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Paddler's Weight vs. Stability and Performance?
Having noticed that most touring kayak manufacturers recommend shorter
lighter kayaks for lighter weight paddlers (without really specifying why) I am looking for an explanation of the effect of a person's weight on initial stability, secondary stability, and performance. Given two identical touring kayaks and two people of equal abilities, if one weighs 125 lbs and the other weighs 190 lbs what would be the differences, if any, on stability and performance? Chuck |
Paddler's Weight vs. Stability and Performance?
Every boat has a designed load range and an optimum load range, which is
the engineering view of the world. I have a personal story. I one time tried to use a friend's sit-on-top kayak. Now I am a big guy, around 200 pounds, and heavier than he is. I found that if I took my feet out of the water and put them on the deck, I went over almost instantly, in calm water. With my feet and lower legs in the water, the boat sat higher in the water, and was reasonably stable. However much my legs weighed, maybe 30 pounds (15 kg), it was the difference between stability and instability. After swimming a few times, and failing to get far from the dock, I gave him back his kayak. I think that the extra weight took away the initial stability that the kayak was intended to have. My guess is that the boat was designed for smaller people, but usable for medium sized people. My weight was outside the operating range of that design, making the boat unusable by me. I would make a guess that overloading a boat is generally much more detrimental than underloading a boat. A large boat paddled by a small person probably will be slower and more subject to wind and wave than a smaller boat paddled by the same person, but probably still safe and usable. My two cents. Richard Wright wrote: Having noticed that most touring kayak manufacturers recommend shorter lighter kayaks for lighter weight paddlers (without really specifying why) I am looking for an explanation of the effect of a person's weight on initial stability, secondary stability, and performance. Given two identical touring kayaks and two people of equal abilities, if one weighs 125 lbs and the other weighs 190 lbs what would be the differences, if any, on stability and performance? Chuck |
Paddler's Weight vs. Stability and Performance?
On 6/18/04 10:41 PM, in article
, "Richard Ferguson" wrote: Every boat has a designed load range and an optimum load range, which is the engineering view of the world. I have a personal story. I one time tried to use a friend's sit-on-top kayak. Now I am a big guy, around 200 pounds, and heavier than he is. I found that if I took my feet out of the water and put them on the deck, I went over almost instantly, in calm water. With my feet and lower legs in the water, the boat sat higher in the water, and was reasonably stable. However much my legs weighed, maybe 30 pounds (15 kg), it was the difference between stability and instability. After swimming a few times, and failing to get far from the dock, I gave him back his kayak. I think that the extra weight took away the initial stability that the kayak was intended to have. My guess is that the boat was designed for smaller people, but usable for medium sized people. My weight was outside the operating range of that design, making the boat unusable by me. I would make a guess that overloading a boat is generally much more detrimental than underloading a boat. A large boat paddled by a small person probably will be slower and more subject to wind and wave than a smaller boat paddled by the same person, but probably still safe and usable. My two cents. Richard Thanks for the reply - nothing like a real world example to support the theory! I am on the lighter end of the adult weight spectrum (130 lbs) so I guess that I would see more stability out of most kayaks than a heavier person. The trade off - I suppose - would be the sitting higher in the water I would be more subject to the wind and waves? Chuck |
Paddler's Weight vs. Stability and Performance?
