The successful design of any boat is a subtle blend of art and science. While there is a lot of room for personal preference and choice, there is also much that is constrained by the immutable laws of physics. The artistic choices are in determining the desired performance and then shaping the boat such that it interacts with the hard-and-fast rules of nature to achieve those goals.
While there are obvious differences between a super-tanker crossing the Atlantic and a kayak bobbing on a bay, the water and gravity effecting the two boats are exactly the same. The science developed for large ships is precisely the same as that which relates to small boats like a kayak or row boat. The difference is the design goals of the kayak designer vs those of the ship designer. While a tanker designer needs to worry about carrying a heavy load across an ocean and into defined spaces like the Panama canal, a small boat designer may want a kayak that carves down a wave or is stable for fishing.
The reason a tanker looks different from a kayak is not due to some difference in the way physics applies to the two boats, but the fact that their intended purposes are different. The task of designing any boat should first start with determining that intended purpose. The same applies when choosing an existing design for your own use.
Even if you are not designing your own boat, you still need to decide what you want out of your boat before choosing a design.
A sunny summer day on a small lake or pond just cries out for a small boat. A recreational kayak can be just the thing. Intended to be small and easy to handle, not requiring much skill, comfortable and stable, recreational kayaks provide the ideal craft for poking around, watching wildlife, or just floating care-free on a mirror smooth pond.
Recreational kayaks tend to be short because a short waterline helps make the boat more maneuverable which is nice for poking into little coves and streams. Short boat are also easier to transport - they may even fit on the back of a pickup truck - and can be made lighter weight than longer kayaks. These boats tend to be relatively wide. This is in part due to the fact that since they are short, they need extra width to give them buoyancy. However, wider boat tend to be more stable, and the added width helps give more room for comfortable seating.
The trade-off with a short, wide boat is they are not generally as "sea-worthy" as longer narrower boats. If you get caught out in rough conditions you may find a recreational kayak hard to handle and get back to shore safely. They generally are not equipped with the safety features of other kayaks.
While sea kayaks were originally designed for use on the ocean, they are at home on any large body of water or in situations where you want move efficiently for a long distance. Sea kayaks are the nearest ancestor of the original kayaks created by the Inuit and Aleut peoples of the Arctic. These original kayaks were often quite narrow and fairly long. This help them move efficiently through the water even when the wind blows and the waves get large. This makes them well suited for any body of water where the conditions may get rough or if you just want to be able to go a long distance with a minimum of effort.
Sea kayaks are typically designed with a cockpit small enough to accept a spray skirt. This helps keep water out of the cockpit. They also often are equipped with bulkheads that provide additional safety and a dry place for storing lunch or camping gear.
Due to their length sea kayaks are typically heavier than recreational kayak. The length also makes them less maneuverable for investigating small coves and streams. They will not be as responsive as a whitewater boat when attempting to navigate fast moving water.
Traditionally kayaks were solo boats, but a tandem or double kayak is a great way for two paddlers to get out on the water together. Working together in one boat two people can typically go faster than each would go alone. This especially useful if one paddler is less experienced or not as strong a paddler. Instead of being left behind by the stronger paddler, they both end up going faster.
Multi person kayaks can either have one large cockpit or a separate cockpit for each paddler. A single large cockpit can be handy if you want to paddle the boat solo as it lets you position yourself in the middle to balance the boat. The separate cockpits allow the use of individual sprayskirts and generally just do a better job of keeping water out of the boat.
Tandems or kayaks designed for more than one paddler tend to be longer and wider than solo boats. This accommodates the need to carry more weight. The added width also provides more stability which is nice when two individuals are each doing their own thing.
The distance between the seats will effect how easy the boat is to paddle. With the seats close together, if one paddler stops paddling the other may end up whacking into their paddle. Increasing the distance will minimize this problem. This lets each paddler move at their own rate. However, it should be noted that the boat will move a lot easier if everyone paddles in sync with each other.
There are a lot of different kinds of racing kayaks, but almost all have the characteristic of being fast. There are several things that contribute to speed. You will often here about "hull speed" in relation to boat speed. This term is a misnomer in that it implies that there is a speed a hull will go. What it really indicates is the speed at which the hull starts to become rapidly more inefficient.
What hull speed suggests is that longer boats can go faster than shorter boats. This does not mean that they are automatically faster, just that a longer boat starts losing efficiency at a higher speed than a short boat.
The other big factor in speed is "wetted surface area" or how much surface area is in the water. This is important because as you move through the water the water must slide across the surface of the boat. The more surface there is to slide against, the more friction there will be and the slower the boat will go.
The easiest way to reduce the wetted surface area is to make the boat narrower. As a result, there are two primary options for making a racing boat fast: make it long and make it narrow. Therefore, most racing kayaks you see will be as long and narrow as they can get away with. There are typically constraints on these factors. For example, if you make a boat too narrow, it can be so unstable as to be impossible to keep upright. The other big constraint is racing rules.
Most race organizers will set maximum lengths and minimum widths in an effort keep to their events as fair as possible. Their goal is to make the race a test between individual paddlers, not a test of the fastest boat. However, it is the kayak designer's task to try to develop the fastest boat permitted within the rules.
This battle between race organizers and boat designers can create some funny looking boats, but the most common solution creates a boat with a plumb bow and stern. In this way, the boat can get as long a waterline as possible within the prescribed overall length. You will also often see a fair amount of "flare" in the cross sectional shape of the boat. This permits a narrow waterline beam with a wider overall beam that meets the rule specifications.
