Defining laminar flow

OLD WOTSISNAME’S girl friend, Gloria Mundi, is surprisingly knowledgeable about boats and sailing. Perhaps the most surprising thing about her is that she IS Old Wotsisname’s girl friend, considering how little he knows. But it takes all kinds to make a world. Whereas he has a solid concrete barge, Gloria owns a pretty little 25-foot wooden Vertue which, in a fit of funniness, she named Sick Transit.

Gloria hasn’t been sailing long, but she has absorbed a lot of theoretical knowledge. The other day, out of the blue, she e-mailed me with a simple question: “What is laminar flow?”

Well, we all know what laminar flow, is don’t we? Don’t we? Surely we do. It’s kind of how water or air sticks to the surface it’s flowing past. It’s kind of how friction builds up and slows a boat down. It’s kind of ... well, it’s not easy to explain, actually, so we’ll kind of paraphrase the wise words of the British scientist and sailor, C. Marchaj.[1]

Laminar flow, he says, is the pattern of motion of particles in a zone of water that a yacht draws along the surface of its hull. It’s a pretty thin zone. On the smooth hull of a racing yacht, it may be only 3 mm thick, but it consists of layers of water sliding over each other parallel with the surface of the hull.

On a hull like OW’s, coated with weed and barnacles, and pitted with flaked- off cavities of old anti-fouling paint, this zone is much thicker, and will greatly increase the drag, or resistance to the boat’s forward motion.

Now, the water immediately beyond the laminar flow becomes disturbed and gives place to a much thicker zone of turbulent flow that is also drawn along with the hull.

“It is the drag induced by these layers which causes the resistance to rise so sharply as the speed of a vessel increases,” says Marchaj.

There. That’s exactly what I meant to say, Gloria. He took the words right out of my mouth.

[1] C. Marchaj, Sailing Theory and Practice.

Today’s Thought
What fates impose, that men must needs abide;
It boots not to resist both wind and tide.
— Shakespeare, III Henry VI.

Tailpiece
The Bishop, out for a walk, noticed a very small girl struggling to reach the bell push on a front door.
“Allow me to help you, my dear,” he said, ringing the bell for her.
“Outta sight, man!” cried the girl. “Now run like hell.”

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