Tuesday Morning Tactician – The Shape of Speed

Tuesday Morning Tactician

Round Is A Shape

To know sail theory, it’s important to understand to aspects: lift and flow. Flow is the movement of air over the sail that generates lift, which is the force that causes the boat to move. Modern day sails are shaped like an airplane wing; basically, an airfoil that changes the air pressure of the wind as it flows over the surface of the sail. Air on the leeward side of the sail must travel further and faster than the air on the weather side of the sail to meet at the back of the sail at the same time in order to maintain pressure balance. This is known as the Bernoulli Effect, where an increase in the wind speed on the leeward side of the sail causes the pressure to drop, effectively “sucking” the sail forward (with assistance from the centerboard or keel).

Size Matters

The sail is the boat’s motor. To maximize the horsepower available when sailing in different conditions, you must change the draft, or depth, of the sail. The optimal draft position and size may change with the conditions. A deep draft leads to more power and accelerations, whereas a shallow draft leads to higher speed and higher pointing. Generally, speaking, draft is created by sewing flat panels of cloth together using tailoring and/or broadseaming. Tailoring means a flat sail is built with a curve in the luff, like a windsurfer sail, whereas broadseaming means the panels themselves are cut with curves on the connecting seams, like a Thistle sail.

Control Issues

The draft of the sail is adjusted by adjusting one or more of the “controls” on the boat. While they may vary from boat to boat, the core controls remain the same:

  • Outhaul: The outhaul is one of the more basic of all the controls. It is used to control the depth of the draft in the lower 1/3 of the mainsail. While possible to adjust while racing, this control is generally set prior to the start of a race and remains static unless there is a significant change in the conditions during the race. To flatten the mainsail, the outhaul is pulled tight and, vice versa, to create more depth the outhaul is eased.
  • Cunningham: The cunningham is used to control the location of the draft. As the wind increases, the draft in the mainsail moves towards the back of the sail. Since the draft should be somewhere between 40% and 45% behind the luff of the sail, the cunningham is used to pull the draft back forward.
  • Boom Vang: The boom vang is used for two purposes, to maintain the shape of the mainsail when eased and to induce mast bend. By maintaining the shape of the mainsail, the sail can be eased without powering up and trimmed without depowering. By inducing mast bend, effectively depowering the sail.
  • Pre-bend: The bend in the mast can induced before adjusting any other controls, by making adjustments to rig tension and mast butt placement to pull the bottom ½ of the mast forward toward the bow. Pre-bend is needed when a flatter sail is needed.
    Sheets: The sheets are a great way to make gross adjustments to the mast bend and draft.

Play The Slots

The headsail offers little in terms of control, however this does not mean the sail shape is without adjustments. An important aspect to consider when adjusting the shape of the headsail is the “slot, or the space between the headsail and mainsail. The slot should be open to allow proper flow to be achieved by the mainsail. This is achieved by adjusting the leads and sheets to maintain a uniform distance between the headsail and the mainsail up the entire length. If the slot is too open the jib is reaching maximum efficacy, whereas if the slot is too narrow the flow over the mainsail is cut off.
One way to control the slot is by adjusting the leads, or headsail sheeting blocks, fore and aft. When the lead is moved aft, the bottom of the sail is pulled towards the stern which flattens the sail and opens the leech. When the lead is moved forward, the bottom of the sail is pulled down which creating more depth in the headsail and closing the leech.

Gearing Up

Changing gears while sailing is like shifting gears in a car, meaning the conditions and situations are always changing, and sails need to be adjusted accordingly the maximize the horsepower available. A racing sailboat generally has a 5-speed transmission:

  • 1st Gear: This gear is used when the boat needs to starts moving, with the sheets way eased and lots of twist induced at the top of the sails.
  • 2nd Gear: This gear is used when the boat needs to accelerate, such as in the last seconds of a well-executed start, right after a tack, or driving through a wave set. This is when the mainsail is eased slightly with no overbend wrinkles and the headsail is out a couple inches.
  • 3rd Gear: This gear is used when the boat needs to travel at its top speed, requiring the flattest draft allowed by the conditions, but not necessarily the tightest trim.
  • 4th Gear: This gear is used when the boat needs to travel at its top speed and point high. To achieve this, the mainsail is trimmed in an extra 1 to 2 inches with lots of overbend wrinkles.
  • 5th Gear: This gear is used in heavy air, when the boat needs to depower. To achieve this, all the control lines are pulled tight and maximum mast bend is induced to create the flattest draft possible. The headsail may be lead out or back to open the top of the sail.

Call To Action

Putting this knowledge into action is critical to getting the most horsepower out of your boat the at the conditions warrant. Each condition may call for an adjust to one or more of the controls.

  • Light Air (0-6 knots): Let’s just say that light air lack the proper motivation, and will not be able flow over the sail if the draft is too deep. Maintaining a lightly flatter sail in very light wind will make the boat move faster in the long run.
  • Medium Air (6-14 knots): Adjusting your sails in medium air is based upon a relationship between you and the other boats on the race course. If you feel underpowered, create more depth in the sails. If you are lacking height or top speed, flatten the sail. Continue to adjust the controls and sheets achieve a good balance between speed and height.
  • Heavy Air (14+ knots): This condition may lead to excessive heel, which is almost never good on a sailboat. Heeling creates an imbalance between the air, boat and water, which in turn impacts speed and pointing ability. In heavy air, the draft should be flattened to keep the heel of the boat consistent and manageable.

Wrap It Up

In summary, a sail’s shape creates flow which induces lift and generates forward movement. This shape is adjusting using the controls available and these controls should be adjusted to maintain a proper balance of speed and pointing ability. No one setting is perfect, as sailboat races are held on an ever changing landscape of wind and water. The top teams are constantly making adjustments to find new ways to make their boats go faster. It’s recommended that you look around at what the top sailors in your fleet are doing, talk to your local sailmaker, try new things, take notes and most importantly, have fun and accept the challenge.

Make it a great yacht, or not… the choice is yours.

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