Does the final stability of a kayak depend on how much forward
propulsion the boat is undergoing, all things being equal. In other
words if edging a boat dead still on flat water gets me to the edge of
the combing before the boat wants to capsize, will the same boat
permit further edging when it's underway? A bicycle heeled over at
speed is stable and unstable heeled at rest. While a kayak does not
experience centrifugal forces, does the hydrolic surge force of
propulsion provide a similar effect?

Gene

Gene Cosloy wrote in
m:

Does the final stability of a kayak depend on how much forward
propulsion the boat is undergoing, all things being equal. In
other words if edging a boat dead still on flat water gets me to
the edge of the combing before the boat wants to capsize, will the
same boat permit further edging when it's underway? A bicycle
heeled over at speed is stable and unstable heeled at rest. While
a kayak does not experience centrifugal forces, does the hydrolic
surge force of propulsion provide a similar effect?

Gene

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).

Now, if there is some force from the moving water acting on the boat,
I would expect them to be fairly small. Perhaps small enough that you
wouldn't be able to distinguish between the amount of lean you were
able to do. It is, after all, a pretty coarse measure that you are
trying to take -- degrees of tilt from the horizontal judged by eye
alone.

But I could be wrong. g

I'd be interested in seeing if you get a more informed answer.
--
Darryl

It might be less stable at speed. Just running this question though my head
makes it hurt - the hallmark of a good question. What I came up with is that
at speed and on edge, the curve of the kayak would tend to make it rise up
out of the water. This might make the boat less stable because less of the
boat is wetted. On the other hand water moving past an object pushes on it,
holding it somewhat in place (Like a sweeping brace). So in other words I
have no idea --- hope that clears it up ;-

I'd be interested in seeing if you get a more informed answer.

Obviously not from me.


Ken
--

"Darryl Johnson" wrote in message
news
Gene Cosloy wrote in
m:

Does the final stability of a kayak depend on how much forward
propulsion the boat is undergoing, all things being equal. In
other words if edging a boat dead still on flat water gets me to
the edge of the combing before the boat wants to capsize, will the
same boat permit further edging when it's underway? A bicycle
heeled over at speed is stable and unstable heeled at rest. While
a kayak does not experience centrifugal forces, does the hydrolic
surge force of propulsion provide a similar effect?

Gene

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).

Now, if there is some force from the moving water acting on the boat,
I would expect them to be fairly small. Perhaps small enough that you
wouldn't be able to distinguish between the amount of lean you were
able to do. It is, after all, a pretty coarse measure that you are
trying to take -- degrees of tilt from the horizontal judged by eye
alone.

But I could be wrong. g

I'd be interested in seeing if you get a more informed answer.
--
Darryl

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.

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

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.

Gene,

I've read the other posts and believe that talking about hulls without
mentioning the kayaker may not really be the point. While the physics of the
posters may be right or wrong (I can't definitively state that a moving hull
is more stable than one that is stationary, though I believe this to be so).
I can state, however, that if you are moving forward in your kayak, you are
also moving your body as you paddle (or even if you cruise between strokes).
These small muscle adjustments may not seem to be much at first, but they
add up to a significant amount of righting force (note that the boat has
little tendency to be tippy until you add a paddler in the frist place,
hench changing the center-of-gravity by a significant amount). As one
individual said to me, "there are no tippy boats, just tippy paddlers,"
(meaning, of course, that any boat can be paddled by the properly skilled
person).

When stationary, just like on a bicycle, kayaks tend to be less stable.
Stand on one foot and you are less stable than you are on two. Close your
eyes, and you tend to become unstable very quickly. Hop on one foot, eyes
closed, and you can do that for quite a while. It isn't the bike that is
unstable, it's the human. In the absence of eyes, the dynamic changes the
body makes during motion are very effective at keeping balance, so hopping
is easy, standing is less so.

I remember in the film, "Baidarka," the boat was uncomfortably unstable to
the paddlers when they were launching, but once under way, they found it to
be a real joy. While part of this was definitely hull design (the bifurcated
hull improved stability while moving, something the boat designers hadn't
considered), the dynamic motions of the paddler as they autonomously
adjusted for balance were probably more significant.

Rick

"Gene Cosloy" wrote in message
m...
Does the final stability of a kayak depend on how much forward
propulsion the boat is undergoing, all things being equal. In other
words if edging a boat dead still on flat water gets me to the edge of
the combing before the boat wants to capsize, will the same boat
permit further edging when it's underway? A bicycle heeled over at
speed is stable and unstable heeled at rest. While a kayak does not
experience centrifugal forces, does the hydrolic surge force of
propulsion provide a similar effect?

Gene

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

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

"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