Performance Articles page 2
page 2

Performance
"You don't seem to be pointing very well". It evokes the same kind of response that finger nails on a black board produce. The unsuspecting crew member that mutters this all too obvious observation usually receives a less than enthusiastic response.

Anyone can point a boat higher into the wind, but you usually loose speed. So a more useful discourse has to involve something which I call "performance".

Performance is the result of the delicate relationship between apparent wind angle, apparent wind speed, sail power, boat speed and ballast. Performance is the greatest factor in determining the results of a sailboat race!

Lift and Drag
Every thing that affects your boat's performance can be expressed in terms of lift and drag. Whenever lift is produced, drag is the natural consequence. They go together hand-in-hand. Fortunately for sailors the ratio of lift to drag is not constant. It is possible to increase lift faster than drag, and vice versa. Understanding this relationship is useful in trouble-shooting performance problems.

Keels
Keels create lift because they slide through the water, both forward and to leeward. We call the leeward direction "leeway". It may not be perceptible, but all keels and centerboards have some leeway while sailing upwind. The orientation of your keel to this leeway determines the angle of attack of your keel.

When you point your boat away from the wind the angle of attack of the keel decreases. That means both lift and drag are reduced. Less drag enables the boat to speed up. The increased speed forces more water over the keel, which increases lift without significantly increasing drag. In this case, the ratio of lift-to-drag changes. More lift and less drag enhances performance. For many, it is counter intuitive to head down in order to gain progress to windward. This is a common mistake many sailors make in light air.

Too Low
There is a limit to how fast your boat will go, however. Drag increases exponentially with boat speed. So there is a point depending on boat speed where lower angles will not significantly decrease drag. At top speed, you will have similar drag no matter what direction you point (unless you can plane). As you approach this point, upwind performance declines the lower you sail. Some people find themselves going very fast, but they can't hold their lane. By the time they get to the windward mark they are back in the cheap seats.

Too High
If you turn towards the wind, the angle of attack of the keel increases. Eventually the flow across the windward side of your keel starts to breakup, or stall. A stalled foil quickly looses lift and dramatically increases drag. Boat speed drops, which further provokes stall. Performance quickly degenerates at too high an angle.

The Groove
Most sailors have an intuitive notion of a "groove" while sailing on the wind. Whether they understand the concept or not, a good helmsperson is better at keeping the keel's angle of attack between the two extremes described above. In light-to-moderate wind speeds, great sailors are able to keep their keels closer to the "verge of stall", usually without ever letting them stall. The verge of stall is usually the point where the maximum lift-to-drag ratio is found.

Rudder angle is the primary indicator for monitoring the lift of your keel. A mushy or leeward rudder angle is a symptom of a stalled keel, or a keel that is not generating enough lift. Many people get so distracted during the race that they forget to feel the rudder pressure. They then haveno way of knowing what the underwater surfaces are doing. They inevitably end up outside the "groove" for long periods of time.

If your boat has a wheel, it is important to have some indication of the rudder angle. Most boats sail with an optimum rudder angle of about three degrees.

Sails
Sails provide power by creating lift and drag. We can alter the sail's lift-to-drag ratio through sail controls.

Light air
In light air the goal is to maximize lift and minimize drag. Simply put, deeper sails tend to create lift faster then than drag, but if they get too deep the leeward aft edges start to stall. We monitor this stall with leech tell tails and visual clues such as distance off the spreader, and top batten angle. Increasing the angle of attack such as raising the traveler or sheeting the Genoa increases lift faster than drag, but if you go too far, the wind will strike the leading edge of the sail at too wide an angle and the leading edge will start to stall. When the leading edge stalls, the entire leeward side of the sail is vulnerable to stall. That is why most sailing handbooks suggest sailing with the windward tell-tail just lifting. In light-to-moderate air, the best sail trimmers are very good at keeping their sails at the verge of stall.

Heel
Lift generated by upwind sails is not directly oppostite of the lift generated by the keel. The degree to which the lift is not directly opposed is vectored into forward motion. As the wind increases, there comes a point where the opposition of these forces create heel. Too much heel is disastrous to performance for a multitude of reasons, but let it suffice that too much heel increases drag and reduces lift in both keels and sails. It's nature's way of equalizing the forces.

Reducing Heel
Centerboard boats can reduce heel by reducing their exposed underwater surfaces. Keelboats do not have such flexibility. Heel has to be controlled through minimizing the lift and drag that the sails produce. As it turns out, flat sails reduce lift and drag fairly effectively. Decreasing the angle of attack reduces both lift and drag. A flogging sail, however, creates almost no lift at all, but it generates an large amount of drag. Great sail trimmers are able to flatten their sails and maintain an appropriate angle of attack, but they rarely if ever let their sails flog. Their goal is to minimize drag and maintain the appropriate amount of lift suitable to the boat's ballast. It is better to have a little too much heel and keep it consistent than it is to over heel and then over flatten over and over again.

Feathering
As it gets windier, the force of the sails continues to overcome the boat's ballast. Instead of lift pushing the boat abeam to leeward, the drag on the already flat sails becomes more influential. Drag wants to push the boat aft and to leeward. This increases the angle of attack of the keel. Somehow the excessive lift of the keel has to be relieved in order to control heel. Great helmspeople can feather their keels at extreme angles of attack so that a portion of the windward side of their keel becomes stalled. Too much stall and speed will diminish. Not enough lift will be produced and the bow will drop off to leeward. The sails will overpower the ballast. The boat will want to spin up into the wind. The result is usually a series of minor broaches as the boat cycles through successive stalls and overpowerments.

A Delicate Balance
Great sailors maintain a steady angle of heal in overpowering conditions because they monitor how much of the keel is stalled and the distribution of power across the sails. They concentrate on rudder angle and heel. If the keel wants to lift too much they experience too much rudder angle. The boat will want to climb into the wind. If they don't intervene by easing the main, the rudder will stall and the broach cycle starts anew. If they ease the mainsail too far, the boat is thrown out of balance and the jib pulls the bow to leeward. The cycle starts again. If the keel begins to loose too much lift they sense it early because the rudder suddenly feels mushy. They can ease the sails quickly and head off to reattach the flow around the keel.

In extremely heavy air, great sailors are constantly adjusting their sails as they attempt to delicately balance their sails over a partially stalled keel. The groove becomes almost impossibly narrow because the immense forces transform the rudder into an increasingly impotent tool. Teamwork is put to the test, as changes in course become impossible without sail adjustments.

Low and Slow
If you ever find yourself low and slow there is a high probability that something is stalled. The challenge is to find out which foil or foils is the culprit, and find a way to reattach the flow.Performance Articles page 2

 

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