Midwinter 2004
By Tom Snyder
As a kid I raced a Blue Jay, which was a cute little wooden centerboard sloop. She could sail. (And turn. Closing within inches of a dock at full speed one could throw the tiller hard over and come to an instant stop. Fill the boat first with four midwestern cousins and you come off looking pretty cool.) I raced that little girl and voyaged too, eventually sinking her off of Great Misery Island. I was 12, and I stayed with the boat, thank God, because I didn’t own a life vest. Incidentally, never complain about a lobsterman who ignores you or gives you the finger because most often he’s the guy out there saving 12-year-old fools.
The point is, or will be, this: I don’t understand sails. At 13, I quit sailing when I discovered girls, guitars, skin problems, and then intoxicants, depressants, socialism, capitalism, computers and lower back pain. At 50, rediscovering boats, I know nothing about sailing except the “feel” part. But now with a bigger, slower-to-react boat, it’s time to go from intuition to real knowledge. To get a Blue Jay going upwind you tried not to pinch it too much – and you didn’t even know exactly what pinch meant. But to sail an Island Packet to windward you have to have several advanced degrees.
So now let me switch from this mature narrative to my preferred state of ranting and exaggerating and flirting with absurdities. Here is my guide for the adult sailor who wants to know the Newtonian backstory to a day of sailing. Don’t be put off by the theory and technical talk. It will all make you a better sailor.
Understanding Your Sails
Your sail behaves like an airplane wing.
A few simple additional notes:
The symmetric cross section of a jet wing requires all of the lift to be generated by angle of attack rather than through shape distortions. Keep that in mind when sailing. Angle of attack is important, but it is not everything. Forget that at your peril! You must think of your vessel as an exploding source of vectors, all of which must be balanced and controlled to create the largest possible resulting forward vector.
Beginning with the antagonistic forces, consider the dreadful effects of shearing as air moves along your sail. The result is turbulence. Turbulence robs the entire system of energy, like dragging claws of lethargy. Only you can prevent turbulence, but where exactly is this nearly invisible cloud of complexity? It is somewhere aft of your mast and you must locate it to confront it.
How far aft? This would be a good time for you to remember “Reynolds Number,” which you learned in college physics. Probably all that you remember is that you should keep that number shy of 1,000,000, so you will be glad to also remember that you can quickly compute this number by multiplying the length along the sail by the velocity of the air past the sail and then dividing that by the density of the air times the viscosity of the air. (Viscosity means stickiness.) Keep that number as low as possible.
Next, consider drag created by induced vortices BEHIND the sail. And please, please don’t fall into the trap of confusing turbulence with eddies as a result of separation from too great an angle of attack. Separation between the airflow and the sail is like a disease, and one that the prudent mariner can ill afford. If nothing else, keep it laminar.
Now for the “Slot Effect.” You’ve read about it, and you’ve explained it to the crew – the famous slot effect whereby the opening between main and jib effectively funnels the air into a wind tunnel that creates staggering fluid dynamic effects. Well, forget it. It’s wrong. There is no slot effect. The reason that two sails work better than one, and this has been proven at MIT, is because two sails have more sail area than one sail. This is a highly counter-intuitive idea and leads to sailors clinging to the slot effect myth. But think of it this way – a sail of 5 square feet plus another of 5 square feet would create a total of 10 square feet, which is fully twice as much as 5.
Finally, consider sail shape, one huge source of your lift. Glance at the “polar plot” that you have paid an engineer to develop for your boat. Notice the catastrophic reduction of boat speed as you approach 30 degrees off the wind. It’s nobody’s fault, of course, but it serves to remind us that controlling sail shape is now our only hope. For reasons too complex to explain without fractals, increases and decreases of curvature in sails are best determined by both point of sail and wind speed.
So, at the risk of appearing glib, let me reduce all of the formulations to this rule of thumb: very, very light wind means flatter sails; very light wind means fuller sails; light wind means flatter sails; moderate wind means fuller sails; more moderate winds means flatter sails; extremely moderate wind means fuller sails.
Conclusion: Sailing is like anything else. It’s hard and it’s complicated and intimidating. So don’t try to do it with instinct. Do it with the skills that made Western European culture famous: obsessive analysis.
Tom Snyder sails out of Peaks Island, Maine.
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