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U.S. Pat #6871608
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Updated 01/2008
DESIGN FOR BETTER KAYAK STABILITY: WHY AND HOW
Stability is
defined as resistance to change, deterioration, or displacement, and it
is synonym to reliability and dependability. In naval terms it means the
ability of a watercraft to maintain equilibrium or resume its original,
upright position after displacement, as by the sea or strong winds.
This article discusses lateral stability and not directional
stability i.e. tracking, which is discussed in other articles on this
website.
WHY IS LATERAL STABILITY IMPORTANT?
Lateral
stability is a key factor in kayaking and
kayak fishing since it enables prevention of accidents as well as
increases the well being of kayakers and kayak fishermen.
This article explains the basic terms used in kayak design in the
context of stability, and how the patented W kayak offers a degree of
lateral stability previously thought to be unattainable in kayaks.
Before going further the author of this article would like to stress
that in his opinion the idea of relying on the kayaker's skills in
performing the 'Eskimo Roll' as a primary resource in safety terms has
largely failed since the overwhelming majority of people who paddle
kayaks in
recent decades has ignored it, and increasingly so. The reason for this
is that rolling is basically a method of recovery and not a means of
prevention. This explains why most manufacturers and kayakers apply
common sense and prefer to prevent accidents rather than focus on
unreliable recovery techniques.
Primary and Secondary Stability
Primary (Initial) stability refers to what the kayak feels like when
used in flat water - Does the kayak convey a basic sense of ease and
confidence as far as its stability goes?
Secondary stability refers to how easy it is to stabilize and control
the kayak once it's already heeled, or generally speaking in adverse
conditions where it is either constantly and/or suddenly being tilted
on its side - either because of an external force or because of
something the kayaker did.
Both primary and secondary stability are important but while
primary stability relates mainly to how the kayak passengers feels,
secondary stability is what mostly affects their safety and performance in paddling and fishing.
Any further discussion about these terms would be futile without
determining who's inside the boat, since in most cases the passenger
weighs several times more than the kayak itself, and he/she is the key
factor that affects the way the boat reacts to destabilizing forces -
whether external or internal.
Flat water racing kayaks can be as 18" or 19" narrow, while some
fishing kayaks have a beam that's over 40". The first are
designed for use by highly skilled and relatively small kayakers that
can't stabilize such kayaks without keeping their paddle in the water,
while the latter are required to offer good stability mostly to bigger
and less skilled paddlers that occasionally happen to be fighting big
and strong fish, and often stand up in their kayak when paddling and
fishing if they happen to be using W fishing kayaks.
Therefore,
primary
stability has much to do with comfort and secondary stability is what
helps you from getting your kayak overturned in real life conditions
- whether you're surfing with it in five foot waves or struggling to
pull a hundred pound bass onboard.
HOW TO MAXIMIZE KAYAK STABILITY?
1.
The first stabilizing method is not necessarily the most popular one,
and it consists of minimizing the destabilizing effect of the kayaker's
weight on the kayak in traditional (monohull) kayaks, and making use of
this weight and other attributes in W kayaks. In order for
this method
to be effective this weight needs to be applied as low as possible,
preferably much lower than waterline.
In traditional, monohull, sit-in kayaks the designer who wants to apply
this method would try to lower the kayaker's center of gravity (CG) by
designing a deeper hull and placing the kayaker's lowest parts as
closely as possible to the bottom of the kayak.
In this case the designer's efforts will be limited by the fact that
traditional kayaks must have a shallow draft or else they won't offer
sufficient freeboard, and by the modern kayaker's need for a padded
seat, which places him/her at about a couple of inches distance higher
than the hull's lowest point.
This
approach is mostly passive and regards the kayaker as a load having
certain physical properties such as height, width and weight.
Applying this method of stabilization in sit-on-top (SOT) kayaks, which
have gained roughly one third of the kayak market today is not possible because the SOT kayaker must sit several inches above
waterline in order to enable water to drain down from the deck through
the scupper holes, and try to prevent the deck from being often flooded by water coming from below through those holes.
The W kayak is not restricted with issues of freeboard and draft,
and it enables the kayaker to apply his own weight directly to the
lowest point of each hull through his feet, especially in the standing
or riding positions (see user manual)
where the legs carry most of the weight. This stabilizing method
works less effectively in the sitting position, which is also less
effective ergonomically and biomechanically - similarly to the
traditional sitting position in kayaks.