Richard Ferguson wrote:
Every boat has a designed load range and an optimum load range, which is the engineering view of the world. I have a personal story. I one time tried to use a friend's sit-on-top kayak. Now I am a big guy, around 200 pounds, and heavier than he is. I found that if I took my feet out of the water and put them on the deck, I went over almost instantly, in calm water. With my feet and lower legs in the water, the boat sat higher in the water, and was reasonably stable. However much my legs weighed, maybe 30 pounds (15 kg), it was the difference between stability and instability. After swimming a few times, and failing to get far from the dock, I gave him back his kayak. Your issue was not likely one of absolute weight, but center of gravity. The only way one can be too heavy for a kayak is if you literally push it under water. However, the higher your center of gravity, the less stable the boat will be. Also, the heavier you are, the more effect you will have when you shift your weight or lean. People who are tall and heavy will find a given boat to be much less stable than a person who's short and light. I think that the extra weight took away the initial stability that the kayak was intended to have. My guess is that the boat was designed for smaller people, but usable for medium sized people. My weight was outside the operating range of that design, making the boat unusable by me. That's highly unlikely. With most kayaks, the stability increases as you push the boat deeper into the water, up to the point that the gunwales submerge. I would make a guess that overloading a boat is generally much more detrimental than underloading a boat. A large boat paddled by a small person probably will be slower and more subject to wind and wave than a smaller boat paddled by the same person, but probably still safe and usable. Actually, the opposite is true. A heavier person pushes a boat down farther in the water, increasing the length of the waterline. While this adds more surface friction, it also increases the theoretical hull speed of the boat, making it possible to paddle it faster before "hitting the wall", so to speak. I paddle low volume boats and am at the upper end of the recommended weight range for all of them. I also build skin-on-frame boats that are even lower in volume. All of my boats perform well and have the added benefits of a better fit and less windage. |
Paddler's Weight vs. Stability and Performance?
Wright wrote:
Having noticed that most touring kayak manufacturers recommend shorter lighter kayaks for lighter weight paddlers (without really specifying why) I am looking for an explanation of the effect of a person's weight on initial stability, secondary stability, and performance. Given two identical touring kayaks and two people of equal abilities, if one weighs 125 lbs and the other weighs 190 lbs what would be the differences, if any, on stability and performance? It all depends on their center of gravity. If the heavier person carried most of their weight in their legs and butt (as many women do), they could actually be more stable in a given boat than a lighter person who carried their weight higher. On the other hand, a tall, broad-shouldered, barrel-chested 190# man would feel much less stable in a given boat than a shorter, 125# woman. Weight ranges specified for kayaks are generally much lower than the actual carrying capacity of the boat. In many cases, a paddler weight is specified, but not a cargo weight. The manufacturer assumes that some amount of extra gear will likely be carried and builds in a margin for it. Overall, the majority of people end up buying kayaks that have too much volume for them. It's actually hard to buy a boat that's too small, since you simply won't fit. Some disadvantages of a boat with too much volume a - They will feel less stable, since the bigger boat raises your center of gravity higher above the water. - They have more windage and are more easily pushed around by wind and waves. This can hamper your ability to control the boat. - Their higher decks make rolling and other advanced skills more difficult. As a general rule, you want the smallest volume boat that will comfortably accommodate you and the gear you anticipate carrying. In terms of performance, there's a trade-off between length and speed. As a general rule, kayaks are limited to a specific theoretical hull speed by their wavemaking resistance, which is a function of the waterline length (THS in MPH = 1.34 x the square root of the waterline length). The longer the waterline, the higher the theoretical hull speed. Most touring boats have a THS in the range of 4-6 mph. However, longer boats have more surface area in the water and therefore more friction. At low paddling speeds (~2-3 knots), surface friction is the main source of drag. As your speed increases, wavemaking resistance becomes the dominant factor up to the point that you simply cannot paddle any faster no matter how much effort you put into it. So what does this mean in practical terms? A shorter boat with less surface friction will be more efficient at lower speeds, but it will limit how fast one can paddle. A longer boat will require more effort at low speeds, but it can be pushed to a higher top speed, provided that the paddler has the horsepower to do so. For a small paddler with a proportionately lower power output, a shorter boat can be advantageous as long as the THS limit is not a problem for the type of paddling they intend to do. |
Paddler's Weight vs. Stability and Performance?
