Construction Of Racing Sails: Feet
Categories:
Skipper’s Corner | Sails
By Rod Carr
Proper sail feet are configured to improve performance over the range of wind and wave conditions that you encounter on the racecourse. The mainsail foot is especially important in maintaining boat balance and power through the variability of wind that is encountered during a day of sailing competition.
MAINSAIL FEET
If we analyze the impact of adjusting the clew outhaul in or out along the mainsail boom, as an example, we find that pulling the clew farther from the mast flattens the sail. But more importantly it changes the angle at which the wind exhausts the sail and the centerline of the boat. Since the bottom of the mainsail is large, and located a goodly distance aft of the boat's center, this adjustment can make a significant change in the amount of weather helm that the mainsail is forcing onto the boat. So rather than have to carry more rudder trim when the wind pipes up, one can simply flatten the sail a bit and get the same effect without damaging, speed-reducing drag. The flattening takes place in the lower third of the sail so it doesn't disturb the important power generating camber in the upper half of the mainsail, and it does not make a visible change in the twist that has been set in the sail by the mainsheet and vang.
For a mainsail to allow this quick and easy adjustment, it needs to be built with a minimum of seam-induced shape in and near the foot. Sails with all kinds of foot "shelves", vertical seams and so on below the bottom batten prevent flattening of the sail as described above. So to provide our customers with a sail amenable to optimization on the race course, CARR HT sails are typically constructed with broad seams at the batten positions from the head of the sail down, but not at the lowest batten position. This construction provides the controllable foot we need, and simultaneously increases the aerodynamic smoothness that is so important for efficient airflow over the sail. More seams means more parasite drag and less effective drive available to the boat.
When the wind drops, a quick adjustment of the mainsail clew toward the mast will reintroduce extra camber into the foot of the sail for an increase in power, and it will reintroduce that additional power down low in the sail. It will also add a tendency for additional weather helm that has been reduced by the reduction in wind speed.
JIB FEET
Smooth but adjustable jib feet are equally important to the overall performance of the boat. Optimizing the interaction between the jib and mainsail requires substantial control over the shape of the jib leech and the opening between them referred to as the "slot". The topping lift is the major control over jib twist and influences jib leech shape along its entire length. Fine-tuning of the leech shape is enhanced if the bottom third of the jib can be tuned to work effectively with the mainsail shape downwind of it.
Here a smooth sail foot with direct control of its camber by the clew outhaul is a benefit. Sails with too much "built-in" camber rapidly lose the ability to be adjusted. CARR HT jibs are constructed with smooth feet, with the bottom 1/3 of the sail uninterrupted by seams or battens. Rather than having to pull the jib in to balance the boat when the wind pipes up, adjusting the mainsail as described above reduces the new weather helming tendency and leaves the jib free to continue to develop full power. As with the mainsail, the smoothness of the lower jib area encourages aerodynamic efficiency without additional parasite drag or constraints on adjustability found in sails with complicated foot construction.
Proper sail feet are configured to improve performance over the range of wind and wave conditions that you encounter on the racecourse. The mainsail foot is especially important in maintaining boat balance and power through the variability of wind that is encountered during a day of sailing competition.
MAINSAIL FEET
If we analyze the impact of adjusting the clew outhaul in or out along the mainsail boom, as an example, we find that pulling the clew farther from the mast flattens the sail. But more importantly it changes the angle at which the wind exhausts the sail and the centerline of the boat. Since the bottom of the mainsail is large, and located a goodly distance aft of the boat's center, this adjustment can make a significant change in the amount of weather helm that the mainsail is forcing onto the boat. So rather than have to carry more rudder trim when the wind pipes up, one can simply flatten the sail a bit and get the same effect without damaging, speed-reducing drag. The flattening takes place in the lower third of the sail so it doesn't disturb the important power generating camber in the upper half of the mainsail, and it does not make a visible change in the twist that has been set in the sail by the mainsheet and vang.
For a mainsail to allow this quick and easy adjustment, it needs to be built with a minimum of seam-induced shape in and near the foot. Sails with all kinds of foot "shelves", vertical seams and so on below the bottom batten prevent flattening of the sail as described above. So to provide our customers with a sail amenable to optimization on the race course, CARR HT sails are typically constructed with broad seams at the batten positions from the head of the sail down, but not at the lowest batten position. This construction provides the controllable foot we need, and simultaneously increases the aerodynamic smoothness that is so important for efficient airflow over the sail. More seams means more parasite drag and less effective drive available to the boat.
When the wind drops, a quick adjustment of the mainsail clew toward the mast will reintroduce extra camber into the foot of the sail for an increase in power, and it will reintroduce that additional power down low in the sail. It will also add a tendency for additional weather helm that has been reduced by the reduction in wind speed.
JIB FEET
Smooth but adjustable jib feet are equally important to the overall performance of the boat. Optimizing the interaction between the jib and mainsail requires substantial control over the shape of the jib leech and the opening between them referred to as the "slot". The topping lift is the major control over jib twist and influences jib leech shape along its entire length. Fine-tuning of the leech shape is enhanced if the bottom third of the jib can be tuned to work effectively with the mainsail shape downwind of it.
Here a smooth sail foot with direct control of its camber by the clew outhaul is a benefit. Sails with too much "built-in" camber rapidly lose the ability to be adjusted. CARR HT jibs are constructed with smooth feet, with the bottom 1/3 of the sail uninterrupted by seams or battens. Rather than having to pull the jib in to balance the boat when the wind pipes up, adjusting the mainsail as described above reduces the new weather helming tendency and leaves the jib free to continue to develop full power. As with the mainsail, the smoothness of the lower jib area encourages aerodynamic efficiency without additional parasite drag or constraints on adjustability found in sails with complicated foot construction.