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.

On 19-Jun-2004, Brian Nystrom wrote:

(THS in MPH = 1.34 x the square root of the waterline
length).

Minor nit - that formula's for knots, not mph.

By the way , the guy did say he practically sank the sot. In that
case, stability would be limited and lifting his legs _might_ have
done the trick regardless of the CG change. His comment about the
elliptical section suggests that the sot has a bit of tumblehome.
That would mean that if he's significantly close to sinking, he
has no righting moment increase on heel. Any reduction in
stabilization ("sponson" effects, roll damping from his legs, CG
position etc) can undo everything.

While your argument is completely correct for the typical situation,
I think his explanation is a bit vague/difficult to envision - hence
he might be partially correct without being able to express it in
standard terms. The bigger problem is that he's viewing the general
characteristics of kayaks from his single, possibly atypical,
experience.

Mike

Mike explained my situation better than I could. With the sot close to
sinking, which it appeared to be, the stability situation changes a lot.
I can easily imagine that this might not be typical, even for an sot,
which I suppose is why people are having trouble with my experience and
analysis.

This was not a narrow high performance type of sit on top, just looking
at it, you could see that it was an inexpensive recreational type of
kayak. I know a narrow boat when I see one, and this was not a narrow
boat. Brian and Marsh have basically ignored my statement that this
was not a narrow high performance boat that could logically be expected
to be inherently unstable for an inexperienced user, but a wider
recreational boat that should have been very stable, but was very
unstable for me. Mike at least is listening to me.

Richard



Michael Daly wrote:
On 19-Jun-2004, Brian Nystrom wrote:


(THS in MPH = 1.34 x the square root of the waterline
length).


Minor nit - that formula's for knots, not mph.

By the way , the guy did say he practically sank the sot. In that
case, stability would be limited and lifting his legs _might_ have
done the trick regardless of the CG change. His comment about the
elliptical section suggests that the sot has a bit of tumblehome.
That would mean that if he's significantly close to sinking, he
has no righting moment increase on heel. Any reduction in
stabilization ("sponson" effects, roll damping from his legs, CG
position etc) can undo everything.

While your argument is completely correct for the typical situation,
I think his explanation is a bit vague/difficult to envision - hence
he might be partially correct without being able to express it in
standard terms. The bigger problem is that he's viewing the general
characteristics of kayaks from his single, possibly atypical,
experience.

Mike

Sorry to be a little cranky on this thread. After messing around in
boats for 35 years or so, I know what boat behavior is normal and what
is not.

Let me ask a question of Brian and Marsh: If not overloading, what
could cause an ordinary (not narrow) recreational sit on top kayak to
become extremely unstable? I know that narrow boats are unstable, which
just makes common sense, but this was not a narrow boat. In some
private email, Marsh mentioned 17 inch wide boats; 17 inches seems very
extreme to me, and I would expect to swim often if I was in such a boat.
So tell me, how can one make a wide boat unstable, if not by
overloading or standing up?

I agree that in most circumstances, weight makes a boat more stable. I
am currently in the process of learning to pole my canoe, which means I
stand up in my canoe. I started out with a little extra weight in my
canoe, and swam once the first day. The second day, I added more
weight, securely tied town, and the boat felt a lot more stable.

Richard



Richard Ferguson wrote:
Mike explained my situation better than I could. With the sot close to
sinking, which it appeared to be, the stability situation changes a lot.
I can easily imagine that this might not be typical, even for an sot,
which I suppose is why people are having trouble with my experience and
analysis.

This was not a narrow high performance type of sit on top, just looking
at it, you could see that it was an inexpensive recreational type of
kayak. I know a narrow boat when I see one, and this was not a narrow
boat. Brian and Marsh have basically ignored my statement that this
was not a narrow high performance boat that could logically be expected
to be inherently unstable for an inexperienced user, but a wider
recreational boat that should have been very stable, but was very
unstable for me. Mike at least is listening to me.

Richard



Michael Daly wrote:

On 19-Jun-2004, Brian Nystrom wrote:


(THS in MPH = 1.34 x the square root of the waterline
length).



Minor nit - that formula's for knots, not mph.

By the way , the guy did say he practically sank the sot. In that
case, stability would be limited and lifting his legs _might_ have
done the trick regardless of the CG change. His comment about the
elliptical section suggests that the sot has a bit of tumblehome.
That would mean that if he's significantly close to sinking, he
has no righting moment increase on heel. Any reduction in
stabilization ("sponson" effects, roll damping from his legs, CG
position etc) can undo everything.

While your argument is completely correct for the typical situation,
I think his explanation is a bit vague/difficult to envision - hence
he might be partially correct without being able to express it in
standard terms. The bigger problem is that he's viewing the general
characteristics of kayaks from his single, possibly atypical,
experience.

Mike

Define "wide". In my previous posts, I was referring to boats in the
typical touring width range, which is ~20"-24".

Richard Ferguson wrote:

Sorry to be a little cranky on this thread. After messing around in
boats for 35 years or so, I know what boat behavior is normal and what
is not.

Let me ask a question of Brian and Marsh: If not overloading, what
could cause an ordinary (not narrow) recreational sit on top kayak to
become extremely unstable? I know that narrow boats are unstable, which
just makes common sense, but this was not a narrow boat. In some
private email, Marsh mentioned 17 inch wide boats; 17 inches seems very
extreme to me, and I would expect to swim often if I was in such a boat.
So tell me, how can one make a wide boat unstable, if not by
overloading or standing up?

