A bulb keel is a keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

, usually made with a high aspect ratio foil

Foil (fluid mechanics)

A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag...

, that contains a ballast

Sailing ballast

Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...

-filled bulb at the bottom, usually teardrop shaped. The purpose of the bulb keel is to place the ballast as low as possible, therefore gaining the maximum possible amount of lever

Lever

In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...

age and thus the most righting moment

Moment (physics)

In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...

. A example of a class of boats that use a bulb keel is the International 110

International 110

The International 110, often just called the 110 is a one-design racing sailboat designed in 1939 by C. Raymond Hunt. The 110 was revolutionary for its time, and it is still raced, and has a competitive Portsmouth yardstick number of 89.3.-Design:...

 racing class, which uses a 300 lb (136 kg) cast iron bulb keel on a boat whose minimum racing weight is 910 lb (414 kg).

Since bulb keels work best on long, thin keels or daggerboard

Daggerboard

A daggerboard is a retractable centreboard used by various sailing craft. While other types of centreboard may pivot to retract, a daggerboard slides in a casing. The shape of the daggerboard converts the forward motion into a windward lift, countering the leeward push of the...

s, they are generally not used on sailboat

Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...

s intended for shallow waters, but are most often found on offshore racing craft. Daggerboards built like bulb keels are often referred to as "lifting keels", and they can be retracted into the boat to reduce the draft, and to allow the boat to be loaded onto a trailer. Lifting keels are also more likely to be found on craft built for speed, as a winch

Winch

A winch is a mechanical device that is used to pull in or let out or otherwise adjust the "tension" of a rope or wire rope . In its simplest form it consists of a spool and attached hand crank. In larger forms, winches stand at the heart of machines as diverse as tow trucks, steam shovels and...

 or a set of pulley

Pulley

A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

s is required to provide purchase to lift the heavy keel.

Some boats with lifting keels go so far as to allow the keel to be canted side to side, allowing the bulb to be placed to windward to reduce heel in high winds; this type of keel is called a canting keel

Canting keel

A canting keel is a form of sailing ballast, suspended from a rigid canting strut beneath the boat, which can be swung to windward of a boat under sail, in order to counteract the heeling force of the sail...

, and is used in the newest Volvo Ocean Race

Volvo Ocean Race

The Volvo Ocean Race is a yacht race around the world, held every three years. It is named after its current owner, Volvo...

's Volvo Open 70

Volvo Open 70

The Volvo Open 70 is the current class of racing yachts designed for the Volvo Ocean Race. It was first used in the 2005-2006 race and use a Canting keel which is capable of canting transversely up to an angle of 40 degrees.According to the VO70 rule, the yachts can be made...

 class yacht

Yacht

A yacht is a recreational boat or ship. The term originated from the Dutch Jacht meaning "hunt". It was originally defined as a light fast sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into the shallow waters of the Low Countries...

s. It is also possible to shape the bulb into a horizontal foil, called a winged keel

Winged keel

A winged keel is a sailboat keel, usually of moderate aspect ratio, that uses a nearly horizontal foil, the "wing", at the bottom to provide additional performance. Note that the wing is upside down with relation to the boat...

.