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US Patent 6871608

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DESIGN FOR BETTER KAYAK STABILITY: WHY AND HOW

CONTENT

What is Stability
How to Maximize Stability Outriggers
The Importance of Lateral Stability
Tunnel Hulls
Primary (Initial) and Secondary Stability
Conclusion

The purpose of this article is to elucidate the general principles of kayak stability, and to explain what enabled us create the world's most stable kayak for fishing and paddling, based on the invention described in US utility patent number 6871608 entitled 'Twin Hull Personal Watercraft'.



Before going further, we'd like to recommend you watch this short demo video:






WHAT IS STABILITY?


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 SO 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 (INITIAL) 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 on board.


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 free board, 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 free board 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.

One Simple Question to Ask Yourself


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. 

How kayaks stability is improved by hard chines
Figure 1

This figure shows a cross section of a W500 Kayak and its 4" (10 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 4 inches below waterline.



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 shown in Figure 2:

Figure 2 compared lateral stability in two fishing kayaks
       Monohull        W500 Twinhull

Figure 2


This figure shows a monohull kayak (left) and a new, W500 kayak (right) of identical length and width - Both kayaks are viewed from the bottom.
The vertical, interrupted lines represent the center line of each of the two kayak forms.

The white colored areas represent those buoyant parts in the kayak that are sufficiently distant from its longitudinal axis to effectively contribute to its stability.
Although the monohull kayak on the left is wide for its length, the white areas in it still make just a small part of its overall volume.
In contrast, the white areas in the W kayak on the right represent 100% of its total volume, and they are several times bigger than the white areas 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.

This is how the W kayak is capable of offering its unrivaled 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

Comparison of chines in 3 fishing kayak models
A.                                          B.                                         C.
                         
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

Stability: tunnel hull vs. regular monohull
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 dis-balancing 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.




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, and kayak outfitted with outriggers.  Such Hyper Stability is currently achieved with a hull that's only 29" wide, which is the width of some touring kayaks.




Appendix A

Food For Thought About Truth In Advertising


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.
 



Appendix B

More Food For Thought About Truth In Advertising


About Add-On Kayak Outriggers and Integrated Outriggers

Advertisers often cross the line between fact and fiction, and sometimes they cross the line between true and false.

For example, the owner of a company offering a fishing kayak featuring folding outriggers at its rear end has claimed on a promotional video that his product (quote) : "...offers the equivalence in stability of an eight foot wide boat"  (I.E. it's as stable as a bass boat...)
Such claim is so disconnected from reality that anyone can easily understand that it's false, of course.
But generally, crafty marketing hype is everywhere, and too many people are fall for it, and them fall out of their kayak, unfortunately.

What's the real effect of small outriggers attached to the kayak's stern?

Generally, small outriggers offer some initial stability but too little secondary stability. This means they give the kayak user some sense of security on flat water and when the kayak is not tilting on its side, but this sense of security is misleading, since a small outrigger is not sufficiently buoyant to support the heavy weight applied on it when the kayak is tilted on its side, and therefore such outrigger may not prevent an accident.
Practically speaking, as soon as the kayak user seriously loses balance, for example in case they're attempting to stand up, or if the kayak is hit by a motorboat's wake, they cannot rely on extra lateral stability that's enough to prevent their kayak from tilting further. This problem is particularly acute in SOT kayaks, since their user is already seated or attempting to stand on a deck that's several inches above waterline, and therefore they're insecure and less stable to begin with.

The folding outriggers in that fallacious promotional video are integrated with the hull itself, and therefore offer less stability than outriggers added on the kayak's sides.
Why is that? -
The reason for the poor performance offered by outriggers integrated with the hull's rear end (stern) is that the buoyancy they add on the sides of the stern is in fact taken away from the stern itself:  When the outriggers are deployed sideways the stern splits in two, and its two halves are repositioned outwardly, so the kayak no longer features a proper stern. 
This means the user benefits from no additional support whatsoever when they lean forward (hours 10 to 2), or sideways (hours 8 to 10 and 2 to 4), or if they lean backward (hours 5 to 7). The only gain in (initial) stability is in case the user happens to lean in the angles in which the outriggers are deployed (hours 4 to 5 and 7 to 8).
Bottom line: The gain in initial stability when the outriggers are deployed is in a range of angles that add up to about 1/4 of the total circle (just about 4 hours out of 12) around the kayak user.
This almost insignificant advantage is offset by the fact that it is impractical to paddle that kayak when its outriggers are deployed, since the amount of drag they add is considerable, and the main hull in this position loses most of the hydrodynamic advantage offered by its initial length (I.E. hull speed). The combination of these two negative factors is critical to the kayak's speed, or lack thereof. It is probably the slowest and most difficult fishing kayak to paddle, and this is no small feat in a category of kayaks generally known as 'barges'... Interestingly, since the little stability added to this kayak also makes it so hard to paddle, anglers are effectively prevented from using it for sight fishing and fly fishing in case they intend to cover any significant distance.

The vertical metal frame... -
Some SOT fishing kayaks, including the one discussed in the previous paragraph feature a tall vertical metal frame, supposedly in order to provide some support for the angler who's attempting to fish standing up there.
Practically, that vertical metal frame adds leverage to the angler's weight if they lean on it or grab it, which makes the kayak tilt further in case the angler loses balance and the kayak is tilting sideways.

Again, the only fishing kayak that really offers fishermen to stand up and fish with 100% confidence and safety is the Wavewalk, as anyone can see by watching the movies on this page.





More articles on the subject of kayak stability:




Children kayak fishing User Manual: Launching, paddling, fishing, recovery


This Invention Is Protected By U.S. Utility Patent Number 6871608

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