Tuesday, February 14, 2006

First Race

We've all been there. The first race in a new boat. Some of us may be too old to remember it well. Some of us may still remember it with embarrassment.

Over at the Laser Forum Skipper Johnson captures the experience of that first race perfectly.

3 comments:

Carol Anne said...

Well, at least I didn't make so many mistakes in MY first race!

My second will be this coming Saturday; rumor has it Larry won't be on his boat, so the boat to beat will be ... Goat Rodeo.

OG said...

Hmmm... I remember my first time in a boat!

I was 3 years old and my Dad had just built me my first boat. A plywood Sabot.

The thing is though, the first time I was allowed in it was only in the pool! I was on a time trial. From the shallow end to the deep end.

I won!

Can this count as my first race???

Sailingaray said...

Oh, a very timely question. I'm not sure how we ended up talking so quickly about sailing boats on a reach when trying to answer such a fundamental question. Although the bloggers do mention other very increasing factoids, they play a role much later in understanding the concept. There are two camps these days. In my opinion both camps play a role in helping us to understand the ways of wings and/or sails. Why should we keep two theories around? Maybe an analogy is handy. Einstein proposed that light acted both as a particle and as a wave. So far, no one has proved him wrong and both are used.

The first aerodynamic explanation is the good 'ole Bernoulli concept. You nailed the description. Lift is generated by the change in the speed of air as it moves across the two sides of the sail or wing. On one side, the pressure lowers and the other side sees ambient pressure, or slightly higher. Differentially, a force is created in the direction of the low pressure on the leeward side.

The second theory is the Circulation Theory. In this theory the apparent wind approaching the lifting body (sail or wing) sees a low pressure area and bends dramatically to get to that low pressure. It is this bending motion that transfers energy to the sail -- conservation of energy. Circulation theory seems to have it biggest value in explaining slot effect. Slot effect? On a wing, slots are found as either leading or trailing surfaces. On sails, the jib forms a slot with the main sail. Forming a good, efficient slot is apparently a black art. However, the circulation theorist will show you wind tunnel results to prove that the Bernoulli (Bernie) Law falls apart when used to describe a slot. Typically Bernie says the air speeds up even more in the slot, producing more lift. Circulation theory predicts, "Nope, it slows down." Wind tunnel results say, "It slows down."

The other bloggers were correct. A huge difference between sails and wings comes as a result of their use. The angle of attack difference between sails and wings is huge. An aircraft wing sees very low angles of incidence (I think in the order of 3 degrees). A wing lives in a perfect world (high efficiency). A sail must work over a large angle of attack because the difference in apparent wind between upwind and downwind is dramatic, i.e or the order of 45 to 90 degrees. Sails are inefficient; it is an imperfect world.

Sailboat designers face another awful problem. Say you'd buy yourself one of those beautiful, if not gorgeous, efficient wings, as found on a sailplane. Put it on a sailboat. A glider wing produces very high lift and low drag. Its efficiency comes because of its length versus width. The ratio of these is known as the aspect ratio; a glider has a high aspect ratio. Great! This should mean nothing but good things for a sailboat! Soon you would realize that the boat and its gorgeous wing is constantly tipping over. The long wing puts forces up too high and all of the grog in the guts of sailors couldn't hold the boat flat in the water. To keep the boat from heeling and tipping over the designer is forced to make the sail stubby. See, you give up a lot to own a boat! Most boats are designed to have the approprite amount of heel in 12 knots of wind.

But you can design a sail to have a high aspect ratio -- it does have less drag and it is typically faster -- just don't ever compare it to an airplane wing. Catamarans, such as a Hobie Cat, sport these sails. They are efficient devices and as a result they want to move fast to generate to gain that efficiency. They depend a lot on apparent wind. And wow do they move, but not straight downwind. That's why Hobies sail at 135 degrees to the true wind. Any lower and the apparent wind dies. The boat slows; no more flying around fast. High aspect sails do not want to go directly downwind. You should think, oh they don't push well, but they fly great. Fly means lift. High aspect ratio on a sail mean lifting device.

Now here is my cool little discovery. Drag means push. Here's why. Nature seems to always have an equal and opposite. Lift and drag are always the opposing forces in aerodynamics. If a high aspect ratio sail is a lifting device, then a low aspect ratio sail must be a drag device. Drag devices want to be pushed. What's another example of a drag device? Or, low aspect ratio sailes want to be pushed because they work a bit more on drag, dowind than a high aspect ratio sail. A parachute. A short, but wide sail -- it's a drag device. It will sail straight downwind. Often out comes the symmetrical spinnakers. Even these sails can benefit from apparent wind, so they are never sailed competitively directly downwind.

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