Paddling Under a Load
by Wes Kisting
Recently, I received an e-mail asking for clarification about my article Weight, Drag, and Performance. Since the publication of this article, many readers have written in to confirm that they, too, have experienced performance benefits after adding weight to their kayaks. Nonetheless, since few other articles have elicited so many reader questions, I thought it fitting to post my recent e-mail exchange below. The letters are presented in sequence. Portions of my reply have been edited or expanded for purposes of conciseness and clarification.
To be clear, neither the original article nor this recent exchange aims to suggest that adding weight to a kayak will "always" or "necessarily" enhance its performance. The intent is simply to refute the popular but facile assumption that an empty or lightly loaded kayak "always" paddles faster and more efficiently than a more heavily loaded one, and thus, will arrive at its destination sooner. This claim simply does not hold true for many kayaks under real-world conditions. I have sought out knowledgeable advice to try to understand why and to correct the patently false claim that additional weight always reduces performance.
I have really enjoyed your articles on kayaking.
I have one question about "Weight, Drag, and Performance": In the paragraph that starts with "Momentum is the opposite of inertia" it wasn't clear to me what you meant. I agree that a more massive boat takes more energy to accelerate due to it's mass and that this extra energy is not wasted because the boat takes longer to slow down in between strokes. It also takes more effort to accelerate due to the added friction and this energy is not recouped. So extra weight may help with stability/tracking, but it does lead to more energy loss via drag which is not recovered on the "halting phase" of the stroke.
You may have meant this and I misunderstood. If so I apologize. In either case, your articles are great.
Dear Heavy Thinker,
Well, first a qualifier: I was trying to explain several complex concepts that I discussed with a concrete canoe-building team and a physics professor. I'm a professor of English myself, so I don't claim to have a perfect grasp of the technical explanation. But I did know that my experience was not lining up with the popular claim that weight is "always" detrimental to paddling performance, so I asked some knowledgeable folks for insight.
In theory, you are correct about the points you mention in your letter. However, these rules cannot be applied and considered in a vaccuum because the properties of fluids affect real-world kayak performance in significant ways. It is technically true that a heavier kayak produces greater drag. If we were talking about moving the load on glass-calm water (which is rarely the case), then the heavier kayak might often prove slower assuming all other things are equal. However, on a wavy surface, added weight can significantly improve straight-line tracking; it also gives the kayak more "power" (momentum) to punch through waves and other diverting forces such as wind or currents. Essentially, each ripple or wave the kayak encounters has its own energy to overcome, and this energy increases exponentially with its size. In a lighter boat, more force must be supplied by the paddler to punch through waves, maintain speed, and stay on track. In contrast, a more heavily loaded boat possesses greater momentum, meaning that more kinetic energy is built up and stored in the hull, resulting in a greater tendency to glide in a straight, steady direction and a higher resistance to diverting forces like wind and waves. True, more effort is required initially to accelerate the heavier kayak, but this effort is not entirely "wasted" as many people think. A loaded kayak can easily maintain the same cruising speed as it can unloaded. In fact, once brought up to speed, the energy required to maintain a steady "glide" in the loaded kayak is negligible compared to the unloaded kayak. Yet the loaded boat possesses a much higher kinetic energy that can power through diverting forces with less penalty to speed or direction. In many conditions, it is even true that the loaded boat can maintain its efficient glide despite a slower paddling cadence. You're correct that paddling energy is not technically "gained back" during the halting phase of the stroke (I realize that phrase is somewhat misleading), but the additional energy expended to accelerate a heavier kayak does become advantageous again in the form of greater momentum. It is not "wasted," but rather stored and re-released in a way that typically yields better tracking and a slower paddling cadence as I mentioned. I know of no accurate means to measure whether this slower cadence comes at the cost of negligibly greater effort per stroke (as some people claim), but even if it does, I find the slower cadence of a loaded kayak less fatiguing over the long-haul than a faster cadence that theoretically requires less effort.
