hearts and minds

August 6, 2006

II. Momentum and Energy – by clyde winter

Filed under: Paddlesports — Hearts & Minds @ 1:42 pm

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.

Go HERE to see the table of contents and other articles in this series. I appreciate any comment you care to make.

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7 Comments »

  1. This is eloquent.

    Comment by Howard H — March 23, 2007 @ 10:45 am | Reply

  2. […] turning maneuvers to achieve much better control, trajectories, and paddling performance, in “Energy and Momentum – Use It or Lose It“. https://clydewinter.wordpress.com/2006/08/06/iimomentum-and-energy/ Go HERE to see the table […]

    Pingback by I. The Peripatetic Pivot Point – by clyde winter « hearts and minds — June 21, 2009 @ 11:14 am | Reply

  3. It might be useful to add that ferries require edging. So, for example, if you leave an eddy on river left pointed up stream with a slight angle toward river right with your boat flat on the water you will almost certainly get flipped if the current is at all strong. I don’t know the physics of edging. I only know it is a lesson I learned the hard way for both front ferries and back ferries.

    BTW, I think back ferries are very useful and just as important as front ferries but are rarely taught to beginners.

    Comment by Tom C — June 23, 2009 @ 4:19 pm | Reply

    • Absolute agreement, Tom. Thanks for the comment. The back ferry is the heart of the flanking turn, so useful and important and basic for descending a pushy river in control. What could be more valuable to teach to a beginner?

      Comment by clydewinter — June 23, 2009 @ 5:18 pm | Reply

  4. Interesting article. What you describe as the “flanking turn” is perhaps better known to river paddlers, especially river trippers as a “set” or “setting around a bend”.

    I agree that a back ferry is an extremely useful maneuver, especially when paddling loaded canoes or canoes that have taken on water (and thus have a lot of real or potential momentum). I think one reason that back ferries are not used and taught more often is that they are actually quite a bit more difficult to learn. When looking downstream it is considerably more difficult to adjust the angle of your boat relative to the current. The current acting on the boat that affects the ferry is the current acting on the stern behind your head. It is quite easy to “lose the angle” and get blown downstream before you know it.

    Also, adjustments in ferry angle for a back ferry require a completely different approach than those used in an upstream, or “forward ferry”. When adjusting a ferry angle, you must always correct at the “loose” end of the boat, otherwise known at the “eddy-resistance” or “ball-bearing” end of the boat. For an upstream ferry, the angle is adjusted at the stern using stern correction strokes. The bow (“frontal resistance end”) is pinned by the differential current and is difficult to pull back upstream if the angle is being lost, whereas the stern is easy to draw downstream with the current. For a back ferry just the opposite applies. The bow is now the “loose end” and adjustments in ferry angle must be made at that end using bow draws, bow prys, or reverse J strokes.

    In a tandem canoe, the roles’ of the paddlers is reversed for a back ferry from what they would be for a forward ferry. In a forward ferry the bow paddler provides pure upstream power and the stern paddler adjusts angle as necessary. For the back ferry the stern paddler provides pure upstream power with backstrokes and the bow paddler must adjust angle as needed. A lot of less-experienced tandem boaters don’t appreciate this role-reversal and begin pointing fingers at each other as the ferry fails.

    It is common for beginning whitewater paddlers to confuse their heading with their actual course made good. A common sight is to see a beginning paddler attempt to “eddy out” from brisk current. They often correctly adjust their heading by “pointing” their boat toward the eddy, but fail to appreciate how much downstream momentum they have, and that their momentum is carrying them in a quite different direction from where their bow is pointed. As a result, they fail to put in enough effort, with strong forward strokes, to overcome that momentum and pierce the eddy line. As a result, they are “rejected” and skate along downstream on the eddyline, watching their intended eddy go by.

    Another concept that is important to river paddlers is the momentum of their boat relative to the water. A boat floating with the current and at equilibrium with may have a lot of momentum relative to the ground, but it has none relative to the water it is flowing along with. Ferries only work when the boat is either moving upstream or downstream relative to the water. Static strokes (hanging draws, pries, jams, or Duffecks) used to sideslip or turn the boat also do not work if there is no movement of the boat relative to the water.

    Beginning whitewater paddlers are often taught to “lean downstream”, and it is a good general rule. But there is no need to lean a boat downstream that is flowing in equilibrium with the water. The boat has to be leaned one way or the other when the boat is moving upstream or downstream relative to the water. If you were sitting in your canoe 1/2″ above the surface of a brisk current and were suddenly dropped into it, unless you compensated with a downstream lean, as you contacted the water the current would roll the hull out from underneath you and you would capsize upstream. But if you are floating sideways in current and are in equilibrium with it, you need not lean regardless of the speed of the current. If you suddenly encountered a patch of much slower moving water, you would actually need to upstream to avoid capsizing downstream.

