Know Your Kayak Under the Water (part 2): Stability Applications

There are many considerations between practicality, safety and desire when we shop for a kayak.  We may look for features that empower us for a realm of high adventure, or opt for a more modest craft to spend a few blissful hours in a tranquil paradise.  Whatever we choose should have features that reach a balance between our aspirations, skills and confidence on the water.  Unfortunately, there is no high tech miracle that will deliver all these things for every paddler.  But instead a game of give and take that forces us to sacrifice coveted qualities we desire for the performance we want.  Hull design is all about tradeoffs.  But kayak designers are using some innovative techniques to seemingly cheat the laws of physics.   In part 1 of this series, we examined the merits of primary and secondary stability and learned the importance of selecting the proper measurement of each to cultivate our skills and piece of mind while considering the consequences of the trade-offs for our choices.  In this article we will examine some real kayaks and identify performance characteristics from their hull features and uncover some tricks designers are using to deliver performance while minimizing sacrificial tradeoffs.

The first kayak we will analyze is a popular recreation kayak targeting novice and casual paddlers with a bit more prowess to take them a bit beyond novice conditions: The Tsunami from Wilderness Systems is the choice of a wide range of paddlers from the very novice to intermediate and delivers a surprising performance when pressed.

A kayak that fills this role must feel comfortable and secure to paddlers whom have never paddled a kayak, and those engaged in a secondary activity like birding or fishing.  So a high degree of primary stability is required.  In exchange for this up-front stability, a substantial wetted surface must be deployed underwater to provide an adequate angle of support for a comfortable stable feel (see Fig A).   The downside to providing this comfortable stability is the substantial drag from the broader wetted hull surface in addition to instability in rough conditions from the primary stability attempting to right itself on the slope of waves.  But it is a designer's job to cheat the laws of physics anyway they can to reclaim performance.  And the designers at Wilderness Systems had a few tricks up their sleeves.

   As we saw in part 1, the theatre of battle between the forces of stability and instability plays out in the form of rotation about the longitudinal axis spaning the length of the kayak.  To remain stable, the kayak must apply righting forces in the form of an opposing torque to this axis to counter the rotational destabilizing forces much like a wrench applies torque on a bolt.  As we know, a longer wrench shaft will apply more torque on the axis.  Moving the righting force away from that axis will allow the hull's beam to be used as leverage to magnify the forces of the primary and secondary stability as illustrated in figure B.  But designers pay a high premium in wetted surface drag if they extend primary stability to the entire width of the hull.  Figure A illustrates a cross-section at the center of the hull's length where the always deployed primary stability support is consuming wetted surface (WS) area.  However, secondary stability is much less costly to the drag of the kayak as it resides undeployed at and above the waterline.  So the kayak's streamline qualities will benefit most from this leverage if secondary stability resides at the furthest distance from the center axis.  As we can see from the figures above, the designers at Wilderness Systems took a bite out of the primary stability area and lowered the secondary stability to quickly deploy when the kayak leans, taking over at the point where the center of gravity pushes the primary stability to the point of capitulation (see figure B).  We can also see the wetspace drag is reduced from this design as the wetted area is reduced.   The handoff to secondary stability will also lend more stability in waves as the destabilizing effects from primary stability are reduced.  But one drawback to locating the secondary stability this low to the water is a jump in the amount of wetted surface drag when a heavy payload makes the kayak sit lower in the water as secondary stability  sitting passively above the waterline is deployed prematurely to bolster buoyancy.