I believe the difference is actually perceptible in the paddling experience: If you paddle as hard as you can in an unloaded kayak, you will reach a point at which the kayak feels as though it is simply plowing water (on the verge of getting onto plane, but unable to do so). At this point, much of the paddle stroke is wasted as excess energy that, if relaxed, immediately results in a slowing of the boat. The experience is, of course, exhausting. In a more heavily loaded kayak, if you paddle as hard as possible, it takes slightly longer to reach top speed and the top speed may be slightly slower (though not always); however, the excess energy of each stroke feels as though it is being transferred to the hull. I don't know how to explain this feeling except to say that there is a distinct sense of "solidness" to the hull (including less wander at the bow) and less of that "fizzy" feel to the paddle blades as they exit the water. As with the unloaded kayak, it will feel exhausting to maintain top speed for long in the loaded kayak. Regardless, in both cases (unloaded or loaded), an expedition paddler will need to slacken his pace until the kayak settles into its efficient cruising pace (frequently referred to as "glide"), which is typically between 3.5 and 4.5 knots. Here is the important detail: According to my GPS, the unloaded and the loaded kayak easily cruise at the same speed (measured to one-tenth of a knot), but the loaded kayak is often able to maintain that speed at a slower paddling cadence. The difference in cadence varies depending on conditions, but in choppy waters, it is sometimes remarkable how much less exertion is required to keep the kayak moving and on-course. It is possible some of this difference is merely my subjective perception, but it accords with years of experience in which, according to GPS data, my average moving speed was measurably higher during heavily loaded expeditions than during my day-trips in an unloaded kayak.
A few years ago, when my article first appeared, I noticed an exchange on a popular kayak forum that typified the problems with how people think about this issue. On the forum, several paddlers were attempting to rebut my claims by comparing the difference between paddling a loaded or an unloaded kayak to that between pushing a heavy or a light wagon up and down a series of hills (presumably the hills were meant to be analogous to waves). They complained that, no matter how you cut it, it will always take "more energy" to push a heavier wagon uphill, and that both wagons will coast equally easily downhill. (Strangely, they ignored the fact that the heavier wagon would coast faster downhill and, therefore, coast further up the next hill than the lighter wagonbut that's beside the point.) Such logic showed a lack of adequate attention to the properties of fluids. Water may indeed produce waves as steep as hills, but despite wave patterns, any body of water always still occupies a horizontal plane which, in fact, is constantly sliding back and forth under the kayak. This is very different than a series of hills, which remain perfectly fixed and immovable. In reality, a kayaker never truly pushes his kayak "up" anything. A kayaker only needs to maintain forward momentum and a straight course, both of which typically improve with weight (excepting, of course, those cases in which the added weight significantly exceeds the optimal design waterline for the hull design in question, but that would be a packing error on the part of the expeditioner). The waves, meanwhile, will do most of the "heavy lifting" in a way that cannot be said about wagons going up and down a series of hills. More importantly, the wagon analogy also effaces the tracking issue. If you had your choice between pushing a loaded wagon with a nice straight axle, or pushing an unloaded wagon (20 pounds lighter) with slightly more wobble to its wheels, I bet you would find the loaded wagon far easier to push over a great distance. Anyone who has ever had the misfortune of pushing a shopping cart with a slightly wobbly wheel can understand why. Regardless, no land-based hill analogy can adequately reflect the forces at work when paddling on a fluid body. For one thing, the analogy distorts the actual role of friction by replacing (significant) water drag with (negligible) air resistance. And we still haven't even begun to address the effects of wind or currents. As long as hydrodynamic forces keep getting excluded from consideration, people will continue to think (erroneously) that weight must always be detrimental to performance in a kayak. It is simply not "always" true (nor does it seem to be "most often the case") that a heavier kayak is slower and less efficient.
Hope that helps to clarify.
Thanks for making such a thorough explanation. I think you're right. When I was thinking about it I hadn't considered that the frictional force increases with speed. From the friction vs speed curves I've seen published by sea kayaker the friction does increase with speed very quickly.
That means that even if the heavy kayak and light kayak have the same average speed, the energy lost by the light kayak due to friction will be greater since it will spend some time with velocity far above the average where friction will sap more energy. The end result is the light kayak which spends some time faster than the average and some time slower than the average, will lose more energy due to friction than the slower kayak which spends the entire paddling cycle near the average speed, even if the average speed over the cycle is the same for the two boats. That is true even for kayaks on glassy smooth water.
A qualifier of my own, I only started kayaking a few months ago, so I don't have a huge inventory of personal experience. Regardless, I enjoy thinking about these problems and have a physics background, but no experience in hydrodynamics. Thanks again for discussing interesting kayaking questions.
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© 2009, Wesley Kisting