Everything You Need to Know About How a Keel Works

You’ve probably heard the phrase, “even keel”. Those two words summarize a vital function on a boat. Its primary purpose is to keep the vessel upright. You’ll see it on all types of boats from sailboats to cargo ships. The concept involves a little bit of physics and hydrodynamics to understand its role.

No one knows when sailing started. Early evidence from Chinese, Egyptians, and the Vikings tells us that these people knew the value of a keel and why it was necessary for safe and efficient travel. Some call it one of the profound advances in the history of sailing. Without it, sailors would keel over instead of getting underway.

The Functions and Design of the Keel

The keel sits on the bottom-most part of a boat in the center of the hull. Many refer to it as the backbone which you take literally and figuratively. The classic example is a wedge-shaped piece with a rounded tip, not unlike a fin. Fishes probably provided the inspiration for the design. Another common example is the full keel which runs the length of the vessel.

The keel serves two purposes. First, it keeps the vessel on a straight course without adding drag or resistance. It actually counters it from coming from the sides to tip over or heel a boat. Second, added weight or ballast within the structure keeps the boat vertical and helps right it if it capsizes, providing an additional counterbalance. You can think of it as the yin to the sail’s yang on a sailboat.

Together, these functions bring stability to the design. The fin keel is what you’d typically see on smaller sailboats. Their smaller size makes it easier to maneuver, especially in shallower waters and avoid grounding. They also allow for higher speeds because they have less impact on the water resistance. The thin, blade-like form lets them cut through the water with ease.

The full keel is something you’d see on cargo ships and larger sailing yachts. The size of the vessel dictates the need for a more substantial keel. You’ll also find them on ocean-going crafts too. As you may expect, it has a greater effect on drag and handling. However, it also brings stability to the forefront. That’s a significant advantage for boats traveling on big waters.

The keel is usually a part of the hull instead of being affixed to it. For larger vessels, it’s the first thing that manufacturers make. They’re typically made of metal which adds to the weight. The strength and durability of the materials are imperative, given the magnitude of the forces the keel encounters. The ballast ratio of a sailboat describes the percentage that it makes up in proportion to the overall weight.

The figure is an excellent indication of how likely the boat will tip. An average number is around 30 percent. Sailboats meant for ocean waters often get closer to 40 or even higher. That tells you that both the keel and ballast are substantial parts to a boat.

Other Types of Keels

The fin and full keel aren’t the only kinds of keels. Other designs serve similar functions with vessels that sail in different water conditions in boats with various purposes.

The bilge keel, for example, consist of three components. Together, they allow a boat to stay upright, not unlike a three-legged stool. You’ll see them in coastal areas to protect the craft during low tides. On the water, they provide additional stability and keep the vessel from rolling in rough waters. They also give a boost in speed if the boat is moving windward.

The bulb keel is a different riff on the fin type with an attachment for the ballast to keep the weight lower. You’d typically find them on racing boats that sail offshore. It balances the challenges of big-water boating with the speed of a fin model.

The wing keel is another kind you’ll see on high-performance craft. It resembles the tail of an airplane. It reduces your center of gravity for better leverage like the bulb type. It’ll also minimize draft and improve efficiency.

The fin keel with skeg adds a second element with an extension from the hull. The added section protects the rudder and propeller from damage. It helps keep the vessel on course. The downside is that it’s added weight and surface area which can slow the boat. While it reduces damage to other components, the gap between the two can pick up debris.

A power boat often has a skeg only on the hull which serves the same purpose. The idea is that it will deflect any floating objects in the water before it can get to the prop. If it gets damaged, the craft will have a noticeable wobble when underway.

The lift keel differs from the previous ones in that it’s not a fixed feature of the hull. Instead, you can retract it as necessary. It has the least draft of any type. However, the fact that it’s mechanical means a higher risk of damage to the device and hull. It can compromise handling if the conditions change suddenly.

A variation on this theme is the centerboard. It consists of a base with an opening that allows the blade or dagger to drop when needed. You’ll often see them on catamarans and other multi-hulled vessels that don’t have a traditional keel.

How the Keel Works When Sailing

The keel comes into its own when the sailboat is underway. There are a lot of forces at work, involving the structure of the craft, sails, and keel. The water and wind act as opposing elements. The wind pushes you forward at the same time as the water resists it.

The keel is making sure you’re not going sideways. The flat surface and its perpendicular position to the deck make it possible. When the wind pushes the sail in one direction, the keel goes the opposite way. The forces are matched even though the latter is smaller. The density of the water makes up the difference.

This element also supports tacking. That is the maneuver that sailors must use if they are trying to go upwind. Instead of taking a straight path with the wind behind the boat, they use a zigzag pattern. This action allows them to harness the wind and water energy to create forward thrust. The reason they go back and forth is that they must draw the two forces together to keep moving.

The keel and ballast help with this task when the sailboat heels. The former provides resistance while the latter is the counterweight. You can get a feel for how this works by knowing the angle of vanishing stability (AVS) of the sailboat. This figure describes the angle your boat can tip before it capsizes, and you start crunching numbers with a boat payment calculator.

The base number is 90, which is the horizon. Anything above it tells you how much it can go below it before you fall in the drink. For example, an AVS of 130 gives you 40 degrees before you’re going overboard. The number is also an indication of how quickly your keel and ballast will save the day. The higher it is, the faster the boat will right herself.

These figures speak to the many elements impacting the design of a sailboat versus a boat with an outboard motor.

Sailing is a challenging sport that requires physical and mental strength.Over 740,000 have fallen in love with its demands and rewards. The advances in design have made the sport safer and more enjoyable. It’s odd to think that so much rides on something that you’ll only see when it’s in dry dock. If anyone asks you, what is a keel on a boat, you can say it is its backbone.