This approach in W kayaks takes into account the kayaker's physical
attributes such as size and weight, as well as his/her physiological
attributes namely his/her natural propensity and obvious capability to
balance himself/herself through the use of the legs, feet etc.
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One Simple Question
For a clearer understanding of this point we recommend that the readers ask themselves the following:
-"Would I consider sitting in the traditional, L kayaking position when
surfing, riding a horse, riding a snowmobile, an all-terrain vehicle
(ATV), a jet ski etc.?"
The correct answer would obviously be "Definitely not!", and this is because all
these sporting activities require active and efficient balancing, which
is best achieved through the use of our legs, and for this purpose the L
kayaking position is among the worst imaginable. |
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Figure 1
This figure shows a cross section of a W Kayak and its 5.5" (14 cm) draft when loaded with a 200 lb (90 kg) passenger.
The red arrows show where the kayaker applies his weight with his feet at the lowest point in each hull's bottom - in this case 5.5 inches below waterline.
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2. The most common solution for increasing kayak
stability is widening its beam, although the wider the kayak the less
efficient paddling it becomes. Very wide kayaks are practically
impossible to paddle for any reasonable distance.
Improving initial lateral stability is
achieved by placing maximum buoyancy as far as possible from the
kayak's longitudinal axis. In monohull kayaks (both regular and 'tunnel' hulled) this is
achieved through a wider beam, but even the widest monohull kayak still has most
of its buoyancy concentrated along its longitudinal axis - as demonstrated in Figure 2:
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Figure 2
This
figure shows a monohull kayak (left) and a new, W kayak (right) of
identical length and width - Both kayaks are viewed from the top.
The
vertical, interrupted lines represent the longitudinal axes of each
kayak, respectively.
The yellow colored areas represent those parts in the kayak that are
sufficiently distant from its longitudinal axis to effectively
contribute buoyancy that may counteract its heeling.
Although the monohull kayak on the left is very wide for
its size these areas still make just a small part of its overall volume.
In contrast, the yellow areas in the W kayak on the right represent
100% of its total buoyancy, and are several times bigger than those in
the traditional kayak.
| In sum, all monohull kayak
designs (SIK, SOT and Tunnel hull) use just a small part of their
buoyancy for effective stabilization, while the W design uses all its
buoyancy for this purpose. |
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Monohull W
This is how the W kayak is capable of offering its unmatched initial stability and some of its legendary secondary stability.
3. Another common solution for increasing lateral
stability is through minimizing the kayak's propensity for rolling and
overturning
by increasing resistance to rotary motion: This can be achieved by
giving the kayak a form that generates resistance from the water
through the need to displace water when the kayak is tilting on its way
to roll. This method is useful mainly in dealing with primary stability.
Figure 3: Comparison Of Three Kayaks' Cross Sections
Kayak A:
The bottom part of this traditional kayak's cross section is round, and such a
kayak would be called 'round bottom' (think of a virtual wheel, or a
barrel). Such kayak offers practically no resistance to rotary
motion, and therefore is particularly unstable.
Kayak B: The bottom
part of this traditional kayak's cross section is angular, and such a kayak would
be described as having 'hard chines'. The chine is the nautical term
for the line where the side and bottom of the hull intersect. Such
kayak would have to displace some water when in lateral rotary motion
and thus offer more resistance than kayak A, and therefore would be
more stable than kayak A.
Kayak C is a W
Kayak: The bottom part of this kayak must displace big quantities of
water when heeling (tilting) and forced into rotary motion, and thus it offers maximal resistance
to rotary forces.
'Tunnel' Hulls
A tunnel hull is a name given to a monohull with usually one 'tunnel' going along its longitudinal axis - from bow to stern. The tunnel is submerged, including its 'ceiling' (top side).
Tunnel hull kayaks are not stabler than other monohull kayaks (I.E.
common SIK and SOT) of similar size and proportions, as will be
explained here.
Tunnel hulls have been in use since the late part of the 1870s, and the concept has
already been implemented and tested in various canoe and kayak designs over the years.
A tunnel hull kayak is another form of monohull kayak - It is not a multihull kayak (see figure 2), so unlike a multihull the tunnel hull does not distribute more buoyancy on its external sides than a
regular monohull does (see figure 2).
In other words, most of the tunneled hull's buoyancy is wasted when it
comes using it to increase lateral stability, which is also the problem in other monohull
designs (E.G. SIK and SOT).
