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#1
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Darryl Johnson wrote:
Gene Cosloy wrote in Does the final stability of a kayak depend on how much forward propulsion the boat is undergoing, all things being equal. Interesting question. Here's why it never ocurred to me to ask it: I have assumed that the kayak's stability would be independent of forward motion. Therefore, if this assumption is valid, you will be able to edge the boat to the same degree whether it is standing still or moving forward at any speed. The bicycle gets its stability from the gyroscopic forces on the rotating wheels (not centrifugal). Gyroscopic forces turn out not to be required for bicycle stability. A study done by David Jones (The Stability Of The Bicycle; Physics Today, April 1970, 34-40) used a variety of modified bicycle designs to determine the key elements in stability. A bicycle without gyroscopic forces still turned out to be stable and easily rideable (as are bicycles with a ski substituted for the front wheel for use on snow). Now, if there is some force from the moving water acting on the boat, I would expect them to be fairly small. I'd also expect the effect to rather small. But I would expect the movement to help rather than hinder stability. The upper part of the bow curves outward and if this surface is brought into contact with the water by leaning the boat then the forward movement of this curved surface should tend to produce an additional righting force. |
#2
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On 4-Jun-2004, Peter wrote:
A study done by David Jones (The Stability Of The Bicycle; Physics Today, April 1970, 34-40) used a variety of modified bicycle designs to determine the key elements in stability. An earlier analysis of this stuff is in Timoshenko's book on Dynamics. It is indeed centripetal and (D'Alembert) centrifugal forces that provide stability to a bike. The degrees of freedom in a bike are coupled - if you turn the front wheel to the left or right, the bike leans a bit accordingly. This changes its direction which generates a counteracting centrifugal righting force. But I would expect the movement to help rather than hinder stability. The upper part of the bow curves outward and if this surface is brought into contact with the water by leaning the boat then the forward movement of this curved surface should tend to produce an additional righting force. OTOH, the water flowing over the surface may generate a normal force. Water flowing over the surface results in less pressure on that surface. hat that does in the end depends on the orientation of that force relative to other forces. I've been told that longitudinal stability is enhanced with motion, but I have no idea what it would do to rolling stability of a heeled craft. It seems to be below my threshhold of awareness, regardless. Mike |
#3
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Michael Daly wrote:
On 4-Jun-2004, Peter wrote: A study done by David Jones (The Stability Of The Bicycle; Physics Today, April 1970, 34-40) used a variety of modified bicycle designs to determine the key elements in stability. An earlier analysis of this stuff is in Timoshenko's book on Dynamics. It is indeed centripetal and (D'Alembert) centrifugal forces that provide stability to a bike. The degrees of freedom in a bike are coupled - if you turn the front wheel to the left or right, the bike leans a bit accordingly. This changes its direction which generates a counteracting centrifugal righting force. Sure, there are plenty of references that make the theoretical claim that gyroscopic forces result in bicycle stability. But Jones tested those claims with an experimental arrangement where the gyroscopic effects were canceled out by using a second counter-rotating wheel next to the regular wheel. The resulting bicycle was found to still be very stable and easy to ride whether the second wheel was turning backwards (no gyroscopic effects), forwards (double the effect), or stationary. Bicycles are still stable even in the absence of gyroscopic effects. The steering geometry was found to be more significant, especially the amount of trail between where the steering axis hits the ground compared to the position of the contact patch of the tire. But I would expect the movement to help rather than hinder stability. The upper part of the bow curves outward and if this surface is brought into contact with the water by leaning the boat then the forward movement of this curved surface should tend to produce an additional righting force. OTOH, the water flowing over the surface may generate a normal force. Water flowing over the surface results in less pressure on that surface. hat that does in the end depends on the orientation of that force relative to other forces. I've been told that longitudinal stability is enhanced with motion, but I have no idea what it would do to rolling stability of a heeled craft. It seems to be below my threshhold of awareness, regardless. As I wrote before, I expect the effect to be small at the relatively low speed of kayaks. But the heeled-over surface of the upper part of the bow has the leading edge higher than the trailing edge and when pushed forward through the water it will tend to push the water downwards which requires a corresponding upward push on that part of the boat. This force will provide a net torque helping to keep the boat upright. |
#4
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Peter wrote in message ...
