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