I agree that in most circumstances, weight makes a boat more stable. I
am currently in the process of learning to pole my canoe, which means I
stand up in my canoe. I started out with a little extra weight in my
canoe, and swam once the first day. The second day, I added more
weight, securely tied town, and the boat felt a lot more stable.

Richard



Richard Ferguson wrote:

Mike explained my situation better than I could. With the sot close
to sinking, which it appeared to be, the stability situation changes a
lot. I can easily imagine that this might not be typical, even for an
sot, which I suppose is why people are having trouble with my
experience and analysis.

This was not a narrow high performance type of sit on top, just
looking at it, you could see that it was an inexpensive recreational
type of kayak. I know a narrow boat when I see one, and this was not
a narrow boat. Brian and Marsh have basically ignored my statement
that this was not a narrow high performance boat that could logically
be expected to be inherently unstable for an inexperienced user, but a
wider recreational boat that should have been very stable, but was
very unstable for me. Mike at least is listening to me.

Richard



Michael Daly wrote:

On 19-Jun-2004, Brian Nystrom wrote:


(THS in MPH = 1.34 x the square root of the waterline
length).




Minor nit - that formula's for knots, not mph.

By the way , the guy did say he practically sank the sot. In that
case, stability would be limited and lifting his legs _might_ have
done the trick regardless of the CG change. His comment about the
elliptical section suggests that the sot has a bit of tumblehome.
That would mean that if he's significantly close to sinking, he
has no righting moment increase on heel. Any reduction in
stabilization ("sponson" effects, roll damping from his legs, CG
position etc) can undo everything.

While your argument is completely correct for the typical situation,
I think his explanation is a bit vague/difficult to envision - hence
he might be partially correct without being able to express it in
standard terms. The bigger problem is that he's viewing the general
characteristics of kayaks from his single, possibly atypical,
experience.

Mike

Michael Daly wrote:

On 19-Jun-2004, Brian Nystrom wrote:


(THS in MPH = 1.34 x the square root of the waterline
length).


Minor nit - that formula's for knots, not mph.

Thanks for correcting that, Mike.

On 18-Jun-2004, Wright wrote:

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?

If you made it through the nonsense that followed, Brian's answer is correct.
Low-placed weight increases stability, high-placed weight decreases it. Hence
women and shorter men tend to see more stability than tall, big people. Adding
weight (gear) up to a reasonable load limit tends to increase stability.

If you pick up a copy of Sea Kayaker magazine, you'll find that they include
a graph of stability (righting moment vs angle of heel) with every review of
kayaks. I recently posted about this and can add that they plot four curves.
There are curves for light and heavy paddlers both with and without gear.
A light paddler with gear always shows the greatest stability, while the
heavy paddler without gear always shows the least stability.

Mike

On 6/21/04 1:51 PM, in article , "Michael
Daly" wrote:

On 18-Jun-2004, Wright wrote:

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?

If you made it through the nonsense that followed, Brian's answer is correct.
Low-placed weight increases stability, high-placed weight decreases it. Hence
women and shorter men tend to see more stability than tall, big people.
Adding
weight (gear) up to a reasonable load limit tends to increase stability.

If you pick up a copy of Sea Kayaker magazine, you'll find that they include
a graph of stability (righting moment vs angle of heel) with every review of
kayaks. I recently posted about this and can add that they plot four curves.
There are curves for light and heavy paddlers both with and without gear.
A light paddler with gear always shows the greatest stability, while the
heavy paddler without gear always shows the least stability.

Mike
Thanks for the reply. It certainly makes sense that more gear weight (up to
a point) low in the boat would make it more stable. The heavy keel on a
sailboat would be an extreme example of that. Here is the part I don't quite
follow. I have seen the reviews in Sea Kayaker magazine and they assume, if
I am understanding them correctly, that the center of gravity for both the
lighter and the heavier paddler is 10 inches from the low point of the seat.
In other words the center of gravity is at exactly the same place for both
paddlers (without any gear). Yet the curve for the lighter paddler
indicates more stability. Is this because weight below the center of
gravity and weight above the center of gravity are not created equal? That
is, the 50% of the weight of the heavier paddler that is above the center of
gravity must cause somewhat more instability than the 50% of the weight of
the lighter paddler? Is that correct, or am I misreading the graphs?
Chuck

On 21-Jun-2004, Wright wrote:

I have seen the reviews in Sea Kayaker magazine and they assume, if
I am understanding them correctly, that the center of gravity for both the
lighter and the heavier paddler is 10 inches from the low point of the seat.
In other words the center of gravity is at exactly the same place for both
paddlers (without any gear). Yet the curve for the lighter paddler
indicates more stability.

Interesting - I never noticed that comment about CG position in the reviews.
Since these are static and not dynamic stability calculations, the mass at
the CG is all that matters. If both the heavy and light paddlers have their
mass at the same point, then the only difference is the quantity and the
stability curves do not reflect the difference in distribution.

Since that CG (10" up and 10" forward) is above the center of bouyancy, any
heeling will have it contribute to the overturning moment. The heavier
paddler will still have the lesser stability at a given angle of heel. If
the CG was determined for real paddlers, taking into account different CG
positions, the moments calculated would be slightly different.

Don't forget that these figures still ignore the fact that a real paddler
will tend to keep his/her body roughly vertical if the kayak heels. This
greatly changes the overturning moments.

Mike