Neither of the these solution necessarily result in the best boat possible for the purpose if there were no design constraints imposed, but they can make the boat quite fast within those constraints.
The other consideration in a racing boat is stroke mechanics. Despite kayak designers best efforts to make a fast boat, in the end there are really only fast motors. In order to go fast, the person paddling a kayak needs to be strong and have good technique. The design of the boat will often include features that help the motor maintain a good and strong paddle stroke.
The boat should not get in the way of the stroke, and the cockpit should let the paddler move as needed to paddle with full power. This often means the boat is quite narrower in front of the cockpit for a clean start of the stroke. The cockpit may be long to permit the paddlers legs to move.
How fast is your boat? Do you know? Does the question even make sense? Isn't any boat going to go faster if you apply more power to making it move? What people really should ask is" "how efficient is your boat?"
What efficiency means is that for a certain amount of energy applied to making the boat move, you will get a certain amount of motion. A more efficient boat will move more with less effort. This could mean that for a given speed a more efficient boat will require less effort, or for a given effort, a more efficient boat will move faster.
Efficiency is typically measured as "drag". Drag refers to how much slowing force is created by the boat while moving through the water. More drag means that the boat will be forced to slow down more rapidly. A boat with less drag will go faster for a given amount of effort.
There are two main sources of drag on a boat. The one people with a little knowledge of boat tend to think of first is wake drag. This is a force created by the effort required to make the waves in the wake of a kayak. Most people are familiar with this idea through the term hull speed
These plans are product of Nick's years of boating experience and extensive research into what makes boats work. Each design starts out as an idea spawned on the water. Time out in boats will suggest how a boat could be tailored to perform better in specific conditions. The idea may be for something that works better poking into a salt marsh or for dropping down the wave face of hurricane surf. Inspirations for solutions come from traditional Inuit and Aleut kayaks as well as more contemporary powerboats and sailboats.
After an concept is born Nick will sketch up some ideas on paper, attempting to get the basic shape developed. From there the sketched ideas will be transferred to the computer and a naval architecture CAD package. CAD does not design the boat, it is just a fancy tool for drawing and analyzing the result. The design is initially roughed out and analyzed for basic hydrostatics, i.e. how and where it floats. The shape is then tweaked as needed and reanalyzed. This process is repeated as long as necessary until the basic concept appears workable.
Then the design is further analyzed for more complicated hydrostatics such as stability and the various form coefficients such as the prismatic. The effort here is to bring these performance related parameters into line with the initial design goals of the desired boat. This iteration continues with needed adjustment and further analysis until Nick is satisfied that the boat should perform as desired.
Then starts a process of further hydrodynamic analysis and alterations looking a factors such as drag and how the boat reacts in waves. This may involve some significant changes to the boat which will require returning to some of the earlier iteration processes to keep the design focussed on the design goal.
Finally the design will be refined for aesthetics and to assure the boat is buildable using the intended method. With stitch-and-glue design this means making sure the plywood panels will bend into the intended shape. With strip-built designs, which can almost always be built, the goal is to assure that the shape is not unreasonably difficult to build. If this requires major changes it may mandate a return to earlier steps in the process.
All of these tasks often take Nick a year or more. The whole process is fed by constantly returning to the water and using different boats in a wide variety of conditions to see how they respond. If the newly built boat does not handle well Nick does not release the plans. If the design works well final drawings are prepared and the design is made available to boat builders. Testing of the design continues for as long Nick owns the boat.
It typically takes a full year to really get a full understanding of how a boat handles. This gives the opportunity to test it in a wide variety of conditions. Nick paddles his boats year around along the coast of New England, putting over 1,000 miles on most new designs in the course of its first year. If any issues come to light in the design, adjustments will be incorporated. And during the year new ideas for new boats optimized for different conditions will start to percolate leading to new boats in the future.
One of the most important aspects of kayaking comfortably and easily is your paddle. An efficient kayak is of no use if the paddle you use is inefficient, or if you use your otherwise efficient paddle in an inefficient manner. A good paddle can make the worst boat seem better and good technique will get the most advantage out of the worst paddle.
The goal of the paddle is to push against the water so that the boat goes forward. This rather simple principle is not as simple as it seems because the water is a fluid that doesn't want to be pushed against. The act of trying to push against water tends to make the water move away from the force. And again, the goal is to make the boat move forward, making the water move backwards is a waste of effort.
Unfortunately, it is the nature of the physical world that for every action there is an equal and opposite reaction. In other words, the act of pushing a boat forward will produce the inevitable reaction of pushing something backwards. In this way the law of conservation of momentum is kept happy. Even if you put you paddle all the way down to the bottom and push off mother earth, you will actually cause the earth to rotate infinitesimally away from you. Luckily, it is unlikely that everyone on earth will paddle in the same direction simultaneously
There are several standard ways for boats to be propelled through the water. The old fashioned paddle wheel rotates like a tire and just pushed the water straight back, thus making boat go forward. The propeller works in a different way. It spins such that the blades rotate at right angles to the direction of motion for the boat. The blades of the propeller are angled such that they slice and climb through the water like a screw into wood. Paddle wheels apply power via dragging through the water. Propellors operate via lift through the water. However in basic principle they both push against the water and as a result the water starts to move.
It is through the motion of the water that inefficiency manifests itself. The effort of moving the paddle through a stroke serves to convert the mornings breakfast into motion. In an ideal system all the energy you expend would be converted directly into the kinetic energy of the boat moving through the water. Unfortunately, anything else you cause to move will also use some of the energy you applied to the paddle. Just moving the paddle through the air expends energy, but for the purpose of this discussion we will just say that a lighter paddle will require less energy to move. Otherwise we will ignore the loss of energy in moving the paddle.
