The Types of Sailboat Rudders

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Full Keel Rudder

Sailboat Full Keel Rudder
Photo © Tom Lochhaas.

On a sailboat, as the rudder is moved to one side by means of the tiller or steering wheel, the force of the water striking one edge of the rudder turns the stern in the other direction to turn the boat. Different types of rudders have different advantages and disadvantages. The type of rudder is often related to the boat’s type of keel.

Rudder on Full-Keel Sailboat

As shown in this photo, the rudder of a full-keel boat is usually hinged to the aft edge of the keel, making a continuous surface. The engine’s propeller is usually positioned in an aperture between the keel and rudder.

Advantages of Full Keel Rudder

The primary benefit of this rudder configuration is the strength and protection provided to the rudder. It is hinged at top and bottom, well distributing the forces on the rudder. Rope (such as lobster pot warps) or debris in the water cannot snag on the rudder.

Disadvantage of Full Keel Rudder

Because the sideways force of the water on the rudder is entirely behind the rudder’s pivoting point at its leading edge, putting all the force on one side of the rudder, it takes more energy to move the rudder. This is one reason why larger boats seldom have tillers—because it can require much force to “push” the rudder out against the water streaming past the keel.

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Spade Rudder

Sailboat Spade Rudder
Photo © Tom Lochhaas.

Most fin keel boats have a spade rudder, which extends straight down from the aft hull section. The rudder post comes down through the hull into the rudder itself, allowing the entire rudder to rotate to either side, pivoting around the post.

Advantages of Spade Rudder

The spade rudder is self-standing and does not require a full keel or skeg for its mounting. The rudder post inside the rudder can be moved aft from the leading edge (see next page on Balanced Rudder) so that the force of the water is not all on one side when the rudder is turned. This requires less energy to steer than with a keel- or skeg-mounted rudder.

Disadvantage of Spade Rudder

A spade rudder is more vulnerable to debris or objects in the water, which may strike the rudder and exert a force on the rudder post, the only structure supporting the whole rudder. Even the force of water when the boat “falls” off a wave can exert damaging stress on a spade rudder. If the rudder post is bent, the rudder may jam and become useless.

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Balanced Spade Rudder

Sailboat Balanced Spade Rudder
Photo © Tom Lochhaas.

Note the clear air space at the top of the leading edge of this balanced spade rudder. The rudder post is several inches back from the front of the rudder. When the rudder is turned, the leading edge rotates to one side of the boat while the trailing edge rotates to the other side. While the turning action on the boat is the same, the forces on the helm are more nearly balanced, making it very easy to steer.

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Skeg-Mounted Rudder

Sailboat Skeg-Mounted Rudder
Photo © Tom Lochhaas.

Some fin keel sailboats have a skeg-mounted rudder like the one shown. The skeg offers the same advantages as a keel mounted rudder: the rudder is protected from objects in the water and has more structural strength than a rudder mounted only on the rudder post.

It also has the same disadvantage: because it is not “balanced” as a spade rudder may be, with water forces distributed on both sides, it requires more force to turn the rudder.

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Outboard Rudder

Sailboat Outboard Rudder
Photo © Tom Lochhaas.

An outboard rudder is mounted outside the hull on the boat’s stern, such as shown in this photo, rather than below the hull using a rudder post or hinges to the keel or skeg. Most outboard rudders are turned with a tiller rather than a steering wheel since there is no rudder post to which to gear a wheel.

Advantages of Outboard Rudder

An outboard rudder does not require a hole through the hull for a rudder post and thus is less likely to cause trouble if damaged. The rudder can often be removed or serviced while the boat is still in the water. Hinges at the top and bottom of the rudder section may provide more strength than a single rudder post.

Disadvantages of Outboard Rudder

Like a spade rudder, an outboard rudder is vulnerable to being struck by or caught in objects or rope in the water. Unlike a spade rudder it cannot be balanced in the water flow, so the force of water is always on one side of the pivot point, requiring more energy for turning the rudder.

A rudder is often related to keel shape