April 6, 2014

The rough's better than the smooth

THOSE OF YOU who roll your own antifouling paint on the bottoms of your boats will know how difficult it is to get a really good finish. Racing skippers who insist on finishes like glass spend many man-hours and many bags of money to achieve less underwater resistance this way. But they may be wrong.
I have mentioned this before but, sigh, nobody takes any notice of what I say:  Years ago golfers discovered that balls with dimples in them have better aerodynamics and travel farther than smooth round balls.  I have therefore always held that the dimpled (well, all right, rough) surface of my antifouling paint actually makes my boats go faster.

Now some high-powered scientists are finally, finally, agreeing with me. Here’s a fascinating piece from a very smart website at  www.sciencerecorder.com

“When it comes to moving rapidly through water, smooth is generally considered better. That’s why ships have smooth hulls and swimmers shave their legs. However, a new study published in the journal Physics of Fluids suggests that smooth might not be best and that some rough surfaces can reduce drag.

“’A properly designed rough surface, contrary to our intuition, can reduce skin-friction drag,’ said John Kim, a professor in the mechanical and aerospace engineering department at UCLA, in a statement obtained by EurekAlert.

“Scientists had previously tested the idea of using rough surfaces in water with limited success. More recently scientists have started working with rough surfaces that are very hard to wet, a property called superhydrophobicity, the National Monitor explains. In theory this implies that the surfaces can snare air bubbles, producing a hydrodynamic cushion, but in practice they frequently lose their air cushions in chaotic flows.

“The researchers modeled the fluid flow between two surfaces coated with small ridges. They used a superhydrophobic surface design that another UCLA team had already observed could keep air pockets entrapped, even in choppy conditions. The surface was blanketed with tiny ridges arranged in the direction of flow.

“The researchers tested the model in both laminar and turbulent flow. To their surprise, the drag-reduction was greater in choppy conditions.

“The irregular fluctuations and swirling vortices in turbulent flows on smooth surfaces generally increase drag, Kim noted. However, the air cushion generated by the superhydrophobic ridges changed the turbulent patterns near the surface, reducing their effect.

“Drag-reducing surfaces may one day cover the undersides of both cargo and passenger ships. ‘It could lead to significant energy savings and reduction of greenhouse gas emissions,’ Kim explained.”

Today’s Thought
The best scientist is open to experience and begins with romance — the idea that anything is possible.
— Ray Bradbury, LA Times 9 Aug 76

“And you, madam, what’s your husband’s average income?”
“Well, on Friday nights, when he goes to the yacht club, it’s usually between 2 and 3 a.m.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


biglilwave said...

That was an interesting read, thanks.

It makes sense. Fish scales and shark skin aren't smooth.

Edward Jones said...

Yep there is the key "A properly designed rough surface". I doubt my rough bottom is proper in any respect:)

Unknown said...

On racing boats soured out of the water (no antifoul) we aim to get as close to a mirror finish as possible, then run over the hull with fine sandpaper aiming to scratch it as evenly as possible. The theory is that tiny eddies form from the scratches and make for a much more slippery surface than the mirror hull.
It seems to work, but then those who go to the effort probably spend more time on other aspects of their boat and sailing, so coulee be expected to be faster.