What Is a Sailboat Keel and How Does it Work?
Keels are an essential part of any boat's design and determine handling properties, durability, and comfort. To understand why they matter, we must understand how they work first.
The Basics of Sailboat Keels Explained
What is a keel?
The keel is a structural member of a boat which runs length-wise across the hull from front to back. It is the lowest part of any hull and the backbone of a ship or boat. Sailboats use deep-running keel designs which extend out of the hull. They can be made from timber, metal, or fiberglass.
What does a keel look like?
Sailboat keels typically look like a long fin, or flat blade, at the bottom of the hull, which runs down for about 1.5 meters. While most modern sailboats use fin keels, the full keel is integrated into the hull, resulting in a more triangular shape that is shallow at the front and deeper at the back.
Since keel designs differ, the exact shape also differs. Here is a clear diagram showing the differences between different sailboat keel designs:
Where is the keel on a boat located?
The keel is located in the center of the bottom of the boat's hull, combining both hull sides. Sailboat keels extend out from the hull downwards. Depending on the exact keel design, the extended part can run from stem to stern, or just narrowly in the center of the boat's hull.
What does a keel do?
Keels help boats maintain course by countering the current and wind. They also stabilize boats by reducing roll and heel angle. Roll is the tendency of boats with round hulls to roll over. Heeling is the tendency of sailboats to heel due to the force of the wind on the sails.
How does a keel work?
Keels provide sideways resistance to reduce slippage. Sailboat keels usually carry iron or lead ballast in their tip to act as a counterweight to the wind's force on the sails, which reduces heeling. The keel's length acts as a lever, increasing the ballast's effectiveness and improving its stabilizing properties.
In other words, the deeper the keel runs and the heavier its ballast is, the more stable the boat will be.
Slippage simply means falling of course due to the current and wind.
The keel ballast lowers the center of gravity of the boat and counteracts the buoyancy of its displacement hull. As the boat tilts, the center of gravity and center of buoyancy move downwards, reducing the heel angle. This is called righting moment. More righting moment results in less heel angle.
Keels also increase the wetted surface of a boat. Wetted surface is simply the amount of surface underwater. The larger the wetted surface, the more resistance there is. Sailboats are among the boat designs with the largest relative wetted surface. A larger wetted surface increases sideways resistance which reduces slippage to leeward, helping to maintain course.
The Importance of Sailboat Keels
Do all sailboats need a keel?
All sailboats need a keel. Without it, a sailboat would very easily capsize due to a combination of hull design and wind force on the sails. Sailboats have displacement hulls, which tend to roll easily. Wind force is multiplied through the mast, which acts as a lever, and could capsize the boat if it didn't had a keel.
The displacement hull design is also used on canoes, and if you've ever tried to enter one a bit too quickly, you'll know how quickly they roll over.
However, there are sailboats that use a different hull design, which reduces the need for a keel or enables them to use alternative keel designs.
Do all sailboats have a keel?
Nearly all sailboats have some sort of keel. However, not all sailboats have a typical, deep-running fin keel, since not all need one, and in some cases, a shoal draft is desired.
For example, some sailboat designs use leeboards or removable keels, like daggerboards, which enable a boat to enter much more shallow waters.
What is a boat without a keel called?
Boats without a keel are called sailing dinghies. These small boats (under 20 feet) use a flat-bottomed hull design to stabilize instead of a keel. This makes them useful for entering shallow waters and beaching.
Examples of sailboats without a typical keel
- Barges, which use flat-bottomed hulls to reduce heel
- Sailing dinghies, which have a flat-bottomed hull and no keel
- Sunfish and lasers, which often use removable daggerboards
Two examples of sailboats with alternative keels are Dutch barges and Cornish Crabbers. Dutch barges use leeboards, while the Cornish Crabber uses a swing keel, which can be lifted or lowered according to water depth.
Sailing dinghies, sunfish, and lasers typically use no keel or come with daggerboards, which can be removed. They capsize easily and often, which is why you'll see so many toppled sailboats drifting across the lake at sailing camps.
Can you sail without a keel?
You can sail without a keel, however, you could easily capsize and would experience a lot of roll and heel. To prevent capsizing, you would need a different hull design. Historic sailboats without keels use flat-bottomed hulls to prevent excessive roll and capsizing.
The Dutch barge design actually uses a very flat bottom instead of a full round displacement hull, which makes it a bit faster, stiffer, and counters heel just by the hull design alone. But even these sailboats require some sort of stabilization, which is provided in this case by their leeboards.
Can a sailboat rest on its keel?
Most keel constructions can support the weight of the sailboat, but depending on the design, proper precautions need to be taken. Boats resting on a fin keel or full keel will require additional support. The bilge keel (or twin keel) is specifically designed to rest upright in tidal waters.
Fin keel sailboats can sag over their length over time. Other keel designs spread the weight over a larger surface, like the full keel.
Keel Design Basics
The most common sailboat keel types
There are dozens of keel designs and variants out there. The most common ones are the full keel, fin keel, and centerboard. Each of these designs has different properties. Full keels handle better in rough conditions, while fin keels are easier to maneuver in and out of slips.
Discussing them all here would be beyond the scope of this article, but I've described the most common keel types and their properties in my illustrated guide. If you want to learn more about the different designs, I recommend you read that.
How deep is a keel?
On average, keels are between 1 and 2 meters deep, in order to get through the surface drift layer of the water. Some keel designs run less deep, like full keels, which require less depth due to increased weight and wetted surface, or alternative designs like leeboards, which only touch the water's surface.
The water's surface drift layer is the layer of water directly affected by the wind.
How heavy is a sailboat keel?
Most sailboats carry around 35-40% of their total weight in their keel. With an average cruising sailboat weighing 15.000 pounds, the average keel weighs between 5.000-6.000 pounds. However, the total required weight is not just determined by sailboat weight, but also by sail area and the length of the keel.
Deeper keels provide more leverage, reducing weight need. A larger sail area increases weight requirement.
What material are sailboat keels made of?
Sailboat keels are typically made of reinforced fiberglass layers, steel, or wood, depending on the hull material, boat model, and build year. Keel ballast in the tip is usually made of lead, iron, concrete, or even water.
How are keels attached to boats?
Some keels are integral to the hull's structure, like the full keel or modified full keel. Other keels are bolted onto the hull, like the fin keel, wing keel, or bulb keel. With this design, the top of the keel slides into a vertical keel box inside the hull and is bolted in place using keel bolts.
What is a skeg keel?
A skeg keel is a type of rudder design in which the rudder is integrated into the keel design, for example with full keels or modified full keels, therefore providing protection against damage from collisions and waste.
What is the chord of a keel?
A chord is a term used to describe the length of any edge on a keel, whether it's the leading edge, trailing edge, or top edge connecting to the hull. The top edge is referred to as the 'root end', while the length of this edge is called the 'root chord'.
The difference between the rudder and the keel
While the keel and rudder design can overlap, the rudder is generally the most aft part, while the keel sits in front of the rudder. The rudder is used to set a course, while the keel's function is to provide directional stability and prevent roll.
The difference between the keel and the hull
While in some keel designs the hull and keel are integrated, like, for example, full and modified full keels, with other designs, the keel is bolted onto the hull, like the case with fin keels. The hull is the bottom of the boat, while the keel is the fin below the bottom of the boat. The hull provides buoyancy, while the keel provides stability.
Keel replacement cost
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