    This is exactly what happens when a paddler exits current into an eddy. The water the boat is entering is now actually flowing upsteam relative to the boat (and often relative to the ground). To compensate, it is necessary to lean the boat upstream relative to the main current, to avoid tipping over downstream as the eddy line is crossed.

    When running rapids, I try to remain aware at a subconcious level, at least, of whether my boat has momentumn relative to the water, and whether that momentum is upstream or downstream. This relative momentum determines what maneuvers and strokes will be effective or ineffective at any particular instant in time.

    Comment by pblanc — June 29, 2009 @ 8:33 am | Reply

    • pblanc’s comment is a pretty good article in itself. But there are a few significant mistakes, and I can’t let them pass because somebody might be misled and confused, and that ain’t good. Thanks for contributing to a useful discussion, and for bringing out some things that were not mentioned by my very brief essay. I will make comments and corrections (where necessary) in the order that pblanc brought them up, not necessarily in their order of importance.

      Flanking turns have been employed on rivers by professional river operators for a very, very long time – I would guess, as long as people have floated on moving water. It would be interesting to learn whether any animals besides humans have employed ferrying and flanking turn techniques in rivers. In recent decades, specialized flanking rudder systems were invented for the specific purpose of better, more precise and powerful control while making flanking turns using screw (propeller) propulsion. It doesn’t hurt for modern recreational canoeists and kayakers to understand (for personal safety reasons) that the flanking turn is commonly employed on rivers by tug barge combinations and by large monohull vessels. Altho the manner of propulsion is different, and the relative size of the hull is different, the reasons for doing it, and the physics and hydrodynamics are the same. A flanking turn is a flanking turn, no matter the size of the vessel employing it, but there is no serious harm in using different names (like liberty and freedom) for the same concepts.

      With regards to the back ferry, the difficulties you allude to exist, but can be overcome with practice, and the advantages are so great that I would not hesitate introducing it to anyone who is interested in paddling, as soon as the opportunity presents. One significant advantage to the back ferry is you don’t have to turn the boat around before you can use it. The time thus saved can be critically important. The other advantage is that when maneuvering using the back ferry, you are usually still moving down river (albeit slowly), and what’s coming at you is almost always right in front of your face rather than sneaking up behind your back. And you haven’t yet seen what’s coming down river (including the rocks and shoals and log jams and sweepers, as well as the hydraulic features), whereas you have just looked closely at (in fact you’ve just floated through or by) the currents that will be affecting your boat when you begin a back ferry maneuver. While I wouldn’t mind having eyes in the back of my head, until that adaptation arrives in my genes, I’d usually prefer to be looking where I’m going on the water, rather than looking where I’ve already been. Practice dramatically increases the back ferry comfort level. Arrested development is no development at all.

      When back ferrying, the pivot point of your boat is @ one-third of the distance along the wetted keel, near the stern. When forward ferrying, the pivot point is in a similar location near the bow. What you are referring to as “the loose end” of the canoe (or kayak) is that end of the boat which is opposite the location of the pivot point. It’s important to point out (as you so well did) that corrections in ferry angle are more effective the farther from the pivot point they are applied. I suggest you read my essay, “The Peripatetic Pivot Point” for a more complete lesson.

      When thinking about, or discussing leaning upstream vs. leaning downstream to avoid capsize, it is most important to avoid confusion. Unfortunately, you illustrated a common confusion afflicting some paddlers when you described a situation in which the paddler should, in your judgment, lean “up-stream”, not downstream – or should not lean at all. Your confusion occurred because you were thinking of the general, macro-flow of the river to the sea (and not the micro-flow, immediately adjacent and relative to the boat) when you stated the paddler must lean upstream (sic), or not lean at all, in order to avoid a capsize in those particular circumstances. If you ask a river, or an expedition planner, “downriver”, on the grand scale, is the direction to the sea.

      However, if you ask the canoe or kayak, or the knowledgeable paddler, “downstream” is the direction that the water, which is in the immediate vicinity of the vessel, is moving, relative to the vessel. The lean to avoid a capsize must always be to the vessel’s downstream side. And the downstream side of the boat is always the side opposite of the side on which the local water is actually impinging, at that very moment. You don’t have to know which way the ocean is, in order to react with the proper lean. Instead, you have to know which way the adjacent water is moving relative to the boat – or which way the boat is moving relative to the adjacent water – which, of course, is saying the same thing.

      Therefore, you made several incorrect assertions in your discussion of leaning to prevent capsize. I’ll say it right, in order to prevent the dissemination of confusion and error in my blog. First, leaning “downstream” is not merely a general rule for which there are many common exceptions. It is the rule. Don’t forget it. Don’t try anything different, when your safety or your outfit is at stake. Learn the rule.