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A rocket is propelled by pushing mass backwards.
If the mass were cannon ball you could attain the same speed by pushing
a small mass quickly (top) or a large mass slowly (bottom). The second
option is more efficient as it requires less energy even though the
force is exactly the same.
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As a general principle it is more efficient to generate a propulsive force by pushing a large mass slowly than it is to push a small mass quickly. Propulsion is created by the principle of conservation of momentum. This is the "for every action there is an equal and opposite reaction" idea. Stated mathematically momentum (M) equals mass (m) times velocity (v) [M = m*v] and for momentum to be conserved, the change in momentum (–M) of the boat (–Mb) must equal the change of momentum of the water (–Mw) [–Mb = –Mw]. In other words if you are starting a 100 kilogram kayak from standstill and with you paddle make 10 kg of water move backwards at 10 meters per second, you kayak will end up moving forward at 1 meter per second [10 kg * 10 m/s = 100 kg *1 m/s]. Notice that you could get the exact same result by moving 100 kg of water at 1 m/s, or 1 kg at 100 m/s. This is where the inefficiency comes in.
While momentum is just mass time velocity, kinetic energy (KE) is mass times velocity squared [KE = m*v^2]. Kinetic energy is the amount of energy contained in the motion of the object, and that squaring of velocity is critical. This means that the amount of energy increases faster with increases in velocity than it does with increases in mass. So 1 kg of water moving at 100 m/s has 10 times the energy of 10 kg moving at 10 m/s [1 * 100 * 100 = 10,000 vs 10 * 10 * 10 = 1,000] even though they have exactly the same momentum. The reason this is important is the kinetic energy of the water is energy that used to be your breakfast and could have become kinetic energy of the kayak instead. Kinetic energy in your kayak is a good thing, it means your boat is moving. Kinetic energy in the water is a bad thing because it is gone and there is almost no way to get it back. So you will maximize your efficiency by minimizing the velocity you impart to the water as you paddle.
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As a paddle is pulled straight back against the
water, the water spills around the edges. The water spins in a pair
of vortice.
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A larger blade will effect a larger mass of water
and thus does not move through the water as fast. In these examples
the paddle is pushing the water towards the left, so the boat will go
towards the right. Notice that water does not just move in a straight
line. Since it needs to maintain the equilibrium on both sides of the
paddle it most move out to the edges and then back towards the back.
This circulation creates a vortex on each edge of the blade. The momentum
to the side is conserved by the matching pair of vortices.
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To create the same momentum change with lower velocity you want to maximize the mass of water you are moving. The first and easiest way to do this is make sure you use your whole paddle blade. Putting more blade area in the water will automatically increase the mass of water you have to push against with an immediate increase in efficiency. It is a common mistake for beginner kayakers to just use half the blade. Be sure you submerge the paddle blade completely in the water. You will immediately feel the paddle gain a better grip on the water. As a result, it may feel harder to paddle, but you will actually be using less energy to gain the same result so it is worth it.
Related to this is timing when you apply the power to the paddle. If you apply it to early you will end up splashing. Splashing is just another way of saying you are moving a small amount really fast. So to avoid splashing, you want to want to avoid applying power until the paddle blade is fully submerged. But you don't want to wait too long otherwise your stoke will be over before you really started. So you should try to submerge the paddle quickly so you can apply power as long as possible. Whenever you are not applying power, your boat is slowing down and the slower it gets the more effort required to bring it back up to speed.
There is another reason for waiting before applying power - "Ventilation". Ventilation is what you call it when the paddle sucks air down the back of the blade. Many people call it cavitation, but that is when a propellor spins so fast that it creates a vacuum behind the blade. Neither is any good, but what your average kayaker creates is ventilation. The reason it is no good is you are moving air instead of water. Air weighs a lot less than water and if there is air behind your paddle instead of water the mass of water you are moving is decreased and thus creating the same momentum requires you expend more energy.
There are two ways of creating ventilation. One is by bringing the air down with the blade as you insert it into the water. The other is to pull air down the back of the paddle as you apply power. Both can be avoided by being sure your blade is fully submerged before applying power. A clean entry will not bring down much water and the small area near the top of the blade is less likely to ventilate.
You can also waste energy at the end of the stroke. Again it will be indicated by splashing. If you still have a paddle full of water when you pull it out you will end up throwing it. This will often happen if you are applying power all the way until the paddle is removed from the water. Throwing water at best falls into the "moving a small mass fast" area at worst you are just lifting it and letting it drop straight down providing you with no momentum boost whatever. If you are going to throw water around you at least want it moving in a direction opposite your direction of travel.
To reiterate, your basic paddle stroke needs several things to be efficient: a fast clean entry, power while the blade is deep in the water, followed by a quick clean exit. Then you need to get started on the next stroke as soon as possible before your momentum is frittered away in drag on the boat. But I have left out an important point: where you put your paddle. A force is applied directly through the geometric center of the mass of the object. When a force is applied to the side of the mass it is actually called a "torque". Torques have the tendency to rotate the mass they are applied to. Sitting in your kayak, your center of mass is in the center of the kayak and you are sticking your paddle out to the side. The force of your paddling effort is actually creating a torque on your kayak. All your kayak really only wants is spin around its center. Luckily it is sitting in water which resists the torque. The length of your kayak sticks out ahead and behind you and when you apply the torque the boat drags against the water at the ends. This sucks up most of the force applied to the side of the boat and leaves you with just the component of the force applied up the centerline. So you move in a straight line because you are wasting some of you effort in moving water near the ends of the boat sideways.