Task of lip changes to rear flotation

As noted above, righting leverage is greatest at the widest point of the kayak, which in the Tsunami is located at the mid point in the hull's length.  So all of the stabilizing magic must take place at the mid point in the length where the hull is widest.  The rest of the hull's length will play little to no part in the stability at all since the leverage possible away from the widest beam is minuscule.  Therefore the totality of the hull fore and aft of the middle is better utilized for other tasks like tracking, decreasing water drag, and providing lift above steep waves.  So the protruding lip that provides secondary stability at the widest point serves a very different purpose of providing buoyancy at he bow and stern to lift them over steep waves and prevent the ends from perling.   This lip  fore and aft also keeps water from splashing on the paddler as waves hit the kayak.  Also notice how the the designers reduced the wetted area fore and aft of the middle.  The designers also added a dome area atop the ends to increase the buoyancy of the ends to reduce the tendency of periling into the waves.  The pointed tops allow the ends to cut to the surface of the wave quickly if they perl without shoveling the water.  These robust design measures at the ends is needed to overcome the lack of rocker the designers sacrificed to put more of the hull's waterline length to work in the water.  As we see later, a rocker design is for waves beyond the targeted market for this kayak, so the designers properly passed on a rocker design.   But they saw the need to bolster the ends to provide a capacity for waves, and this is one of the surprise competencies of the Tsunami.

The Gemini from Valley is an entirely different kayak designed for paddlers with a more advanced skill set.  As such, the designers opted to create a hull at the other end of the tradeoff spectrum to provide more performance and less initial stability, delegating the task of stability to the paddler's skill set.   However, the laws of physics stood directly in the path of their objectives.  They wanted to design a kayak nimble enough to play in the surf and be competent for long distance expeditions.  However, these two objectives put the designers at opposite ends of some significant tradeoffs of the hull design.  A single solution was not possible as these two objectives are irreconcilable without severely diluting their desired specialized performance.  So the designers decided to start from a common base design and spin off two distinct kayaks: the Gemini SP for surf play and the Gemini ST for for sport touring.  For the benefit of our discussion, we will examine the design of both of these kayaks in broader detail to understand the choices the designers faced and the implications on the stability of both kayaks.

Gemini SP underside with peeked keel and sidecut

The Gemini SP by Valley is a surf zone play boat, designed to be nimble in the surf and turn effectively when put on edge.  It is not designed for a comfortable ride over long straight distances for hours on end.  It will smash through opposing waves and surf high atop their crest.  In a surf environment,  primary stability is not needed or desired as we know primary stability will erroneously attempt to right the kayak sideways on sloped surfaces, which is never a good thing.   However, secondary stability is much more desired since it carries a delayed reaction and deploys much deeper into the lean, so a wave will pass before secondary stability can attempt to right the kayak on a slope.  Secondary stability will also protect the paddler form a capsize while edging the kayak and leaning into a wave while side-surfing. Tracking is not as important as turning for a surf zone play boat since it must react quickly and need not hold a straight course for very long.  So the designers created a short 14' 10" (452 cm) kayak with a lot of rocker to turn when on edge and stay above the waves with an upward orientated bow and stern.  However, the tradeoff to this rocker design does not allow the load to be dispersed over the length of the hull, resulting in a hull that concentrates the load at the cockpit.  A necessary sacrifice for the the high degree of coveted rocker.   Normally, the laws of physics would be unkind to such a design as the sagging cockpit would plow the water causing significant drag.   But the designers at Valley were not ready to give up on the kayak's prowess on smoother water.  After all, the goal was to create two similar kayaks for different purposes with similar characteristics.  To make the Gemini SP snappy as well as nimble, they needed to streamline the wetted surface beneath the cockpit to reduce drag.  And the only way to do this was to add buoyancy at the keel with a steep peaked bottom to reduce wetted surface by boosting the kayak a little higher from the keel.   To further reduce the wetted surface area the designers gave it hard chines with a cut-out similar to what we saw in the Tsunami (visible in the picture below).  The picture below also shows a benefit in the substantial amount of secondary stability in reserve above the waterline.  The tradeoff for all this is a reduced primary stability which is not desired in a surf playboat, resulting in an initially unstable feeling kayak that novices would find unsettling, but a high performer for its playground in the surf.

Gemini SP rides high with its rocker and ample sec stability

With less wetted surface the Gemini SP shows surprising speed for this type of kayak.  I was surprised one day on the lake when a friend in his Gemini SP was able to keep up with my Epic 18x on a casual cruise on a calm lake.  Claims that Valley highly touts in their promotional material.  