Michael Daly wrote: On 4-Jun-2004, Peter wrote: A study done by David Jones (The Stability Of The Bicycle; Physics Today, April 1970, 34-40) used a variety of modified bicycle designs to determine the key elements in stability. An earlier analysis of this stuff is in Timoshenko's book on Dynamics. It is indeed centripetal and (D'Alembert) centrifugal forces that provide stability to a bike. The degrees of freedom in a bike are coupled - if you turn the front wheel to the left or right, the bike leans a bit accordingly. This changes its direction which generates a counteracting centrifugal righting force. Sure, there are plenty of references that make the theoretical claim that gyroscopic forces result in bicycle stability. But Jones tested those claims with an experimental arrangement where the gyroscopic effects were canceled out by using a second counter-rotating wheel next to the regular wheel. The resulting bicycle was found to still be very stable and easy to ride whether the second wheel was turning backwards (no gyroscopic effects), forwards (double the effect), or stationary. Bicycles are still stable even in the absence of gyroscopic effects. The steering geometry was found to be more significant, especially the amount of trail between where the steering axis hits the ground compared to the position of the contact patch of the tire. But I would expect the movement to help rather than hinder stability. The upper part of the bow curves outward and if this surface is brought into contact with the water by leaning the boat then the forward movement of this curved surface should tend to produce an additional righting force. OTOH, the water flowing over the surface may generate a normal force. Water flowing over the surface results in less pressure on that surface. hat that does in the end depends on the orientation of that force relative to other forces. I've been told that longitudinal stability is enhanced with motion, but I have no idea what it would do to rolling stability of a heeled craft. It seems to be below my threshhold of awareness, regardless. As I wrote before, I expect the effect to be small at the relatively low speed of kayaks. But the heeled-over surface of the upper part of the bow has the leading edge higher than the trailing edge and when pushed forward through the water it will tend to push the water downwards which requires a corresponding upward push on that part of the boat. This force will provide a net torque helping to keep the boat upright. Peter, for whatever it's worth I like your response the best, it does seem more intuitive that the leaned bow and stern passing over moving water would result in lift similar to that of a paddle brace. As Michael also correctly noted moving water is a fluid which does conform to the Bernoulli effect: the same law that produces lift on airfoils. In the case of the Kayak airfoil however the pressure would be greater on the side which was less wetted, tending to push the boat further into the water. However, the speed of the fluid flow may be inconsequential compared to the positive effect of the bow and stern acting like a paddle with it's leading edge high. Oh well, time to get into the lab (water) and test it out!! Gene |
#5
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On 4-Jun-2004, Peter wrote:
Sure, there are plenty of references that make the theoretical claim that gyroscopic forces result in bicycle stability. Umm,... Timoshenko showed that gyroscopic forces were essentially irrelevant. I was confirming your point, but identifying earlier analyses exist than Jones. I wouldn't be surprised if Jones referenced Timoshenko. It's a classic text in mechanical engineering. Whitt and Wilson's book on bicycling science also catalogued many studies on bicycle stability done before WWII. Mike |
#6
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"Michael Daly" wrote in message ...
On 4-Jun-2004, Peter wrote: Sure, there are plenty of references that make the theoretical claim that gyroscopic forces result in bicycle stability. Umm,... Timoshenko showed that gyroscopic forces were essentially irrelevant. I was confirming your point, but identifying earlier analyses exist than Jones. I wouldn't be surprised if Jones referenced Timoshenko. It's a classic text in mechanical engineering. Whitt and Wilson's book on bicycling science also catalogued many studies on bicycle stability done before WWII. Mike Slightly different question: If you're familiar with SeaKayak magazine reviews, they supply some interesting numerical data with respect to the subject of stability. As an example, if given the same or equal conditions, if it takes say only 10 foot lbs of force to heel a boat 25 degrees which represents the point of imminent capsize, that should also mean that it requires the same amount of force applied opposite to right the boat? Now if another boat requires 20 foot lbs to heel the boat the same 25 degrees and with the same result i.e. imminent capsize, and the same amount to resist or right the boat, which boat is more stabile? Which is easier to control? Which would you rather paddle? while the charts and numerical values can be wildly divergent, the reviews narrative descriptions regarding stability frequently appear to be similar. Gene |
#7
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