There is not much you can do to avoid this completely, but by keeping the force as close as possible to the centerline you will minimize the amount of force wasted in keeping the boat going straight. This means keeping the paddle blade close along the side of the boat. This can be accomplished with a short paddle or by holding your paddle nearly vertical as you stroke.
To this point I have made no real mention of how the paddle actually moves through the water. The principle of maximizing the mass of water you push against applies regardless of how you do the pushing. At first it sounds pretty straight forward, you put your paddle in the water and pull it back parallel to the centerline of the boat and like a paddle wheel you are depending on the resistance of the water to create the force to propel the boat forward. If you do this you will probably have a pretty efficient paddle stroke, but there is more to it that just pulling on your paddle.
A common instruction when you are learning to paddle is to" plant your paddle like you are putting it into concrete and pull the boat to the paddle". Fortunately, water is not concrete, but the fact that it isn't does cause some problems. When you pull on your paddle it starts to move the water. The water on the power face of the blade tries to move away from the blade and the water on the back face tries to follow the paddle. The water in front of the power face needs to go some place as you push it. And it wants to go where ever is easiest. The volume behind the back face of the paddle needs to be refilled with water as the paddle pushes away the water that used to be there. The easiest place to get is from the front face which now has too much. So, water flows around the edge of the paddle. This action is typically what is called "slippage". This is an imprecise term but, it just means that the paddle moves through the water. A more technical term for what occurs at the edge of the paddle is a "vortex". This is a little eddy or whirlpool caused by the water moving from the high pressure area of the power face to the low pressure area of the back face. You will often see the remains of the vortex spinning in the water after your paddle stroke.
| Particulars | Just a fancy term for the measurements and characteristics of a boat. | Click on image for enlargement |
| Over All Length (ft) | Also known as Length Over All (LOA). The total length of the kayak from bow to stern |  |
| Over All Width (in) | Also known as Beam Over All (BOA). The width at the widest point of the boat. |  |
| Water Line Length (ft) | Also known as Length Water Line or Load Water Line (LWL). The length of the boat measured at the waterline. Used to determine "hull speed". In general boats with longer LWLs will be more efficient at high speeds. In other words long LWLs general make boats faster. As the term "Load Water Line" implies this is the length of the waterline when the boat is loaded the design displacement. |  |
| Water Line Beam (in) | Also known as Beam Water Line (BWL). The width of the boat measured at the waterline. This is a primary determinant of initial stability. |  |
| Design Water Line Displacement (lbs) | The Design Water Line (DWL) determines the total amount of weight the boat was designed to carry. This is a some what arbitrary number. The boat can carry more or less, but the other particulars are only completely accurate when the combined weight of the boat, paddler and the gear it is carrying is the same as the DWL displacement. | |
| Draft (in) | How far below the waterline the boat reaches. This does not include rudders or retractable skegs. |  |
| Wetted Area (sq. ft) | The surface area of the part of the boat that is under water. Lower wetted area means less frictional drag. |  |
| Waterpl. Area (sq. ft) | The area of the water plane. The waterplane defined by the waterline outline. |  |
| Total surface area (sq. ft) | The total surface area of the skin of the boat. You can get an approximation of the weight of the bare boat by multiplying by the density of the wood strip and fiberglass layup you are using. A good starting point is 0.7 pounds per square foot for a typical 1/4" layup with 6 ounce glass. |
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| Approximate Bare Boat Weight (lbs) | The weight of the boat before adding seats, hatches, rudders or any other outfitting. This is approximated by multiplying the Total surface area above by 0.7. and assumes 1/4" western red cedar strips with 6 ounce cloth. |
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| Volume (cu ft) | The total volume of the kayak. The volume of water that the boat would displace if it were totally submerged. |
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| Volume (Gallons) | Same as above in gallons. |
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| Prismatic Coeff. |
Prismatic Coefficient (Cp) or Longitudinal Prismatic Coefficient: (The displacement volume) divided by (the volume of a prism equal in length to the LWL whose cross sectional area equals the Midship section area) This is a rough measurement of the "fineness" of the boat shape. A boat with a "fine" shape has quite sharp ends. It is used to estimate drag. The typical range is roughly 0.5 to 0.7. Higher numbers tend to be more efficient at higher speeds and lower number more efficient at lower speeds. The prismatic coefficient was originally developed as term for hull design analysis by Admiral David W Taylor in 1943. |
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| Block Coeff. |
Block Coefficient (Cb) (the displaced volume) divided by (LWL * BWL * Draft) This a rough measurement of how "V" shaped the bottom of the boat. It is useful in determining how well a boat tracks |
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| Midship Area Coeff. |
Midship Coefficient (Cm) (The area of the section at midship) divided by (BWL * Draft) |
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| Waterpl. Area Coeff. | The area of the waterplane divided by the length and width of the boat at the waterline. |  |
| Lateral Plane Coeff. | The lateral plane is what you see below the waterline when looking at the boat from the side. This coefficient is the area of the lateral plane divided by the LWL times the Draft. |  |
| Longitudinal Center of Buoyancy behind Amidship (in) | The fore and aft location of the Center of Buoyancy relative to the middle of the boat. Here it tells you how far behind the middle of the boat the Center of Buoyancy is. Often abbreviated as: B, CB, or COB. |  |
| Longitudinal Center of Flotation behind Amidship (in) | The location of the geometric center of the waterplane area relative to the middle of the boat. This is where you would add more weight for the boat to sink straight down. Often abbreviated as:F, CF, COF, or LCF. |  |
| Center Lateral Area behind Amidship (in) | The location of the geometric center of the lateral plane relative to the middle of the boat. |  |
| Sinkage (lbs/in) | An approximation of how much weight you would have to add to sink the boat another inch. It will generally require more weight to get a full inch, but if you want to go up or down 1/10th inch, one 1/10th of the Sinkage added or subtracted will be pretty close. | 
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| Moments to change trim 1 inch (lbft) | The amount of torque required to push the bow or stern into the water about 1 inch. This is an indications of how quickly the boat will rise over waves. |  |
| Righting Moment at 1 degree (lbft) | How much torque is required to tip the boat a small amount. An indication of initial stability. | 
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| Vertical Center of Gravity above DWL (in) | The VCG is used to calculate the "Moment to change Trim" and the "Righting Moment" above and is important for determining stability. Most of the data on these pages assume a VCG that is 10.5 inches above the bottom of the boat. | |
What could be easier than stability? Just make the boat wide and it will be stable … right? Yet, there are kayaks out there from 20" to 32" wide, all of which the manufacturers say are stable. After all, what manufacturer is going to say, "you need to be born in a kayak to keep this sucker upright"? How can they all get away with this? And what is "secondary stability" anyway? I know from personal experience that this question will provoke a discussion that can go on for days.