As a touring kayak, the Gemini ST sports tourer is designed for covering distances over calmer waters and provide the paddler with a more comfortable experience on the water over a longer span of time.  It is the same length of its twin the SP.   A touring kayak must be more efficient and minimize drag.   Given its very different mission, the ST has much less rocker, letting it disperse its load over the length of the kayak so it rides higher with less wetted surface drag.  The tradeoff is a less nimble kayak that does not edge as well and tends to perl into steep oncoming waves.   Unlike its twin, the ST does not need hard chines or a high peeked keel.  For its mission, the designers have given it softer chines with a flatter, low peaked bottom for more primary stability, but not too much, but allows the paddler to take a break, fish, shoot pictures, or relax without the unstable feeling of its twin the SP.   But the designers at Valley similarly did not want to give up on the nimbleness of the ST.  Without the high peeked bottom and the large cut out of the side, the designers had the luxury to bring down the sides of the hull closer to the water for a faster, more responsive secondary stability with a small cut for efficiency.  These curved sides will also lend some nimbleness to this rocker-less design when edging by putting a curve on the water (see part 3 of this series).  But the lack of rocker leaves the ST more susceptible to perl into sharp waves.  Often, manufacturers will compensate by adding more buoyancy to the bow and stern as we see in the SP.  But unlike its twin, the designers  remained true to their objective and sacrificed the surf readiness flotation volume at the ends for reduced drag and the efficiency of a more streamlined design.

So despite the very diverse performance objectives of the Valley Gemini designers, they created two kayaks rather than one to tackle an impossible spectrum of kayak performance goals in a truly unique way.  The complexity of these solutions underscores the value of hull design knowledge so we are able to understand and make intelligent choices from the abundance of sophisticated technology available.  Practically, we can only test a few kayaks on the water in far from ideal conditions.  We have seen how designers make significant tradeoffs to obtain their performance objectives.  But the motive that drives many kayak designers is to create a kayak that will fetch broad appeal so the company can monetize a successful product.  For other designers, its a labor of love they hope to monetize.  But the desires of a paddler lends purpose to a kayak as a tool leveraged to seek a path to their bliss and dreams.  Ideally, the paddler will seek the the empowering technology they need, grow into its characteristics, and find confidence to carry on to the next level.  A tall task for products of broad appeal.  But as paddlers we have choices and the ability to obtain knowledge of the science that goes into these more specialized and capable craft.  As for any endeavor no matter the discipline, the right tool is needed for the task.

In part 3 we will examine elements of hull design related to tracking and edging then dive into the hydrodynamics of skegs and directional hull features.


Copyright 2013 Lyman Copps

Know Your Kayak Under the Water (part 1)

As kayakers, we rely on our boats to impose our will on the water, exhorting pure human power against the wind, tides, and currents.  Beyond our own endurance, we have our kayaks and their carefully designed characteristics to safely and efficiently ferry us to our destination.  However, most paddlers when considering a kayak acquisition look above the waterline when over 90% of its vital characteristics lie below the waterline.  Recently, I looked through manufacturer promotional material for several kayaks.  I found happy paddlers in emotionally provocative colorful pictures as one could imagine with a detailed list of above waterline features.  But found little to nothing substantive about the all important hull design.  Sadly, most paddlers do not understand the design features of their hull and its intricacies.  Above all, the hull is the very essence of a kayak's designed performance.  As individuals that kayak, we have different demands as diverse as the seasons.  And selecting a kayak compatible with our skills and needs is very important.  If one design was perfect for everyone, all kayaks would look alike, and we would not have hundreds of models to choose from.  But hull design is all about trade-offs.  Features that deliver the performance a paddler desires or needs will often require a sacrifice in another area.  Despite how instrumental your kayak's hull is to its performance, precious little is has written about it, leaving kayakers in the dark on exactly how and why their hulls perform as they do, and what to look for in a hull shape when considering a kayak purchase.  In this series of articles I will bring to light the deep dark secrets of hull design in simple terms.  We will examine facets of stability.  Explore hull shapes and features below and above the water line that affect stability and in later articles examine hull characteristics of speed and efficiency for moving through the water.  But first we will establish a premise for our examination of hull designs with some basic physical principles to help us dissect hull shape features.

