two sides of the plane), and it is confusing for the same reasons.
GBG, quick question
When ferrying across a strong current would you advise heeling the canoe upstream, downstream or that it doesn’t matter?
I’ll make some additional points about so-called relative motion later.
It matters at the start, …
... when you first enter the faster flow, and it matters at the end when you find yourself suddenly entering calmer water or an eddy. At the beginning, I'd be leaning in a direction such that the new current I'm exposed to hits the bottom of my hull. At the end, I'd lean the opposite direction so that my boat doesn't "trip" when suddenly slowing down as it enters calm water again (all that could be explained in terms of relative motion, but I'll leave that alone for the moment). While underway in a broad zone of fast current, no lean is necessary. Once your momentum which at first resisted your boat's acceleration in the downstream direction has faded (this momentum is the one and only cause of "current hitting the hull" from the side), it will no longer matter. Your momentum again becomes a factor when departing the main flow.
Just as a sea kayaker padding in a very fast flow but far from shore can't tell what his true velocity is (I remember someone posting here that he thought he was cruising rapidly forward like always, but his GPS said he was going slowly backward on account of a very fast tide), it won't matter which way you lean once your momentum is gone.
It's clear to me that more consideration needs to be given to the meaning of all the supporting information I've provided, including things explaining why what's in the previous paragraph is true, and also the fact that vector analysis will illustrate for you that what determines your velocity (that's speed AND direction) relative to the river bottom when ferrying are the same factors that are at work for every other direction you can paddle relative to the current (I've been practically pleading with people to give vector analysis a try, and it's so easy that I don't understand why nobody does. Like I said once already, there are no shortcuts, even if the final answer is incredibly easy). Until that other stuff also makes sense, the actual motions involved in ferrying will not make sense either.
Oh, and there's nothing "so-called" about relative motion in this case, so it will be interesting to hear what you have in mind.
I'd love to paddle across the Mississippi with you at a location where the current is really fast in a swath that's hundreds of yards wide, and challenge you to feel the effect of that current as we weave and circle around out in the middle. Sure, you drift with the current no matter which direction you go, but you sure can't feel it or see any effect of it (until such time as a channel marker comes whizzing by, appearing for all the world as if it's being driven through the water by a motor! Oh, and that reminds me. Mark Twain was fully aware of what I'm trying to describe, and there's a remark proving it in "Huckleberry Finn").
Leaning downstream
and presenting the hull to the oncoming water would appear to only be necessary as long as side forces are pushing against the hull, e.g. when entering fast from slow current.
Once the boat has been accelerated by the river to the river’s downriver speed and side forces are thus no longer present, leaning should no longer be necessary.
That is exactly right! nm
Okay I lied. I DO have a message. Consider the formula FORCE = MASS x ACCELERATION
If you (not you, melenas) understand why this formula works, you will not find it possible to continue believing that there's a never-ending push against the boat's side by the current when ferrying, or going in any other direction within reasonably uniform flow, and it's because there's no source of the driving force. The source of that force must be something other than the water that supports you (and since I've already said this a few times, this is another case showing why you can't just ignore the supporting information and still expect to understand what's happening). As I've said before, get yourself a pole and plant it against the river bottom and you WILL be able to ferry in the way most believe it always happens, but you can't do it without some solid connection to something other than the water.
moving sidewalk
Maybe a moving sidewalk like at the airport would be an appropriate example. You only “feel” the acceleration when stepping on or off, thus that’s the only time you need to lean.