Stability is almost always the first concern of the beginning kayaker. Stability is the first thing an experienced paddler will notice about a kayak, and improper stability performance will immediately disqualify a boat for them. Everything people want to know about the stability of a kayak design is contained in their "stability curve".
Sea Kayaker Magazine has been publishing stability curves with their kayak reviews for some time now. Novices look at the curves and are baffled, assuming that the information must be over their head. Skilled paddlers look at the assumptions involved in creating the curve and feel that they are irrelevant to someone who really knows how to paddle. A little experience can make the curves informative regardless of your paddling skills.
The definition of stability seems pretty clear to most people. A boat that keeps them out of the water is stable, one that dumps them in is not. Although that seems pretty clear cut, two people trying the same boat will still have different opinions about its stability. It is useful to start by agreeing on what it means to be "stable". The dictionary definition that applies to boats in water is probably: "designed so as to develop forces that restore the original condition when disturbed from a condition of equilibrium or steady motion." In a kayak we want to return to an upright position after being "disturbed" by tipping to the side. So a "stable" kayak will develop forces that restore the boat to an upright condition after being leaned or tipped.
There are two major forces at work on a kayak at rest in the water. The weight of the paddler, his gear and the boat all add up to a force pushing down towards the center of the earth. This weight is supported by an equal and opposite force from the buoyancy of the water, which pushes up. It is the interaction of these two forces that are involved in stability. The relative distribution of the forces will determine whether a kayak is stable or not.
The buoyancy force of the water is distributed over the whole submerged part of the boat. The water pressure pushing on the outer surface of the boat adds together to support all the combined weight in the kayak. Instead of trying to keep track of a bunch of distributed forces engineers generally find a "centroid" or center of force. If you add together all the distributed forces and apply the result through the center of force, this one force would cause the same reaction as all the little forces acting at once. This technique lets a kayak designer combine all the weights in a kayak into a "center of gravity" (CG) or "center of mass" (CM) and all the buoyancy forces into a "center of buoyancy" (CB). Since the force of buoyancy is equal and opposite to the force of gravity, the designer does not even need to pay much attention to what the actual value of the force is. Instead, they can just remember that on flat water the force of gravity is straight down and the force of buoyancy is straight up, and just look at the relative horizontal locations of the CG and CB.
With a boat in equilibrium, the centers of force will be aligned one directly above the other. In a kayak the center of buoyancy will be directly below the center of gravity. This way the buoyancy is pushing straight up towards the weight that pushes straight down.
If some new condition comes along to disrupt the equilibrium, such as wind, a wave or the paddler reaching for an escaped water bottle, the kayak will start to tip. As you tip, your CG moves in the direction you're tipping. Unless the CB moves in response, your weight will be hanging out beyond the buoyancy forces supporting you and you will capsize. In a stable kayak design, the action of tipping the boat rearranges the buoyancy forces to move the CB in the direction of the tilt beyond the CG, thus forcing the kayak upright again. In a stable boat the center of buoyancy moves side to side faster than the center of gravity.
For a kayak to be stable it should either apply a force to push you back to the upright equilibrium condition, or if you want to lean, it should apply force such that the boat finds a new equilibrium condition before it tips you over. The kayak designer controls this by manipulating the cross sectional shape of the kayak and the height of the seat.
Remember that the goal is to keep the CG vertically in line with the CB. Unfortunately, the most stable position is always going to be with the CG hanging below the CB like a rock hanging from a string. But, since you want to breathe, the CG needs to stay directly above the CB. When you move your body to one side, the CG is going to move to that side, away from the CB. To keep you from hanging upside down, the CB now needs to move under you before you rotate all the way over. As the boat rotates in the direction you are tipping, the hull pushes down into the water on that side while the other side lifts out of the water. This action of adding volume (buoyancy) on the side you are tipping and subtracting volume on the other side will cause the center of buoyancy to move toward the side you are tipping. If the boat is shaped to be stable, the CB will move out to the side faster than the CG.
Notice that the change in buoyancy happens due to changes of volume near the waterline. This is why initial stability is dependant on the waterline shape and width and not on the shape below the waterline. Because it is only the water near the waterline that is initially effected by tipping, it is only the shape near the waterline that effects initial stability.