Of primary importance to our endeavors on the water is stability.   In nearly all watercraft, we look to the design of the hull for stability and must sacrifice streamline efficiency to have it.   However, a kayak will permit the task of stability to be delegated to the skills of the paddler, allowing craft stability to be exchanged for a more streamline performance with lower resistance.  But unless your primary task is powering the craft while providing stability every moment you are on the water, this delegated task may not be willingly accepted by many.   Bird watchers, fishermen, and paddlers out for a relaxing day on the water may desire a kayak that provides a high degree of hull stability.  But at what cost?  And why the tradeoff?

First we will look at what stability actually is.  Our kayaks move and twist on the 2D plane of the water rotating around 2 axes.  Since sea kayaks are long and stand little chance of flipping end over end, lateral rotation is of little consequence. So our only concern is its rotation about its longitudinal axis running the length of the kayak.  When we lean left or right we are applying torque on our kayak to spin about this axis.  As we float upon the water, the weight of our kayak and all its contents is pressed upon the water with a downward force and held in check with an upward opposing buoyancy or (weight displacement force).  If the kayaker is properly centered in the kayak, the center of gravity will go straight down through the axis.  In reaction, the opposing center of buoyancy will move straight through the axis in the upward direction to keep the kayak and its contents in check.  Since these forces pass straight through the axis there is no torque being applied, thus no rotation about it.

If the paddler leans to one side, the center of gravity will move away from the axis and impose a torque upon it.  At this point, the designed features of the hull come in to play to react with an opposing righting force by adding more dry hull volume (floatation) in the water on the side of the lean thus imposing an opposing torque by moving its center of buoyancy off-center in the direction of the lean.   Since the weight of the kayak cannot change, to add dry volume on one side of center requires the kayak to reduce wetted volume on the opposite side.   Buoyancy on the side opposite to the lean is also reduced which helps the center of buoyancy migrate in the direction of the lean.  However, when the kayak runs out of dry volume to put in the water, it can no longer move the center of buoyancy to match the center of gravity.  At that point, an unopposed torque will be applied to the kayak hull and it will capsize.   This is the point of capitulation.

So what can we deduce from these physical facts?  First: a kayak hull has only has a fixed amount of stabilization reserves.  If they are spent early providing primary stability, we can expect the kayak hull to capitulate earlier.  Also, as a long wrench with more leverage can apply more torque  than a shorter wrench, a wider kayak will have more leverage to apply more counteracting torque against a leaning torque.  But widening the beam will dramatically sacrifice speed and increase water drag when the kayak moves.  A tradeoff that must be considered wisely.

 So what are these features that work for us?  What does a featureless hull look like?  Lets examine a featureless hull which is simply a floating cylinder.  Since it is round and featureless, its center of buoyancy will always be in the center and cannot move to either side.  Its perfectly round shape does not allow any more volume to be added to one side or taken from another.   It is the same on both sides all the time.  Consequently, any offset in the center of gravity will generate torque on the cylinder, opposed only by the small forces of the cylinder's inertia, and friction of the water.  Picture yourself standing on a perfectly round floating tree trunk.