      That means much more than just memorizing the two simple words. That means asking, learning, and thinking often about what they mean, on a complex, active, often unpredictable body of water. A boat which is drifting with the general current (in what you refer to as “equilibrium”, whatever that means to you) does not have to be leaned in order to prevent a capsize precisely because that boat does not then have an upstream or a downstream side, which is another way of saying that the boat has no motion relative to the adjacent water. Your mistaken understanding of the rule is most strikingly illustrated in the concluding sentence of your paragraph about leaning in a river. I will take your example, look more carefully at it, and describe it properly:

      Assume that your boat is in a portion of the river where the current is flowing rapidly to the sea, and that your boat is drifting sideways with your starboard side leading. Further assume that your boat is moving in exactly the same speed and direction, relative to the current in the immediate vicinity of the boat. Even though the river is moving very fast at that location, so is your boat, at the same speed and direction, so the water immediately adjacent to your hull is not moving towards any portion of your boat. The water at that moment has exactly the same motion relative to your hull as it would have if you were drifting still in a stagnant pond – in other words, zero relative motion between the boat and the water in close proximity to it. There is no downstream side and there is no upstream side.

      Now imagine that your boat’s momentum carries it into an area where the water is flowing in the same direction, but at a much slower velocity – like an eddy. Your boat does not, and can not, instantly reduce it’s speed to equal the speed of the water it now finds itself encountering. In fact, your boat is now instantly moving faster than the water, and the water is now suddenly moving relative to the boat. The water is moving relative to the boat in a direction that is opposite to the direction that the water is moving relative to the ground. The water is suddenly impinging on your starboard side. Now your boat suddenly has an upstream side and a downstream side. It does not matter which way (relative to the ground) that the water is moving. What maters is which way the water is moving relative to the boat. The upstream side is defined as the side of the boat against which the solid water (don’t be fooled by any frothy white stuff smacking you in the face) is impinging. So, if the solid water is impinging on your starboard side (or better yet, just before it impinges on your starboard side) you need to lean and perhaps brace on the other side, which is the downstream side, which is your port side.

      The “downstream” and “upstream” sides of your boat are defined by the relative motion of the water and your boat at any moment. It simply doesn’t matter what direction the water is moving relative to the ground at your location. It doesn’t matter what direction your boat is heading or moving relative to the ground at that moment. Downstream and upstream has only to do with your boat, and the water next to it, and their relative motion in the moment. In other words, you can determine which way to lean and brace to avoid a capsize as long as you can determine nothing more than the motion of your boat relative to the water immediately surrounding it. You can thus avoid a capsize without knowing anything at all about which way (if any), or how fast, the water body as a whole is tending, or which way (if any) the water immediately surrounding your little boat is moving with respect to the planet.

      This is truly significant, and not merely academic, because you need to know what to do extremely quickly, reacting on the basis of a simple fact that is easily obtainable, without complicated analysis. This is truly significant, and not merely academic, because in really big water, and even in some conditions of visibility where the water itself is not so big, it is very possible for the paddler to have no idea, at least at the critical moment, what the water you’re in is doing in relation to the planet. Tidal rapids and overfalls and ocean currents (especially when no land is in sight) can be pretty mystifying in this regards, and so can very big and powerful rivers – especially in conditions of limited or no visibility, or great distances, or big hydraulics. But a competent paddler can still avoid a capsize. How? – by leaning downstream. The key is to remember and always apply the rule – to avoid a capsize, always lean downstream. The trick is to quickly recognize “downstream” and react, and the way to learn how to do that well is by reflecting upon gradually progressive experience.

      This may or may not be technical nit-picking, but I’d recommend against using the word “momentum” in the way you suggested (in paragraph 6 and the last paragraph of your comment), as in “momentum relative to the water”. I would instead use the expression “motion relative to the water” or “movement through the water”. What you say in your comment is then applicable, accurate, and helpful. But I think it is incorrect, confusing, and misleading to speak of two different kinds of momentum – momentum “relative to the ground” and momentum “relative to the water”. Also (re: your paragraph 2) there is no such thing as “potential momentum”. Potential energy (as distinct from kinetic energy) is defined and valid, but potential momentum is a pure phantom. If you make up and use pseudo-scientific terms (especially without carefully defining them), you make it more difficult for people to come to a common and valid understanding.

      Comment by clydewinter — June 29, 2009 @ 3:02 pm | Reply

  5. an intriguing concept, im going to use this as a resource for my upcoming paper on momentum

    Comment by kinetic energy — August 9, 2011 @ 1:42 pm | Reply


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