I do not buy your argument at all. If the issue was center of gravity,
the trunk of my body, where most of the weight was, did not move when I pulled my legs out of the water. Also my legs were on the deck, just a few inches above the waterline, so the change in center of gravity was minimal. However, the effect on stability was extremely dramatic. (I am less than 6 feet tall, so my center of gravity is not unusually high.) In other words, I can't see how a very small change in the center of gravity could effect a dramatic change in boat stability. There has to be another cause. Remember that this was a sit on top type of kayak, kind of like a surfboard with depresions for the butt and feet. Most of the kayak was underwater even before I pulled my legs out of the water. I think that part of what happened was that the extra weight of my legs put even more of the kayak under the water, and that affected the hull design. It started to look more like a submarine than a conventional boat. One that you said that I might agree with is the idea that weight increases the stability of a boat, up to a certain point. (You mentioned the gunwales). It is really a function of the hull design. If you think of the hull design as kind of an ellipse (or other geometric shape), then as long as the waterline is below the center of the ellipse, the boat is likely to be stable. Once the waterline gets above the center of the ellipse, the effective hull width actually starts to decrease, and the boat becomes less stable. When the hull leans to the left, there is less and less boat above the waterline on the left side resisting the left lean, so the boat rolls. When a boat gets loaded past the maximum hull width, at that point the boat does start to look like a submarine. Conventional canoes are less likely to suffer this kind of instability, since the width of the boat at the waterline, for most hull designs, tends to increase as the boat is first loaded, and most people will never load a canoe to the point where it is close to the gunnels, which is where the canoe hull sometimes narrows, if only a little bit. If you think about the playboat kayaks, they have relatively low volume, which means that much of the hull is below the waterline, reducing stability, which is how they can do some of the tricks that they do, endos, etc. Even they have to have some width at the waterline, because the hull width at the cockpit has to be wider than the person, and the cockpit coaming is several inches above the waterline, so at least the part of the hull around the cockpit tends to provide stability. Conventional touring kayaks have most of the volume above the waterline, so they never hit this form of overload instability, which would probably require a lot more weight than a person and camping gear would provide. Think about the center of volume of the hull, it would always be above the waterline. Since the sit-on-top kayaks have relatively little hull above the waterline, they are more prone to his kind of instability caused by overload, I would think. Richard Your issue was not likely one of absolute weight, but center of gravity. The only way one can be too heavy for a kayak is if you literally push it under water. However, the higher your center of gravity, the less stable the boat will be. Also, the heavier you are, the more effect you will have when you shift your weight or lean. People who are tall and heavy will find a given boat to be much less stable than a person who's short and light. I think that the extra weight took away the initial stability that the kayak was intended to have. My guess is that the boat was designed for smaller people, but usable for medium sized people. My weight was outside the operating range of that design, making the boat unusable by me. That's highly unlikely. With most kayaks, the stability increases as you push the boat deeper into the water, up to the point that the gunwales submerge. I would make a guess that overloading a boat is generally much more detrimental than underloading a boat. A large boat paddled by a small person probably will be slower and more subject to wind and wave than a smaller boat paddled by the same person, but probably still safe and usable. Actually, the opposite is true. A heavier person pushes a boat down farther in the water, increasing the length of the waterline. While this adds more surface friction, it also increases the theoretical hull speed of the boat, making it possible to paddle it faster before "hitting the wall", so to speak. I paddle low volume boats and am at the upper end of the recommended weight range for all of them. I also build skin-on-frame boats that are even lower in volume. All of my boats perform well and have the added benefits of a better fit and less windage. |
Paddler's Weight vs. Stability and Performance?
