Re: too much to read
– Last Updated: Jul-09-09 10:07 AM EST –
Your comment, guideboatguy, and clarion's, were among the best of many that focused and continue to throw valuable light on the subject. I don't think they were ignored. Bottom line, they were the only type of comments that really matter.
Pressure is force per unit area (pounds per square inch, if you will). A little higher pressure on all the square inches in the bow, for example, adds up to a lot of force on the bow. If the pressure is a little higher on the starboard bow than on the port (or if the pressure is higher on the port quarter than on the starboard quarter) there is a lot of force tending to cause (or continue, or accelerate) a yaw to the left. In fact, a continued steady (unbalanced) force will cause an acceleration, not merely continued movement.
A slightly higher momentary waterline on one side of the bow (again, for an example) causes a higher pressure on every square inch of that portion of the hull, from the top of the momentary waterline mark right down to the keel. Unbalanced forces cause movement, and a continued unbalanced force causes acceleration. (The acceleration can be either a change of speed, or an increase or decrease in the rate of turn, or both.)
One thing to be careful about is the tendency to think about moving one end of the boat, while forgetting about the other. It's pretty uncommon to move one end without moving the other. I have found that a way to counteract this "forgetting about" the other end of the boat, is to remind myself that the boat is not a rubber snake, it's stiff and solid. When I move the bow, the stern moves also, and vice versa. And it doesn't usually (except in the case of a perfect side slip) move in the same direction with the same amplitude. And that's why we discuss and consider the pivot point at times in boat handling.
Elsewhere, I have written about how a bowthruster works. A bowthruster moves a vessel very differently than a rope attached to the bow, with tension in the rope, moves the same vessel. (That's the simulation that Cauthier employs.) The forces involved are entirely different. But beyond that, a bowthruster apparently does not work as many people (including me) have assumed it works. It does not work on the principle of equal and opposite reaction to a protracted thrust. It works because it's a pump, displacing water from one side of the vessel to the other, thus raising the waterline on one side, while lowering it on the other. Consequently, the pressure on one side of the hull is greater than on the other side, and the resulting imbalance of force causes the ship to move. Apparently, the observed movement cannot be accounted for by the thrust available from the installed engine/propeller.
And herein is an excellent example of how the peripatetic pivot point does not explain the forces (although it certainly provides important observable clues). The forces, if fully understood and quantified, explain the pivot point.
A bow thruster is most effective when the vessel is making no way through the water. When the vessel begins to move ahead, the thruster begins to lose effectiveness. This can be "explained" by recognizing that the pivot point moves forward as a vessel gains headway, and the bow thruster (which, nevertheless, remains well forward of the forward moving pivot point) loses leverage as a result.
But there is a fly in the soup. The bow thruster, for all practical purposes, becomes completely without effect as the vessel gathers headway of over just a few knots. That wouldn't be the case of a force applied by tension in a line attached to the bow, or if the movement while dead in the water was due to the equal and opposite reaction to the thrust of the motor and propeller. I think that the water slipping past the hull has two critical effects. First, it disrupts the pumping action by obstructing the inlet and outlet of the pump and causing cavitation and turbulence. Second (and probably most important) the mass water movement "washes away" the unequal water line levels and pressures, caused by the thruster, that exist when the mass of water is not rapidly moving relative to the hull. I would guess that either the waterline differences between the starboard and port bow sections disappear, or that any remaining waterline differences between starboard and port become manifest along the entire length of the vessel, not just the bow, as the ship gathers significant headway. Or both. In any case, a bow (or stern) thruster at full power will have virtually no effect on a vessel underway full ahead. It's not because the pivot point has become nothing but an illusion. It's because the forces on the hull produced by the thruster have diminished significantly.
Saying that a vessel underway has no pivot point when it has no rotational motion is like saying that an object has no color when there is no visible spectrum radiation impinging on it. It's true enough, but turning the lights off will not remove the attributes that result in the object appearing the same color when the same lights are turned back on. Similarly, it can be very handy to identify and anticipate, while you are underway, where the pivot point of your boat will be when different forces are applied to and experienced by the hull.
Obviously, thrusters are not going to be mounted in canoes and kayaks. Still, the physics and hydrodynamics and statics involved in the movement of displacement vessels is irrespective of the size of the vessel. And, as Cauthier's article and Hooyer's text (and Melville's studies, and the various interpretations of what transpired aboard the Bounty and the Endurance have shown) we may as well not ignore what we can learn if it might help.
There is speculation on my part in this, about the thruster. The dynamic forces involving a boat maneuvering in water are indeed complex as well as rapidly and constantly changing. Steady state conditions on a vessel in motion are difficult enough to completely describe, let alone quantify. The precise description and quantification of non-steady-state conditions are orders of magnitude more difficult to accomplish. That doesn't mean we shouldn't try, or that we should abandon our imperfect attempts to understand and communicate, or abandon all simple, practical devices that help us get that boat where we want to go, until the "last word", and all the rigorous scientific analyses and equations have finally arrived.