Floating Bodies -101

The shape of a kayak is the heart of its' in the water behavior. If we want to understand why one shape is used, and not another, we need at least a broad understanding of how shapes behave in the water. Any floating body, a vee-section hull, a half-full beer bottle, or a flat plank tilting on a wave, are subject to a few basic principles, and rules of thumb.

The following principles are condensed; with some thought you can uncover a world of implications and conclusions that can be drawn from them.

Displacement, floatation, and buoyancy

Any object, solid or hollow, will float if the volume of water that it displaces weighs more than the object itself does. The difference between the weight of a boat, gear and crew, and that of the displaced water, is the payload that the vessel can handle.

Fresh water weigh 62 lbs per cubic foot, salt water about 64, so 2 to 4 cubic feet of well-distributed enclosed space will easily float most humans. A volume about the size of a loaf of bread can support about 15 lbs.

Streamlining and shape

A floating body actually sits in a hole in the water, having shoved aside, or displaced, a volume equal to its' weight. If the object is moving, then the displacement is an on-going process, and the force required to displace the water will be affected, not only by the amount of volume, but also by the speed of the object. More volume, and more speed mean that more energy is needed to move the water.

Water is both heavy and incompressible. It takes an appreciable amount of energy to move it under the best of conditions. If the flow is blocked, more work will be required. Streamlining is the attempt to mitigate the shock of shouldering aside large volumes of water, all at once.


The total of all weights that tend to push a boat down can be summed up and said to operate at he boat's center of gravity (CG). Conversely, the sum of all floatation is said to operate at the vessels center of buoyancy (CB). The center of buoyancy moves forward or aft when the boat pitches, and shifts inboard and outboard as she rolls. The CG is determined by the distribution of weights throughout the vessel, including her structural components, cargo and crew. Some of these are obviously fixed, others moveable.

Any floating body MUST seek a state of equilibrium between these forces. This condition (equilibrium) is only fufilled when the CG and the CB are located in the same vertical plane. All of a boat's heeling, pitching and rolling is a response to the inexorability with which these centers must stay aligned.

Drag, resistance, and area

The immersed portion of a floating body is subject to the laws of flow and pressure. Resistance results from skin and wave drag, and it operates in the direction of travel. Skin drag is mainly a function of wetted surface area, but the relative smoothness or condition of the surface is important too.

Wave drag is energy diverted from forward motion into wake making. For displacement hulls at normal speeds, skin drag accounts for 80 to 90% of the total resistance exhibited by a hull.

Pressure diffrential, turbulence and lift

In water flow, speed, area, and pressure are all related factors. The idea of "streamlines" is important. Streamlines are bands, or areas of water flow where pressure and speed have uniform readings. Sometimes streamlines are visualized as tubes of flow. A moving hull will be surrounded by concentric layers of streamlines, whose speed diminishes with distance from the hull. This is more of an intellectual construct than the reality, which is closer to a continuum, but breaking it into discrete steps is a helpful device.

If you imagine each streamline as a tube, it will have a cross sectional area, a velocity, and exhibit a given static pressure. Sectional area and velocity are inversely proportional. Flow that is constricted will speed up, and visa versa. Any change of direction or shape of the hull is enough to affect flow and pressure near the skin. Some research has been done about the possible consequences this may have for flexible hulled boats, such as skin kayaks. These boats are a special case in that their hulls are not fixed, but flex and otherwise react to the pressure of flow and waves.

The case of water is further complicated by its' viscosity, or molecular resistance to displacement. This means that any object passing thru water tends to disturb it. This is what causes the familiar boat wake, and other forms of turbulence. Turbulence is the enfant terrible of the fractal universe, being the root cause for the random actions of the visible universe.

As befits such a powerful force, turbulence creates no end of problems and mysteries for boat designers. But, it seems to be a necessary evil: without viscosity and turbulence we would lose, among other things, the ability to sail upwind, or to travel by means of a paddle or screw propellor.

Any flow differential in a viscous material creates pressure differentials, that in turn create lift, which operates at right angles to the direction of travel. Lift is what makes carving or leaning into turns so effective, and also contributes to some forms of directional instability.

Centers of Effort and Resistance

The sum of all forces moving an object through the water may be said to operate at the center of effort. The sum of all lateral resistance can be said to operate at the center of lateral resistance. These centers need to be aligned - one over the other- for the object to exhibit directional stability.


The entire body of naval architecture is based on these basic principles, so any number of conclusions, even conflicting ones, can be drawn from them. This overview barely scratches the surface of a rather involved and complex subject. Interested readers can refer to the attached bibliograhpy for further details.

Readers put off by the scientific approach should take heart. People were designing and building excellent boats millenia before any of these theories surfaced in a coherent form. Experience, observation and intuition are powerful tools, and will takje you quite a ways. Analytical explainations can be helpful, but are not always essential. This is especially true, as history has shown, for smallcraft design.


H.F. Kay
The science of yachts, wind & water
John de Graff inc.
Tuckahoe, N.Y. l971

A good and understandable book, but quite a bit of it is about sails and sailing which is the case for many books of this type. Clear explainations of basic theory.

Pierre Gutelle
The Design of Sailing Yachts
International Marine
Camden, Maine 1979

Good sections on stability and hydrodynamics. Lots of excellent diagrams, but you don't need a math degree to benefit from it.

L. Francis Herreshoff
The Common Sense of Yacht Design
Caravan-Maritime Books
Jamaica, NY l974

This one has been around awhile, written by a master. L.Francis knew a hell of a lot - but you would too if your dad was Nat Herreshoff. This is a yacht book, so there is alot in here not directly related to kayaks, which is not to say there isn't much to be learned here. And there are a few kayak specific points - L. Francis was a bit of kayak devotee oddly enough. L. Francis writes in a semi-victorian manner that you will love or hate, but you can't deny the wealth of knowledge he shares.