One example for illustration
– Last Updated: Sep-09-12 8:19 PM EST –
You understand the principles of controlling the boat in rivers, but I'm not sure you understand the really basic principle I was trying to convey. I'll pick one of your points to try to illustrate the difference.
You said "Facing the current in the boat, the pole is planted but not forward thrust applied - the boat is merely held in place. Now - by simply edging one side or the other, the boat turns back and forth across the current. This demonstrates the pressure differential on the front of the boat as the bow departs from alignment with the current. You can both see it and feel it." This is perfectly true and very easy to visualize, though I must disagree with your wording because thrust IS being applied. That thrust is the force that causes the boat oppose the force of the current as that current goes streaming by. The force supplied by the pole is like that supplied by the string that holds a kite in the wind, In both cases, this is a force supplied by an external source. When held against the current in this way, water is flowing by the boat, and thus changes in the hull's attitude will control what the hull does. The same can be done with steerable kites, but a kite can't fly if simply turned loose in the air, and your boat won't do what you describe if not held in place by an outside force. When the boat is moved against the current by paddling, the force applied comes NOT from an outside source, but from the same water as that which supports the boat, and this is the key difference I tried to describe. In this case (ignoring turbulence, which as I pointed out, is where the control difficulties come from), the boat will act in the same way relative to the water itself regardless of which direction you make it travel.
Have you ever learned vector analysis, such as for studying dynamics of motion? If you know how to do that, this can all be illustrated mathematically and in diagrams - it's extremely simple to do. If not, surely you understand that in a current that moves 3 mph, if you paddle upstream at 4 mph, your total speed relative to the stationary river bottom is 1 mph, and if you paddle downstream at 4 mph your speed relative to the river bottom is 7 mph, but in EITHER case your speed THROUGH THE WATER is still 4 mph. It gets a little more complex when traveling at different angles to the current (here's where you need your trigonometry) but still your through-the-water speed is 4 mph (that's what my floating-swimming-pool example was supposed to illustrate). If you can understand that principle, it's really a very small step to understand that under paddle power alone, the hull responds according to what the paddle does, and from *the frame of reference of the water alone* (ignore stationary objects on shore or attached to the river bottom), the motion of the hull through that water is always the same.
There have been occasional posts here by sea kayakers, describing how they were happily cruising along at a fairly speedy pace, only to discover that their GPS showed their actual direction of travel to be backward because they were paddling against a current that was somewhat faster than their paddling speed. Without nearby visual references, all they saw was water streaming by the hull, just as it always does, so they assumed they were traveling forward. Well, they WERE traveling forward through the water, just not in relation to the ocean bottom. The same would be true for any other direction they chose to paddle in such current. In fact, a favorite vector-analysis problem in introductory physics classes proves that a boat can run rings around an un-anchored buoy, and a trail of "bread crumbs" left in the boat's wake as it does so will form the exact same pattern around the buoy whether there is current or not (only an independent view from a stationary point in space shows the difference between what happens with or without current. The people on the boat can't see this). It's also a neat way of illustrating that when out of sight of stationary objects from which to judge actual speed and direction of travel on the open ocean, no matter how strong the current might be, the current can only be detected by means of navigation aids, NOT by the performance of the boat when traveling in various directions.
I did a little experimental poling from a kneeling position (tippy solo canoe not suitable for standing), to see if I could work my way upstream through a riffle with current that was much to swift for me to paddle against. There were several times I tried to push off the river bottom in the same direction and manner as would be done when pushing with a paddle, and I totally screwed-up my boat control and made some really big mistakes. That's because I was accustomed to pushing with a paddle against water that my boat is traveling through, rather than relying on force applied via a medium which is outside and independent of that water - the stationary river bottom. In the absence of current however, there would have been no confusing difference between relying on paddle/water compared to pole/river bottom, because both would be stationary relative to each other. It comes back to what is illustrated by the old kite-on-a-string example.
Speaking of kites, here's another thought problem to illustrate my original point. You can fly a kite on a windy day if you hold the string while standing on the ground, but not while standing in the basket of a hot-air balloon (while in the balloon, all you can do is dangle the kite directly beneath you. You CAN'T make it fly). If you can understand why this is so, then there's nothing stopping you from understanding all that I wrote in this post and the previous one.