Visualize the end view of a tipping kayak. Picture the narrow slice or wedge of volume being pushed into the water on the side of the tip, and a similar slice lifting out on the other side. As one side is pushed down buoyancy is added on that side and as the other side is lifted buoyancy is subtracted from that side. It is this shifting of the buoyancy which moves the CB. This happens along the whole water plane of the boat. (The water plane is just the outline of the boat at the waterline.) The shift of the buoyancy takes effort. The boat will oppose moving it's CB and it is this tendency of the kayak to resist a change in buoyancy that is felt as the boat's initial stability.
Initial stability is the tendency of the boat to resist tipping a little bit from upright. A larger water plane area increases the volume moving from side-to-side as the boat tips, which requires more effort to move, thus increasing the initial stability. Giving the water plane a greater average width has the same effect.
Notice that the water plane determines the shape and volume of the imaginary wedges and how the CB moves. For small tipping angles, the cross sectional shape of the boat above or below the water plane don't have much effect. This means that the cross sectional shape of the boat does not effect initial stability much. Two kayaks with different cross-section shapes (for example hard chine and soft chine) but similar water plane shapes will have similar initial stability.
It may be contrary to what you have heard people say, but chine shape and whether the bottom is rounded, "V" shaped or flat, will not really effect the initial stability. The shape of the kayak will only effect the stability as it enters or exits the water. While the tipping angle is small parts above or below the water line don't effect the water plane much, so initial stability will not be effected by differences in hull cross sectional shape.
If you are trying two boats with similar water plane shapes and widths and you detect a significant difference in the initial stability it is probably due to difference in seat height or some other factor which changes the height of the CG. The effect of different CG height will be discussed later.
It is only as the tipping angle starts to increase that the cross sectional shape starts to come into play. As the angle increases, parts of the kayak that started above water will enter the water, and parts that used to be wet will become dry.
The center of buoyancy is moved by adding volume on one side (parts getting wet) and subtracting volume on the other side (parts becoming dry). Volume farther away from the original CB will move it faster. One unit of volume 2 inches away is just as effective as two units 1 inch away. The effect of the volume is a "moment" arm dependent on the size of the volume times the distance away. Atlas can move the world with a longer lever because it increases his moment arm.
How much buoyancy force is generated to counteract the tipping force depends on how quickly the CB moves as the kayak tips. Short wide boats create this force by moving a small volume a long way, but narrower boats can create the same effect by moving a larger volume a shorter distance. This righting force or righting moment is often plotted on a stability curve.
As long as the righting moment is positive, the boat will have a tendancy to return upright unless some other force is applied. When the graph goes negative, you will need to apply some bracing force to return upright.
Often the "Y" axis is given in the units of "foot pounds" instead of "GZ". GZ in this case is in feet. To get "foot pounds" just multiply by the displacement weight in pounds. Feet times pounds equals "foot pounds". So if the weight of the boat is 40 lbs and the paddler weights 200 lbs, maximum will be 0.047 ft x (40+200)lbs = 11.28 foot pounds.
Note: The boat being analyzed above is the Night Heron with a 200 pound load, 10 inches above the bottom of the boat, where the boat weights an additional 37 pounds.
Relative to other performance criteria, the stability characteristics of a boat design are fairly easily quantified. The most common representation of stability is the "stability curve". While units may vary, this graph plots a line proportional to the horizontal distance between the center of gravity and the center of buoyancy for various angles of "heel" or lean. This curve describes the "righting moment", or how much torque the kayak creates to force the boat back upright. It can also be viewed as a "heeling moment" or how much force is required to tip a boat to a given angle. These graphs assume that the paddler remains immobile through out the whole range of angles. That fixes the location of the CG relative to the boat. The designer is left to calculate the location of the CB. This requires some fairly complex integration calculations to determine the center of buoyancy of a series of sections of the boat and then integrating these calculations together to determine the CB for the whole boat. While this is hard to do manually, computers are great at these calculations.
While the rigid paddler may seem silly in a boat like a kayak, which depends on the paddler moving for much of its stability, there are good reasons for this assumption. It eliminates differences due to the paddler's skills and the effect of an active paddler can be deduced from the stability curve.
There are several aspects of the stability curve that are worth looking at: the height at a given heel angle, the slope of the curve at any given angle and area under the curve from zero degrees out to a given angle. The height of the curve tells how much force the boat is creating to return upright. The slope of the curve indicates the resistance to further tipping. The area under the curve corresponds to how much energy is absorbed by the boat when it is tipped.
The height of the curve is probably the easiest to understand and is what most people would look at first. A higher curve means the righting moment is greater. This means that it will be harder to tip the boat with the higher value to a given angle and, if two boats are tipped to the same angle, the one with the higher value on the stability curve at that point will start to come upright faster. As long as the stability value is greater than zero the boat will have a tendency to come back upright unless additional tipping forces are applied. So, boats with higher stability curves will generally feel more stable. And boats with positive stability moments out at higher heeling angles will generally give the paddler a little more leeway for tipping. This is easy to understand, but unfortunately is not the end of the story.
Look at the backside of the stability curve where it starts sloping down again and think about what this means. Lets say something hits you with enough force to tip you into this region of the curve and you are lucky and don't tip over. Now another wave comes along and hits you with just slightly more force. You will now be pushed to a place with less ability to push you back. You need more supporting force, and instead you are getting less. Unless you brace you will inevitably go over. Because the curve is sloping downward any increase in tipping will provide a diminishing righting moment. Although the boat still is still providing a righting force, beyond the top of the stability curve this force will not feel very supporting because the slightest additional tipping force will push you down the slope.