Richard Ferguson wrote:
I do not buy your argument at all. If the issue was center of gravity, the trunk of my body, where most of the weight was, did not move when I pulled my legs out of the water. Also my legs were on the deck, just a few inches above the waterline, so the change in center of gravity was minimal. However, the effect on stability was extremely dramatic. (I am less than 6 feet tall, so my center of gravity is not unusually high.) In other words, I can't see how a very small change in the center of gravity could effect a dramatic change in boat stability. There has to be another cause. Nope - with your legs in the water, your CG is definitely lower - and near the pivot point for the cylinder on which you are sitting. When you move your feet up into the well, now everything is at or above the pivot point. A regular kayak keeps a significant amount of your weight below that pivot point and makes the kayak much more stable. I know this oversimplifies the math, but that's my .02 and I'm stickin' to it:-) Remember that this was a sit on top type of kayak, kind of like a surfboard with depresions for the butt and feet. Most of the kayak was underwater even before I pulled my legs out of the water. I think that part of what happened was that the extra weight of my legs put even more of the kayak under the water, and that affected the hull design. It started to look more like a submarine than a conventional boat. Depending on the particular type of surf ski, many of them are about like balancing on a bowling ball. Many of the competition skis are 18-19" wide, and take hours of practice to ride successfully, much less get used to paddling with a wing paddle. The extra height above the water gives you a lot better leverage once you get used to sitting balanced on such a skinny boat, and they are darned fast when paddled right. Marsh Minnesota |
Paddler's Weight vs. Stability and Performance?
Sounds like you guys have made up your mind that CG is the issue, and
that adding weight to a boat can't possibly make it unstable, and that I don't know what I am talking about, so I think I am wasting my breath/typing. Let me make one last try to support my position. 1. My friend, who is kind of a skinny guy, but is about the same height, had no problem with the kayak. He is not an expert kayaker. We later went out offshore into some waves in the Pacific ocean, he in that kayak and me paddling a windsurfer hull, neither of us had problems, we went snorkeling, and then climbed back on and paddled back in. This was not an ultra-narrow high performance narrow sit on top, it was a cheap recreational type of sit on top, which one would expect to be pretty stable. 2. If the center of gravity is the issue, how come my buddy had no problem with the boat, and I could not keep it upright for 10 seconds in flat water, once I lifted my legs out of the water? I am not that bad a paddler, and since he is the same height, his center of gravity would be about the same. But maybe you guys think that I am not only incorrect, but an incompetent paddler. That is probably the only way you can logically defend your position. 3. He warned me before I tried it that I would probably have trouble with the boat, that I was too heavy for the boat. He said that his daughter usually used the boat, and the boat was probably sized more for her than 200 pound men. The only way to settle this issue scientifically would be to take that boat with a medium weight paddler, and gradually add weight to the boat to make the total weight (paddler plus dead weight) equal to around 200 pounds, and see how the boat performed. Obviously I think the boat would become unstable, and you guys think that the boat would not become unstable. Since you have never seen a boat become unstable with heavy loads, you think it can't happen, but it happened to me. As I said, you guys are not going to listen to me, so I will shut up. ;-) Richard Marsh Jones wrote: Richard Ferguson wrote: I do not buy your argument at all. If the issue was center of gravity, the trunk of my body, where most of the weight was, did not move when I pulled my legs out of the water. Also my legs were on the deck, just a few inches above the waterline, so the change in center of gravity was minimal. However, the effect on stability was extremely dramatic. (I am less than 6 feet tall, so my center of gravity is not unusually high.) In other words, I can't see how a very small change in the center of gravity could effect a dramatic change in boat stability. There has to be another cause. Nope - with your legs in the water, your CG is definitely lower - and near the pivot point for the cylinder on which you are sitting. When you move your feet up into the well, now everything is at or above the pivot point. A regular kayak keeps a significant amount of your weight below that pivot point and makes the kayak much more stable. I know this oversimplifies the math, but that's my .02 and I'm stickin' to it:-) Remember that this was a sit on top type of kayak, kind of like a surfboard with depresions for the butt and feet. Most of the kayak was underwater even before I pulled my legs out of the water. I think that part of what happened was that the extra weight of my legs put even more of the kayak under the water, and that affected the hull design. It started to look more like a submarine than a conventional boat. Depending on the particular type of surf ski, many of them are about like balancing on a bowling ball. Many of the competition skis are 18-19" wide, and take hours of practice to ride successfully, much less get used to paddling with a wing paddle. The extra height above the water gives you a lot better leverage once you get used to sitting balanced on such a skinny boat, and they are darned fast when paddled right. Marsh Minnesota |
Paddler's Weight vs. Stability and Performance?
in article , Richard
Ferguson at wrote on 6/19/04 4:44 PM: Sounds like you guys have made up your mind that CG is the issue, and that adding weight to a boat can't possibly make it unstable, and that I don't know what I am talking about, so I think I am wasting my breath/typing. http://www.guillemot-kayaks.com/Desi...tyArticle.html Please read the entire article and understand it before you respond. Let me make one last try to support my position. snip |
Paddler's Weight vs. Stability and Performance?
