Getting Up to Speed on Foils
By Lynn Fitzpatrick and Gino Morrelli
Chances are if you have spent most of your life cruising around on a sailboat, you know a bit about what makes sails work. Foils, canards and wings, however, may mystify you.
We’ve tried to simplify the principles for those of you who have a phobia for math, physics and applied sciences. What follows is enough to make you dangerous. Leave the applications of these principals to experts such as Morrelli & Melvin, who have rocket scientists, naval architects and engineers on staff.
Power Lifting with Bernouilli’s Theorem
Bernouilli’s Theorem is also known as the longer path explanation. A sail is an airfoil. Viewed in cross section, one side of the foil/sail is longer than the other. As the foil moves through air, the air splits at the leading edge (mast or headstay) of the curved airfoil. The air taking a longer path by passing along the backside/leeward side moves faster than the air passing across the front/weather side. The particles moving across the backside/leeward side of the sail are traveling farther in the same amount of time and they have a higher velocity, or speed, than the particles on the front side/weather side of the sail. They also have more room to spread out, forming a low-pressure area.
On the front side/weather side of the sail, the slower air particles are packed together more densely, creating a higher-pressure area. This difference in the pressure on either side of the sail creates a suction force, producing lift/power that makes the boat move forward… therefore sailing sucks!
Rudders, Keels or Daggerboards Create Side Force
All boats have rudders and keels or daggerboards that are essentially vertical airfoils underwater. The symmetric rudder creates lift by turning slightly relative to the flow, also called “angle of attack”. This slight rotation creates just enough difference in the distance the water has to travel around one side of the foil as compared to the other that it creates the same effect as air passing around a sail. The more we turn the rudder, or increase angle of attack, the greater differential in distance that water has to travel around one side versus the other, the more power we get and the faster we turn.
The primary job of a keel or daggerboard is to create enough equal and opposite side force or lift to counteract the forces created by the sails aloft. Sails push one way and the keels/daggerboards push the other way, and essentially spitting the boat forward like a watermelon seed pinched between your fingers!
In a traditional monohull the keel has to do the other equally important job of providing stability by suspending a ballast down low to counteract the heeling/tipping forces caused by the sails aloft. Rather than rely on deep and/or heavy keels, cats’ and tris’ stability comes from placing the hulls far apart.
Lifting Hydrofoils Push Boats Up
The same basic principles that apply to airfoils/sails, apply to all underwater hydrofoils. Rudders are symmetric vertical hydrofoils that turn the boat. Lifting hydrofoils are mounted horizontally and try to lift the boat. A typical asymmetric lifting hydrofoil on a boat is positioned similar to a wing on an airplane with the shorter/flatter side down and the curved side up. Because hydrofoils are much thicker than sails and the density of water is about 800 times the density of air, they are way more powerful for the same surface area.
Hydrofoil assisted boats float on the water while at rest. At low speeds they are not necessarily any faster than boats without hydrofoils. As speed increases, the hydrofoils generate more and more lift and push the hulls higher and higher out of the water. The hull is sometimes completely supported above the water surface by a strut. Free of the drag caused by the hull, a hydrofoil boat can skim over the surface of the water at over two times the speed of boats without hydrofoils.
Curved Lifting Hydrofoils do Both
Both the BMW ORACLE trimaran and Alinghi catamaran use curved daggerboards (hydrofoils) to provide necessary lateral lift/resistance and they also lift the leeward hull up. The same board is used to reduce sliding to leeward or leeway and also provide vertical lift. The current variations that are being experimented with by both teams, tradeoff more vertical lift for less lateral lift or vice versa. Upwind these boats still need a fair amount of lateral lift/resistance to prevent excess leeway due to their enormous sail plans and the fact that they essentially are always going upwind! These boats sail at apparent wind angles downwind that are only about 10 degrees more than upwind! Downwind the sails are never really eased and the apparent wind never goes very far aft… the wind is always in your face on these boats! The apparent wind is so far forward on these boats that you always feel as if you are going upwind.
Combining both jobs, lateral resistance and lifting the boat into one board simplifies the boat but complicates the parts. Both teams are playing with variations of “J” or “L” boards, constant arc boards or even crazy looking “S” boards. Each shape has its slight advantages and disadvantages. They are also playing with different foil section shapes, which can produce better results at different speeds.
Racing sailors are always looking for the optimum combination of maximum lift for minimum drag. Both America’s Cup teams can raise and lower the boards to vary the surface area. They can seek more advantage by jibing them (changing angle of attack relative to the boat) and also rocking them fore and aft. Doing this, however, requires complex hull and board casing construction.
The buildup for the America’s Cup is a time for research and development, design and testing. Each team will have a matrix of foil combinations optimized for forecast wind and sea conditions. Their racing arsenal is much larger than that needed for cruising and performance/cruising multihulls to sail fast, comfortably and safely across a range of conditions.
The Morrelli & Melvin team is currently including curved lifting hydrofoils on our new Nacra F20 Carbon design and curved lifting hydrofoils will be appearing shortly on other new and exciting toys we are working on. We’re participating in BMW ORACLE’s America’s Cup developments and conscientiously incorporating design enhancements into our recreational and commercial power and sailboat designs.