Since have a stability budget, how do we spend it?   If you fish or birdwatch, and paddling is your secondary purpose, or you just want a stable, secure experience in calm conditions, you may want to spend a good part of your stability budget on primary stability.  Primary stability is the instantaneous ability of the craft to apply a righting force to a leaning motion.  Kayaks with high primary stability feel stable initially as any leaning is met with an instantaneous counterforce.   In order to accomplish this, primary stability must be located in the wetted volume of the hull.  High primary stability hulls will have a flattened bottom with possibly a slight "V" or gentle rounded shape.  As such, the hull size below the waterline is larger and drag from water friction is rather high, affecting performance.  Since much of the stability budget is spent on this primary stability, there is less of a secondary stability reaction.  But high primary stability will require more leverage, thus a larger stability budget which must be bought by widening the beam (width) so the hull can achieve enough righting torque on the axis with a longer lever (remember the wrench).  Typically, high primary stability kayaks are wide and short as they do not need an excessive waterline for a kayak that is not designed for blazing speed or cover a lot of distance.  But they are a lot of fun, very practical in rivers and small lakes, swamps, and estuaries and highly maneuverable.  But, a high primary stability exposes the kayak to a serious side effect.  In our theoretical illustration above, we observed the mechanism of stability as a function of the kayak's flotation and the water surface.  We know the kayak will attempt to bring itself level to the surface of the water.  But the surface of the water is often not level (the slope of a wave).  So a kayak with high initial stability can right itself sideways to a small degree; enough to introduce considerable instability in rough water, requiring mitigation with bracing skills from the paddler.  But, for paddlers who rarely venture into rough waters and have no desire to travel far or fast, a primary stability kayak will be a fine investment for a leisurely enjoyable ride.  Performance paddlers will find themselves fighting a sharply increasing  drag as they ramp up speed.  The increase in speed will hit a wall as the kayak reaches its maximum hull speed (explained in a later article).

A kayak facing rough seas will need to minimize the instability side effect from its primary stability, and reserve its stability budget for secondary stability.  Unlike primary stability, secondary stability will not respond instantaneously but apply stability further into the lean.  Secondary stability also exhibits less of the destabilizing behavior in waves since the hull will not react until much further into the lean.  Unlike primary stability, secondary stability assets are in the dry volume of the hull above the waterline.  In the first illustration above, notice how the "V" concentrates most of the flotation in the center, while the flotation at the extremities is pushed out of the water into the dry area of the hull.  This is the secondary stability area in reserve.  Since the dominate flotation force is in the center, the kayak will pivot about it and feel initially unstable until the secondary stability is deployed.  In the second illustration, when the kayak rotates about its axis, dry volume is deployed into the water bolstering flotation at the edge of the kayak, which in turn moves the center of buoyancy to counteract the leaning force.  Since secondary stability assets are stored above the waterline, these kayaks  enjoy an added advantage of a more streamlined hull with much less wetted hull surface resulting in far less drag from water friction when the secondary stability is not deployed.  Secondary stability kayaks cater to more advanced paddlers seeking performance.  In many models, manufacturers will further narrow the beam (width) considerably stripping much of its righting force leverage.  And by this action, delegate much of task of stability to the bracing skills of the paddler in exchange for a considerable increase in performance.  Manufactures may also choose a more rounded hull without a "V".  But the stability principles are the same with more rounded surfaces offering less primary and more secondary stability, with flatter rounded bottoms offering a higher degree of primary stability.  Novice paddlers will find secondary stability kayaks deceptively unstable and unsettling.  With a much more narrow beam, these kayaks will have a much smaller stability budget, but will store most of this tighter stability in reserve for a time when it is really needed.

To illustrate primary stability and secondary stability I presented two mutually exclusive theoretical kayaks.  But in reality, no kayak will have all of one and none of the other.  All hundred or so kayak models will fall somewhere in between catering to many skill levels and a wide range of venues and conditions.  When a paddler chooses where they want to spend their stability budget, they should deliberate long and hard to find the kayak that best suits their needs in the near term and longer term.  Also consider where you are going to paddle and where you want to paddle.  They must also assess their skills and allow room for improvement.  A kayak designed for calm conditions can also perform well in challenging conditions if used with proper skills.  When I purchase a kayak, I am initially a little unstable and grow into its characteristics as my skills improve.   Paddlers for whom the kayak is a vehicle for another purpose or activity may want a lot of primary stability so they can focus on their secondary activity.   Kayakers wanting performance with the intention of piling up a lot of distance will want a performance kayak with a low drag.  Paddling a considerable distance with a higher drag hull can feel like towing a second boat.  A day on the water with a prospective kayak is better than a short test paddle.  When shopping for a kayak, try a lot boats.  You may just fall in love or learn a little more about who your are on the water.

In the next article of this series, we will apply some of our new found knowledge to examine the stability characteristics of a number of actual hull shapes.

Sources:
http://www.rcwarships.com/rcwarships/nwc/stability.html

Copyright 2012 Lyman A Copps