Although you would expect a round bottomed (red) boat to be the least stable, in this case it has the highest overall stability because it flares out a lot above the waterline. And even though the "flared" (blue) shape has similar overall width, the volume distribution of the rounded shape gives it more stability. Any shape that widens above the waterline will tend to have more secondary stability.
Using the same basic hull shapes below are the curves when overall widths are the same. Now, the round bottom is much less stable because the waterline width is much less. This demonstrates why knowing the overall width of a kayak is not that informative. You will learn more by asking for both the overall width plus the width at the waterline.
The upward sloping section of the stability curve feels secure because any increase in tipping force will result in a balancing increased righting force. If you want to lean the boat in this section, you can be confident that the boat will support you. A little lack of attention will not generally cause a swim, it will usually just push you back upright.
A rapidly climbing slope has a more solid feel because it takes more force to effect a smaller change. It is like going up a mountain, where a lot of climbing doesn't get you too far on the map. The importance of the slope of the curve is evident at the very beginning. The slope of the curve at small angles of heel corresponds to the initial stability.
As discussed earlier, initial stability is the resistance of the boat to tip just a little bit. The slope of the line at the beginning of the stability curve indicates this resistance. In fact, the slope of the line at any point along the stability curve indicates how much more force will be required to make the kayak tip just a little bit more. Put another way, the slope shows how much an additional tipping force will effect the boat if it is already tipped. A shallower, flatter slope means that an additional force will have more effect.
A subtler difference in feel can be gleaned by looking at how the slope of the line changes as the heel angle increases. If the slope increases at greater angles it means the boat requires a gradually larger addition of force to achieve the same increase in tipping angle. This boat will feel like it stiffens up as you try to lean it over. If the curve has a progressively decreasing slope, the kayak will require a gradually decreasing change in force to create the same change in tip angle. This will make the boat feel like it gets a little looser as you try to lean it over. All boats will have a part of the stability curve which looks like this, where the line is curving downwards like the top of an "n". It will always occur before the maximum point on the curve. The upward curvature of the boat which stiffens will only be found in some designs.
Secondary stability is generally related to the maximum height of the stability curve. Obviously, a higher maximum righting moment will be more stable because you would need to apply more force to reach that angle. But the angle at which the curve reaches the maximum is also important because that indicates how far you can heel the boat before you begin losing stability. One way of combining the height and angle of maximum righting moment is to look at the size of the area between the curve and the horizontal zero line. This indicates the work or energy required in tipping the boat to that point. A larger area under the curve indicates that it will take more effort to tip the boat.
Most people will not feel secure when tipped all the way to the angle where the curve reaches a maximum. The slope of the curve will always level out before going down hill and a horizontal line on the curve means that a small increase in tipping force can create a large change in how much the boat actually tips. And if you are pushed over to where the curve starts sloping downward, you are on the slippery slope where any added tipping force could cause a capsize.
Each person's perception of a kayak's secondary stability will be effected by how comfortable he or she feels when the curve starts to flatten out. There will probably be a point before the top of the stability curve where a little more tipping force causes too much more tip for the paddler's comfort.
Capsize is not inevitable after the high point on the stability curve. The kayak can be leaned all the way over to the angle where the line crosses zero before the kayak will actively help capsizing. This is the point where the righting moment becomes negative and is where the boat is no longer providing any force to push you back towards an upright position. A negative righting moment means the boats buoyancy and your weight are acting to push you farther over.
Again it is useful to look at the area under the curve. Measuring the area below the stability curve from upright out to the angle where the line crosses zero describes the overall stability of the kayak. A larger area under the curve indicates the paddler can be hit with a larger capsizing force and still recover. The area is a measure of how much energy the kayak can absorb without capsizing. The energy may come from leaning you body, a wave slapping into the side of the boat, or a fish pulling on a fishing rod.
Most paddlers will probably not experience the overall stability in regular use. Usually by the time the boat is pushed over the top of the stability curve, the paddler is bracing strongly or taking a deep breath. However, the overall stability will determine how strong a brace is needed or how much time there is to grab that breath.
Just knowing how to read the stability curve is not going to tell a novice paddler what boat to buy. You must first understand how the curve relates to your paddling style and skills. If you can calibrate yourself by trying several boats and reading their stability curves and then being aware of how their stability changes as you lean the boat, you can learn to relate that knowledge to the stability curves of boats you have not tried yet.
Everyone is a different shape or weight. The stability curve assumes a fixed center of gravity for the paddler. For example the reviews in Sea Kayaker assume the CG is 10 inches above the lowest part of the seat. Obviously, many people will not match this assumption. A barrel chested man will have a different CG from a petite woman. Their weight will be different as well as the relative height they carry the weight.
However, just because you do not fit the assumptions does not mean the stability curve is irrelevant. Changing the weight and the height of the CG will have predictable effects on the stability. A different weight paddler will change how deeply the boat sits in the water. This will change the waterline width, and the relative cross sectional shape. However it doesn't really change the shape of the boat and the form of the curve will stay similar regardless of the weight. Lighter people will probably find all boats somewhat more stable than heavier people would find the same boats. Since the boat is floating higher, the length of the right moment arm tends to be longer for lighter paddlers, but since they weigh less the actual righting moment does not change as much as would be expected. As a result the initial and secondary stability often remain surprisingly similar regardless of weight. The overall stability will be less for heavier paddlers. Once they get over the high point on the stability curve, their weight starts to pull them down more quickly.
