Hydrodynamics is the study of the characteristics and movement of an object in water; hydrodynamics is a branch of the physical science known as fluid dynamics. The hydrodynamics of a craft used in rowing is critical to both the speed of the boat as well as the efficiency with which it can be propelled. Rowing hydrodynamics involves an examination of two separate but related components: the movement of the hull in water and the movement of the oars through the water.
An important aspect in the assessment of the optimum movement of a boat hull through water is the determination of the drag, or the resistance exerted by either the oars or the hull of the boat during propulsion. Drag has three separate components that may be physically calculated: skin drag, or frictional resistance, the drag created between the hull and the water moving past it; foam drag, the turbulence created in the water as the hull passes through it; and wave drag, the loss of energy due to the creation of waves by the movement of the hull.
In determining the ideal hydrodynamics for a rowing shell, consideration must also be given to how the shape or contours of the hull will effect the resistance on the boat due to the air passing over it; the hull of the shell, the bodies of the rowers that extend above the hull, and the oars all are subjected to wind resistance. As a general design consideration, the hull of a racing boat accounts for 90% of the drag created against the boat, while the air resistance accounts for the balance.
The object of racing shell design is to achieve the greatest possible speed, while permitting the rowers to move with a measure of stability across the water. Speed is the product of the power generated to move the boat, less the resistance created by the boat. To generate greater speed, power is the variable; to double boat speed, the boat must receive eight times as much power. For these reasons, in a human-powered craft (such as a rowing shell), it is the resistance of the boat that is the focus of the boat design. As a general rule, a longer and slimmer boat will create less resistance in the water, as it creates less displacement (the shell will ride higher in the water), less turbulence, and less wake. Such designs are also less maneuverable, and subject to the design of the hull and the presence of a keel on the underside of the hull, the craft will be less stable. As hydrodynamics are determined with the optimal rowing techniques in mind, any variation from proper rowing form will have a dramatic impact on the performance characteristics of the boat. The rowing stroke has three distinct components—the drag, the catch, and the finish—and any variation that tends to create an up and down force on the hull will make it more resistant in the water. Smooth, level, and rhythmic strokes ensure that the power is applied evenly to the shell, making it move more efficiently.
The finish used on the hull surface is also a factor in the creation or elimination of excess drag. Hydrodynamics is often the subject of extensive testing with scale models operated in water tanks to simulate race course effects, and computer simulation to test both the hull materials and the finish to be applied in terms of the drag created.
The oars are subject to the same design considerations as the hull. A large flat-shaped oar might be a useful tool to move a large amount of water on each stroke, but the oar will create additional drag as it moves through the water, and it will create a lesser but significant amount of air resistance on each stroke when the rower drives the oar backward prior to the stroke. As a general rule, the blade surface area should be as large as the individual rower can manipulate.
Further physical considerations that stem from the material used in the construction of a racing shell that impact its hydrodynamics are the torque and flex of the hull. The stiffer the hull, the less the boat will tend to flex or bend on the delivery of each stroke; the bigger the boat, the larger the crew, the greater that these forces can be. Any deviation from the direction of the craft due to flex or torque will diminish its potential speed.