Use it or lose it, prn. (Ask your nurse what prn means, bunky, and thank him or her for their service.)
Momentum used well helps you to eddy hop upstream, punch out through surf, catch waves, cross eddy lines and boof drops. Out of control momentum can cause you to miss that very last takeout or scouting eddy, wrap a bridge abutment, snag a log jam, or broach and capsize while surfing. Momentum can make life easy and save you trouble, but it can get you into trouble too. Kinetic energy imparts zest but it can also mess up your boat and your trip.
Linear momentum is mass times velocity. For our discussion, mass is the weight of your boat and everything in it. Velocity is speed and direction relative to the bottom. If your boat is heavily loaded or takes on water, you have to cope with more momentum. If the boat changes speed, momentum increases or decreases proportionately. Momentum changes even when your boat changes direction only. A force must be applied to your boat by something or someone in order to change its momentum.
Don’t confuse the direction of linear motion of your boat with its heading. “Heading” is the direction the bow of your boat is pointing. The direction of linear motion, also known as your “course-made-good”, is the track the pivot point of your boat moves along relative to the ground. Take a moment to comprehend this. Whether in a river, on the ocean, or elsewhere, it is often the case that the direction of our course is not the same as our heading, nor is the speed of our course the same as our speed through the water. The difference between your boat’s heading and speed through the water, on the one hand, and your boat’s actual speed and direction over the ground on the other hand, is known as “set and drift”. (Set is the direction, and drift is the speed of this difference.) It’s usually due to wind and current.
Suppose you’re stroking along, heading the same direction as a substantial south flowing current that’s flowing at five knots, and you’re making three knots through the water. Your heading and speed through the water is south, 3 knots. Your set is south, your drift is 5 knots. Your course-made-good is thus eight knots due south, and you’ve got a big gorilla of momentum and energy accompanying you.
Suppose you spin around to head into and paddle against that current. Let’s say your speed through the water is again three knots. Your heading is now north. Your set is still south, your drift still 5 knots. Your course-made-good is still south (exactly opposite to your heading) though it is cut down to two knots over the ground, and your momentum has shrunk to become a monkey.
Now accelerate, dig hard, and keep heading north straight into the current until your speed through the water equals that of the current. Your heading and water speed is 5 knots north, and the set and drift remain 5 knots south. Your speed over the ground is now zero, and though you’re paddling hard, your momentum also drops to zero.
Next,while still paddling strongly ahead, using a slight lean or an extra thrust on the right, allow the south racing current to strike the starboard (right) side of your bow at a very slight angle. As long as you don’t let the current spin you around and sweep you away, and you hold that slight angle and keep paddling ahead, you will begin to ferry across the current to your left. Your speed over the ground has become maybe a half knot creep due west, and thanks to a well-executed forward ferry, you and your tamed momentum are on your way to a gentle landing at the beach, or to a convenient eddy. (It’s an understatement to say it ain’t easy stemming a five knot current, by the way.)
The heavier the load and the faster it’s moving as you approach a rock or a curve, the more linear momentum that has to be overcome by some other force in order to change course. Controlling your linear momentum allows you to rapidly change course, to move nimbly and maneuver with precision. Linear momentum can be reduced by shaving your load and/or by reducing your speed relative to the bottom.
Perhaps the most powerful basic technique for simultaneously controlling both speed and position is the ferry. To perform the back ferry (while heading downstream), back paddle so that the current is moving downstream faster than you are and use the draw or other paddle stroke to position your hull at an advantageous angle to the current that is overtaking you. The water pressing against that side of your hull will then move your boat towards the other side of the river. For example, if you chose the angle so that the current is impinging on your boat from the port quarter (left rear), you will back ferry to the right. You’re pointed downstream towards the left side, but you’re moving to the right! Very cool move! You’ve cut your momentum close to zero, plus you’re using the river’s force rather than your own to attain desired position. It takes practice, but once you get it down it will get you through tight spots quite gracefully.
The back ferry is an important part of the maneuver known as a “flanking turn”, used by river professionals to get around a tight river bend. The flanking turn also takes advantage of the current usually being stronger and swifter nearer the outside of the river bend (where you’ve placed your bow) than towards the inside (where your stern is). Your position as you approach the bend is important. Close attention to the changing direction of the surface current that meets your hull throughout the entire turn is critical. These concepts are most easily learned first in a solo boat, but if you paddle tandem, when you and your partner have mastered this technique, either of you can give a “Flank Turn” audible any time you approach a tight turn, and the entire maneuver can be accomplished in coordinated elegant silence. Don’t misunderstand – the flanking turn is a dynamic rather than a ponderous strategy, and the particular nature and placement of hazards through any given tight turn will require improvisation and ad hoc trajectory adjustments – and maybe a carefully chosen word or two. (“You f—ing idiot!” should not be used prematurely. Wait till the time is right. You’ll know.)
On a tight river bend with sweepers and other hazards in the strong current on the outside curve, paddlers in an early or arrested state of development too often see what’s ahead – pause – gulp – point their bow at the downstream inside curve and paddle like hell with a lump rising in the throat in an uncertain race against doom. This race is occasionally lost in a high speed broadside crash (the proper term is broach) into a sweeper or rock. If it’s a group grope, one after another hapless boater clutches his gunwales and piles into the mess like commuters on a foggy highway.
Admittedly, ferries and flanking turns may slow your headlong rush downriver, but sometimes that’s essential, and sometimes it’s nice to savor the visuals rather than race past them. After all, are you here to beat the clock and in a big hurry to be done with the trip? Scout before commitment. Plan your entry and intermediate positions and trajectories. Exploit and control your momentum. Acquire competence.
Rotational motion is simply a change of heading. An intentional change of horizontal heading is a turn. An unintentional change is “yaw”. Once a boat moving ahead through the water starts to turn or yaw, forces come into play that tend to increase the rate of turn. So anticipate and correct a yaw promptly. If you’re turning to a new heading, take action to check the swing before you get to the desired heading or you will overshoot. To minimize rotational momentum, carry less weight and keep the ends light. Changing your heading introduces forces which will soon change your linear momentum. Other than banging it straight into a wall, the quickest way to slow or stop a boat (whether you want to or not) is to turn it.
When you are not moving with respect to the ground, you have no momentum and no kinetic energy. However, when you do move over the ground, the faster you go, the more momentum and the more kinetic energy you have.
Let’s say you contact a bridge abutment and come to a dead stop as a result of that contact. Almost all of the kinetic energy you had is absorbed by physical distortion of the boat. Kinetic energy, like linear momentum, is proportional to the moving mass. But kinetic energy, unlike momentum, is proportional to the square of your velocity. (If you’re going twice as fast, your momentum is doubled but your kinetic energy is four times as great. A boat doing four knots carries sixteen times as much kinetic energy as one going one knot.) Momentum carries you into the situation…Kinetic energy does the damage if you blow it. The most effective and satisfying way to use and control momentum and kinetic energy is by learning, improving and eventually mastering and enjoying good technique.
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