Primary (Initial) and Secondary Stability
It's easy to see that with its sides considerably less buoyant than the
sides of a multihull kayak a tunnel hull kayak cannot possibly be as stable.
Interestingly, the tunnel hull
kayak is less buoyant than the
hull of common monohull kayaks (SIK, SOT). In other words, the tunnel
reduces the kayak's load capacity, which decreases both its primary and
secondary stability.
Primary (Initial) Stability:
If the monohull kayak's tunnel is made deep and wide enough, and its
vertical sides have the right form (see
example in figure 4) they can act as additional 'hard chines' and thus add some
initial
resistance to rotational motion. This is far from being comparable to such effect in a catamaran kayak
because the tunnel's sides are shorter than the boat's overall length
while in a catamaran kayak (E.G. W kayak) the hulls' length is equal to
the boat's overall length.
In stability terms it means that on still, flat water certain tunnel
hulled
kayaks could feel more stable than comparable common monohull kayaks,
that is offer a little more primary
(initial) stability than a traditional SIK or SOT design. However, this
potential advantage is likely not to be perceptible since it would be
offset by the
tunnel hull's deficiency in buoyancy.
Secondary Stability:
A tunnel hull kayak may not provide additional stability for
significant
weight
displacement of its passengers, and it wouldn't be useful in moving
water, waves and other adverse conditions: The secondary stability of a
tunnel hull kayak does not exceed that of a regular monohull kayak of
the same size and proportions, I.E. it's considerably less stable than
a multihull kayak.
Figure 4: Cross Sections of Regular and Tunnel Monohulls
Regular Mono Hull Tunnel Mono Hull
Ergonomics as a stability factor
In
a tunnel hull kayak the paddler or fisherman sits with their legs
stretched forward and the trunk only a few inches higher than the
ankles. This position hardly differs from the notoriously non
ergonomic L kayaking position, and therefore hardly offers any improvement as
far as the ability to use the legs for balancing, control and power
generation while it still forces the passenger to rely on a back rest for support,
consequently causing fatigue and discomfort, which are additional
disbalancing factors.
What can a tunnel really do to a kayak?
Incorporating a tunnel in a monohull can be an effective means for improving tracking as the tunnel enables water to flow in a
straight line (I.E. not deflected or 'curved') along the hull, in parallel to the
direction of the boat.
This can be helpful in very wide monohull canoes and kayaks (E.G. fishing kayaks) that track poorly.
Similarly to a rudder, the tunnel has a negative effect on speed.
In motorized boats the tunnel can help the hull plane but this is
irrelevant in low speed boats, especially human powered ones such as
canoes and kayaks, which are the slowest.
'What if' - a quick reality check
Introducing a tunnel in a monohull kayak places the passengers higher
than in a regular monohull kayak without having them benefit either
from significant increase in stability or significant improvement
in their paddling or fishing position.
If the tunnel hull kayak design offered any real advantage in terms of stability
it would enable producing narrower (I.E. faster) yet stabler monohull canoes and
kayaks. Since in reality the tunnel does not produce such effect the various tunnel hull canoes,
kayaks and hybrids are among the widest designs on the market.
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Food For Thought
Well, it's more of a snack really, but the
following anecdote may shed some light on this subject from a different
angle - that of 'marketing hype':
The tunnel hull design for small, paddle powered watercraft has gained
some new life in recent years with one company that promotes it quite energetically.
We found that company's website describing the tunnel hull as being 'extraordinarily stable for a single hull boat', while the same website claims that another small watercraft that company offers 'incorporates a V hull design to provide stability...'
It doesn't take a boat designer to realize intuitively that a kayak
hull whose cross section is shaped like a deep V is in fact unstable,
and the only reason one would incorporate such a form into a hull
design is to try and improve its tracking capability.
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CONCLUSION
In comparison, the W kayak design offers both increased initial and secondary
stability
as well as improved ergonomics resulting in Hyper Stability: The ability to perform things that are impossible with any other
form of kayak, and an overall better user experience than that offered
by any other kayak, including the widest and most stable ones. Such Hyper Stability is currently achieved with a hull that's only 25" wide, which is the width of some fast sea kayaks.
Hyper Stability: Demonstration of Principles
-Click to watch video
The W technology is protected by U.S. utility patent #6871608, and it is patent pending in Canada.
Please feel free to contact us if you need more information - Email: WaveWalk or call 1-617-916-2250
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