Richard Ferguson wrote: I do not buy your argument at all. If the issue was center of gravity, the trunk of my body, where most of the weight was, did not move when I pulled my legs out of the water. Also my legs were on the deck, just a few inches above the waterline, so the change in center of gravity was minimal. However, the effect on stability was extremely dramatic. (I am less than 6 feet tall, so my center of gravity is not unusually high.) In other words, I can't see how a very small change in the center of gravity could effect a dramatic change in boat stability. There has to be another cause. By putting your legs in the water, you did two things: - You lowered your center of gravity. This is not as inconsequential as you think. - You created outriggers. Your legs not only functioned somewhat like a tightrope walker's pole, but their buoyancy acts as - dare I say it - sponsons and increases the boat's stability. Remember that this was a sit on top type of kayak, kind of like a surfboard with depresions for the butt and feet. Most of the kayak was underwater even before I pulled my legs out of the water. I think that part of what happened was that the extra weight of my legs put even more of the kayak under the water, and that affected the hull design. It started to look more like a submarine than a conventional boat. As long as the boat wasn't submerged below the gunwales, the stability would not have decreased. One that you said that I might agree with is the idea that weight increases the stability of a boat, up to a certain point. (You mentioned the gunwales). It is really a function of the hull design. If you think of the hull design as kind of an ellipse (or other geometric shape), then as long as the waterline is below the center of the ellipse, the boat is likely to be stable. Once the waterline gets above the center of the ellipse, the effective hull width actually starts to decrease, and the boat becomes less stable. When the hull leans to the left, there is less and less boat above the waterline on the left side resisting the left lean, so the boat rolls. When a boat gets loaded past the maximum hull width, at that point the boat does start to look like a submarine. Conventional canoes are less likely to suffer this kind of instability, since the width of the boat at the waterline, for most hull designs, tends to increase as the boat is first loaded, and most people will never load a canoe to the point where it is close to the gunnels, which is where the canoe hull sometimes narrows, if only a little bit. You're basically correct, except that you're assuming that a kayak loaded up to the gunwales is less stable than one that's more lightly loaded. That simply isn't the case. If you look at the cross section of a typical touring kayak, you'll see that it flares continuously up to the gunwales. That means that the stability will constantly increase as the boat is pushed deeper into the water. Sit on tops are typically MORE flared than sit-in kayaks. If you think about the playboat kayaks, they have relatively low volume, which means that much of the hull is below the waterline, reducing stability, which is how they can do some of the tricks that they do, endos, etc. Even they have to have some width at the waterline, because the hull width at the cockpit has to be wider than the person, and the cockpit coaming is several inches above the waterline, so at least the part of the hull around the cockpit tends to provide stability. Actually, you're misinterpreting the design. Playboats are actually more stable in some respects (due to their wide beam) and many people find them more difficult to roll than a sea kayak. Because they are so short, they have relatively little fore-to-aft stability, which is something sea kayaks have a huge amount of. Their maneuverability is a function of their short length and a hull profile that encourages planing. Conventional touring kayaks have most of the volume above the waterline, so they never hit this form of overload instability, which would probably require a lot more weight than a person and camping gear would provide. Think about the center of volume of the hull, it would always be above the waterline. See my explanations above. Since the sit-on-top kayaks have relatively little hull above the waterline, they are more prone to his kind of instability caused by overload, I would think. Not generally. Most are wider and have more flare than sit-in boats (surf skis excepted). That makes them more stable, not less. |
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