If you’re new to sailing, you might be wondering what that wing-like object is doing attached to the bottom of your sailboat. Well, that would be called the keel and it serves some important functions so that your sailboat can… sail!
So what are the different types of sailboat keels? The most common types of sailboat keels include full-length, fin, short, wing/bulb, bilge, and centerboard keels.
There’s a lot of useful knowledge when it comes to understanding keels since they take such a considerable role in making a sailboat a sailboat. Knowing the different types of sailboat keels can really make a difference when heading out on the water for a nice sailing session.
Quick Keel Knowledge
Having a sailboat without a keel means you don’t have a sailboat. As a matter of fact, when a sailboat is first constructed and tipped on its side to attach the keel, or laying the keel, this is considered the “birthday” of a sailboat and thus the initial time of construction.
One of the more interesting facts about keels is they find their origins all the way back in ancient China, which allowed them to expand their naval presence all around the world. Of the different types of keels discovered were the use of adjustable centerboards and bilge keels.
Now, a keel is an important part of a sailboat because it provides two very important functions:
- Converts the sideways force of the wind into
- Provides ballast (i.e., keeps your sailboat from tipping).
We’ll get into these important functions of a sailboat’s keel later on. For now, let’s dive into the different types of sailboat keels!
Different Types of Sailboat Keels
There are several common setups for sailboats when it comes to keels. Even though these types of keels differ from one another, they ultimately serve the same purposes. However, it’s important to differentiate them based on use cases and other unique characteristics.
1. Full-Length Keel
A common type of sailboat keel is a full-length keel, which uses length rather than depth to provide a sailboat with a proper amount of life and ballast. When it comes to the location of the rudder on a sailboat with a full-length keel, it’s often attached to the aft (rear) of the keel.
There are several advantages of having a full-length keel. One is that they’re well known for keeping a straight and steady course more easily due to the length and surface area, which makes for a relatively comfortable ride. Also, if your keel runs aground (the keels touches the sea floor), there’s less likely to be a lot of damage due to a spread out the load.
However, there are some disadvantages with a sailboat that has a full-length keel. Due to the advantage of it staying straight and steady more easily, it’s also slower to turn (tack) when the rudder is moving. This means you’ll have a longer delay between the turn of the rudder and the turn of the sailboat. Also, sailboats with a full-length keel are a bit slower than others due to the larger surface area dragging against the water.
2. Fin Keel
Another very common type of sailboat keel is a fin keel, which you’ll most likely find on more modern sailboats. A fin keel has less surface area touching the water when compared to a full-length keel due to it taking on the shape of a… well a fin! This fin, however, sticks out of the bottom of the boat.
Just like the full-length keel, there are some advantages to having a fin keel on your sailboat. The most obvious one when sailing on a boat with a fin keel is the speed since the fin keel has less surface area and a better airfoil shape just like a wing on an airplane. A fin keel also provides for a faster response when tacking, which is great if you need to turn quickly.
There are also some issues with having a sailboat with a fin keel. One is that if you have high powered winds hitting your sails, it’s likely you’ll feel the tilt of your sailboat relatively more than with a full-length keel. Also, since the tacking is more responsive, you’ll need to spend extra attention on the track of the sailboat as you sail along. With a fin keel, you definitely need to be on high alert at times.
3. Wing/Bulb Keel
Wing/bulb keels are another type of sailboat keels that can be found. As the name suggests, they take the shape of a wing at the very bottom of the keel and also can have a fat bulb centered at the middle-bottom of the wing. These types of sailboat keels are more often found are longer and heavier boats.
A boat that uses a wing or bulb keel has several advantages that other boats don’t have. For one, it provides a boat with greater efficiency due to a modified water flow that benefits the forward motion of larger boats. It’s also relatively smaller in terms of surface area and quite hydrodynamic, so there’s reduce friction between the keel and the dragging water.
Similar to the fin keel, the wing or bulb keel has disadvantages related to having to spend greater attention when at the help due to a more responsive tack as well as being susceptible to heavy tilting during relatively strong incoming winds.
4. Shoal Keel
The shoal keel is a type of sailboat keel that’s virtually the same as a fin keel, only it’s a bit more shallow. Due to it being less deep than a fin keel, it has the advantage of being better able at maneuvering around shallow areas. However, since it does have a smaller surface area it’s much more likely to tilt easier when the wind starts to pick up.
5. Bilge Keel
Bilge keels are a special type of sailboat keel due to them coming in pairs. When a sailboat has a bilge keel, there will be two fins that stick out at the same angle from the bottom of the hull. Think of the fins of a shark how they stick out at an angle from the bottom of the body.
There are some nice advantages of having a bilge keel on your sailboat. For one, they allow your sailboat to traverse through shallow water without going aground. Also, if you let the boat dry out on land, it’s very convenient to simply let your sailboat sit on the fins of the bilge keel.
Some of the disadvantages of having a bilge keel on your sailboat include being difficult to free if ever aground since they might get stuck and not being as effective in reducing sideways slippage (leeway) under water. They also produce a lot of drag due to having a large wetted surface, which makes it difficult to sail at greater speeds.
Centerboards are another special type of sailboat keel because they can be lowered and raised mechanically. Being able to extend or retract a sailboat’s keel means your sailboat’s less likely to run aground, which makes it a fantastic option for coastal and offshore sailing.
One of the main advantages to having a centerboard is having the ability to retract it when entering shallow water, removing the chance of running aground. Since you can effectively increase or decrease the surface area of your keel at will, you’ll be able to reduce draft and wetted surface resulting in finer control of the lateral resistance of your keel.
However, there are a few downsides to having a centerboard compared to virtually every other type of sailboat keel. One issue is that they’re rather difficult to maintain. Another is that they have a ballast that is closer to the core of the boat resulting in the need to increase your sailboat’s ballast displacement. All of this ends up being a headache!
Keels Move a Sailboat Forward
How a keel is able to convert the force of sideways winds to forward motion is based in the science of hydrodynamics, similar to how airplane wings work based on aerodynamics. Essentially what happens is:
- The wind hits the sails at an angle
- The wind force causes the sailboat to tip
- The keel resists the lateral part of the wind force
- Energy from the angle portion of the wind force transfers to forward motion
This explanation is pretty basic, but should hopefully provide a clear insight into how a sailboat is able to produce forward motion thanks to the keel.
One of the most important purposes of a keel is to provide ballast for a boat. Essentially, sailing ballast is the ability to resist the lateral forces of the wind based on weight and size of the keel. As a matter of fact, this is a major reason why step 4 in the above list is able to produce forward motion.
Now, most keels are known to be made out of cast iron, steel, lead, and even concrete due to these materials being high in density. Back in the day during “olden times”, decent keels were made out of stone and sand.
When a keel has too little ballast, a sailboat tends to tip (heel) too much during high winds. However, if the keel has too much ballast, the sailboat is at risk of capsizing. That’s why a proper amount of ballast is required for light and heavy winds.
There are multiple ways of creating ballast on a sailboat. Apart from having a high-density keel, your sailboat could have a water ballast which allows you to adjust a certain amount of water sitting in your hull. Another way is “live ballast”, which pretty much means excess weight on the sailboat produced by us sailors.
Positioning dense weight below the hull to produce good ballast is important, which is why keels are the perfect place to put it. In the end, proper ballast provides stability and speed for all types of sailboats and is an essential function of a keel.