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

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

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

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.

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.

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.

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

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

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

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.