Raising or lowering the CG will effect the stability in a predictable manner. The exact change in stability can actually be calculated based on how much the CG is changed, but since it the change in stability depends on the paddlers themselves instead of the design of the kayak, nobody needs to do any math. A paddler with a higher center of gravity than his friends will consistently find boats less stable. Shorter paddlers will always find boats more stable.
You don't really need to know anything specific about how you differ from the assumptions used in creating the stability curves. If you feel the initial stability is insufficient in one design, any design with the same slope at the beginning of the stability curve will feel equally unstable. This only falls apart if the stability curves you are comparing use different assumptions. That is why it is important that all the reviews published in Sea Kayaker put the CG of the paddler a standard 10 inches above the seat.
Even though most skilled kayakers use leaning and the ability to brace as important parts of their stability when negotiating rough water, the assumption of a rigid paddler used in creating the stability curves is still good. Like moving the CG up and down, changing the CG from side to side produces changes in the stability which are independent of the design of the kayak. Although factors like seat configuration may change how easily the paddler can move around, the ability to lean and edge a boat is highly skill dependant. A boat with a steep stability curve will be hard for skilled paddler to lean up on edge but that will be true of all boats with a similar stability curve. If the paddler has the skill or flexibility to lean a boat, that skill will be able to be translated consistently over a variety of boats. By learning how your skills interact with a variety of different kayaks you can learn to translate the stability curve into something meaningful to you.
Stability is a subjective thing. The same boat that is a threatening death trap for a novice may be stodgy and boring to an experienced extreme paddler. You can never be sure of the personal preferences of someone describing a boat. A salesman eager to sell a boat will tend to emphasize the characteristics he thinks will most appeal to a buyer. The stability curve eliminates this subjectivity. It is the unfiltered reaction of the boat itself.
If you are willing to take a little time to learn how to interpret it, the stability curve can be a useful evaluation tool. While there can be no substitute in the final analysis for spending some quality time in a kayak, learning to relate your on-the-water experience with the stability curve can help narrow down your kayak choices. Even just the awareness of different characteristics on the curve, such as the slope, height and area under the curve, will help you know what to look for as you try out boats.
Try leaning boats as you test them, then refer to old issues of Sea Kayaker to study the stability curves. Then try to identify what you liked and disliked about the stability on the stability curve. At the very least you will burn a little more knowledge about kayak performance into your brain which will serve you well as your skills continue to evolve.
"Tracking" has become a somewhat nebulous term when used in reference to sea kayaks. For most people it has something to do with how easily the kayak turns. In this context a boat with "good" tracking is one that is hard to turn and one that doesn't track tends to be easy to turn. Unfortunately, this tends to make people think a boat that is hard to turn is "good" or at least somehow better than one that is easy to turn. However, unless they are doing Olympic style sprint racing, most people do occasionally want to turn, and they would really prefer if it weren't too hard to do. When people talk about tracking they really want to know if they will have a hard time making their boat go where they want it to when they get caught out in a wind. If they have to struggle to maintain their course they think it is "bad", if it is easy to go where they want, it is "good". This quality of being easy to control in difficult conditions is only loosely related to how easy the boat is to turn.
A common problem for paddlers is when they find themselves crossing a bay in a high wind and they can not force their boat to point up into the wind. As a result they are forced to paddle a course that is either far out of their way or is leading them into danger. Any time they try to point their kayak towards the intended destination, the boat immediately gets blown off course or worse, they can not get the boat to turn in the desired direction at all.While this is often a problem of technique it is also a problem of a boat that is hard to turn. The boat that they thought had "good" tracking because it didn't turn easily is making it making their life difficult. The difficulty of turning the boat is combining with tendancy of the wind to make kayaks turn broadside to the wind, and creating a dangerous situation. While it may be true that once they get their boat on course it may be less likely to be knocked off course by the wind, the difficulty in turning is making it so they can't even get to the course they want. A kayak that was easier to turn would be much easier to get on course. While the easily turned boat might take less force to knock off course, at least you can get it there in the first place.
Note that it is not how much force it takes to turn a boat that determines how likely it is to be knocked off course. What really matters is how likely the kayak is to be subjected to sufficient force that it will be knocked off course. A boat that turns very easily but is not subject to sufficent turning forces is going to be easier to control than a kayak that is hard to turn but very likely to be exposed to enough force. An easy to control boat will be well balanced. It is just as likely to be subjected to forces that turn it in the way you want to go as to opposing forces. Balance has very little to do with how easy the kayak can be turned, instead it relates to how likely it will be turned.
Balance comes from the interaction of the kayak shape below and above the water with the air and the water. People often look at how much boat is exposed above the water and try to draw conclusions about how the wind will blow it around. They will see a high bow and assume the boat will be hard to control in a wind. But a high bow can be just fine as long as it is well balanced by the hull shape in the water. Water is much denser than air and as a result a little bit of water resistance can balance out a large amount of wind resistance. It is also a mistake to get too caught up in the "sail" area of a kayak because where there is wind, there are usually waves and waves are quite good at blocking the wind. So the kayak is being exposed to a lot less wind than you might think. Besides, the biggest sail is your body sticking up in the middle of the boat.
The way to make a boat controllable in harsh conditions is to try to balance all the forces. While the bow is being blown down wind, the water is pushing back and the stern is also being pushed around. With careful design these forces can be combined into a balanced boat that lets the maintain control over where they are going regardless of how easily the boat turns.