How Does a Sailboat Keel Work?

Sailboats have the unique characteristic that other boats don’t have and that’s being able to generate forward motion through water without using an engine. While this may seem like an obvious statement, the reason this non-combustible moving force is possible is due to a unique combination of parts of a sailboat, more notably the keel.

With a keel, a sailboat has the ability to resist certain forces initially kicked off by the wind, ultimately resulting in the sailboat cutting through the water.

So how does a keel work? A keel converts sideways force on the sailboat by the wind into forward motion and it provides ballast (i.e., keeps the sailboat from tipping). By canceling out the perpendicular force on the sailboat originally caused by the wind hitting the sail, the only significant leftover force produces forward motion.

Learning about how a keel works, in my opinion, is one of the most fascinating aspects of learning how to sail because, in the end, you truly get a solid understanding of what makes a sailboat a sailboat. Whether your a science geek (like me) or just want to learn at a basic level how a keel works, this article’s for you!

Quick Intro to the Keel

If you’re anything like me, sometimes it’s nice to get a quick refresher into the basics of a topic before diving into the meat. When it comes to learning about how a keels works, it’s a good idea to run through some basic knowledge of what a keel is in the first place.

To put it simply, a keel is a symmetric, wing-like object that’s attached to the bottom of a boat’s hull. While sailboats require a keel to function properly, powerboats don’t necessarily need them since they produce their forward moving force through combustion.

There are a number of different types of sailboat keels depending on the type of sailboat you have. Some of the most popular types of sailboat keels include:

  • Full-length keel
  • Fin keel
  • Wing or bulb keel
  • Shoal keel
  • Bilge keel
  • Centerboard

These different types of sailboat keel have a number of advantages and disadvantages. Based on how you like to sail and which environments you’ll be sailing in, you’ll want to choose the right keel for you.

How a Keel Works

A keel on a sailboat really comes in handy whenever your in a point of sail that doesn’t include running. When you’re sailboat’s in the running point of sail, the wind is coming from behind making the keel a tad bit useless.

However, when sailing under any other point of sail, especially ones heading towards the wind, a keel is absolutely necessary. The main reason for this is because the keel keeps the sailboat from tipping by resisting the lateral force on the sailboat by the wind hitting the sail.

Let’s take a step back and think about a simple example of when your sailboat’s sail caught some wind and began moving forward. Say you’re in your sailboat and you tighten your sails just right so they’re catching the wind without your sails luffing. What follows is:

  1. The wind hits the sail at an angle.
  2. The sail attempts to resist the sideways force and transfer it to a force parallel to the sailboat/hull.
  3. The keel assists the sail in resisting the sideways wind force, ultimately canceling it out.
  4. The sailboat tilts a bit because the cancelation of lateral force isn’t 100%.
  5. The leftover force is the parallel wind force produced by the sail, resulting in forward motion.
  6. The hydrodynamics of the hull and keel allow the sailboat to cut through the water efficiently.

There’s a bit of handwaving going on here, so let me expand on this in more detail.

The wind hits the sail at an angle which means the sail is being pushed around, so it wants to get rid of some of that energy and put it to use. Since a sail acts like a wing due to its shape when being filled with wind, it attempts to transfer it into a propelling force that’s parallel to the sailboat. Think about rolling a marble around a curved object. That marble is being transferred somewhere else.

Now, if your sailboat only had to depend on the wind hitting the sail, your sailboat wouldn’t only move forward move but also at an angle. This is because there’s no other force actively resisting the perpendicular component of the force of the wind. Maybe the keel can help us out!

Unlike the sail, the keel is in a fixed position which is not by accident. Since the keel has a consistent position and shape, has a large surface area, and is made out of a heavy, dense material, the keel is the perfect candidate to fight against being dragged sideways through the water. The path of least resistance for a sailboat is forward and we can thank the keel (and hull) for enforcing this rule.

With the two powerful and opposite lateral forces between the wind hitting the sail and the water pushing the keel, these forces virtually cancel each other out. The only force left over is the wind force that was transferred by the wing-like sail in the parallel direction resulting only in a force that produces forward motion.

By having a hull and keel that are well-shaped, clean, and smooth, your sailboat will be able to more easily cut through the water due to reduced friction, which allows for more of that forward motion energy to be used to… move forward! That (among other things) is a good reason to keep a clean, healthy hull and keel when out sailing.

When You Need and Don’t Need the Keel

As I mentioned before, there are more occasions when you need the keel as opposed to when you don’t. When it comes to the majority of the points of sail, having a keel serves a very important role in ensuring your sailboat’s able to move forward effectively. Also, if you own a powerboat, you might actually not even need one.

When it comes to sailboats, we’ve already learned why your sailboat needs a keel. However, if you’re in the running point of sail, it’s not necessarily essential. Considering that for hundreds of years sailboats were sailing around the world without keels, including during the times of the ancient Egyptians, Greeks, and Romans, we know that it’s possible at least in downwind situations.

Unfortunately for these ancient civilizations, if they ever found themselves having to travel upwind then they’d have to get their oars out and start rowing. Picture those huge, pirate-looking ships with a bunch of square sails and rows sticking out the side of the ship. I’m glad someone discovered the use of keels!

When it comes down to it, you want a sailboat that has a keel and, unless you’re in the market for an 18th-century wooden sailboat, you’ll have one of several types of keels on your sailboat.

Avoid Running Aground

When sailing away, it’s important to realize the depth of the water so as to avoid the keel from running into the ground. When this happens it’s called running aground and is one of the more embarrassing situations a sailor can experience. If a sailor tells you they’ve never run aground, all they need is a little more time out on the water and it’ll surely happen!

Regardless of the type and size of a sailboat, running aground can be damaging to a sailboat’s keel. Depending on whether your keel rubs against mud, sand, or other materials, the outcome can be either “meh” or “uh oh”.

When running aground, your keel can become damaged to the point of losing ballast, reduced hydrodynamics, and potential leaks. This is why it’s extremely important to be aware of the depth of the water wherever you are now and plan to go, but sometimes things just pop up out of nowhere. Paying close and constant attention to charts, detectors, and markers will help ensure that you’re on top of knowing the depth of water at any given moment.

Related Questions

When were keels invented? The use of keels has been traced all the way back to the Chinese Song dynasty (960-1270 AD) through the use of centerboards. During a similar period, it’s been discovered that Norwegian Vikings used keels that resemble more closely to what modern sailboats use today.

Do catamarans have keels? A catamaran is a sailboat that has two hulls, as opposed to a standard monohull sailboat, which results in having a wide beam producing an increased level of stability that does not require them to have a keel.