Re: Fluid dynamics?
Drag Coefficient, designated Cd in the drag equation. There are different forms of drag, here we're talking about form drag which is friction caused by the shape or design of the hull and frontal area of the hull. Basically, drag is induced by the resistance of a hull to free flow of a fluid. As a hull moves through the water the molecules near the hull are disturbed and move around and away from the hull.
(Hull design has a lot to do with how much and how fast does water move away from the hull. To get into that gets away from the question of the effects of dimples on free flow of a fluid. F'rinstance, a hull could be designed to be 'dry' where not much hull is in the water. So, by design, the idea would be to generate lift. A byproduct of lift is induced drag. But a lifting hull would be highly sensitive to small control inputs and not desireable to how most boaters operate.)
Without getting into the math, a good explanation is if the vicosity of the fluid remains constant, to achieve more speed, or economy, which really can be defined as "less drag", then the change has to be in the velocity of the fluid. How to achieve this is the root of the question. Bernoulli showed us that an increase in velocity results in a decrease in pressure.
Dimples or a roughened skin acts to increase the boundary layer which results in a higher velocity closer to the skin. (Without the boundary layer the flow would be turbulent at the skin. And slower, reaching out to the next layer of molecules and on and on until at some distance from the skin the fluid would be once again free flowing.) The boundary layer acts to reduce the 'slowness' of the fluid close to the skin.
Vortex generators (VGs) on planes act to induce laminar flow. But the question here is specific to the boundary layer on which that flow relies. The boundary layer is indeed micro-eddies which create low pressure.
Aerodynamicists use the term, reference drag. This helps them fine tune the basic drag equation for a specific purpose. Is form drag or induced drag more important? It depends on what is their intended purpose. VGs help with induced drag. Boaters are more concerned with form drag. VGs turn the flow whereas dimples, etc allow for a "free stream" flow nearer to the skin, ie, hull.
