Sailboat Rudders

The primary purpose of sailboat rudders is of course to give the helmsman the ability to steer the boat, but a well-designed one will also provide hydrodynamic lift to windward, in the same manner as does the keel.

These twin high-aspect ratio rudders on an all-out race boat are clearly optimised for their hydrodynamic performance.

Placing sailboat rudders into distinct categories is fairly straight forward - they're either:

Outboard or inboard rudders, which can be

Unbalanced, balanced or semi-balanced, and be

Keel-hung, skeg-hung, transom-hung or spade rudders.

Take a stroll around any fair-sized boatyard during the lay-up season and you'll see examples of most of them. Let’s take a closer look at each type and explore the material, construction, and hydrodynamic principles that underpin their design.

Inboard & Outboard Sailboat Rudders

An Outboard Rudder on a heavy displacement, long-keeled, canoe-sterned sailboat.

Fig 1 - An Outboard Rudder on a heavy displacement, long-keeled double-ender.

If the rudderstock passes through the underside of a boat's hull, it's considered an inboard rudder. Conversely, if it doesn’t, it’s an outboard rudder.

Most outboard rudders are turned by a tiller since there’s no rudderstock to which a wheel-steering quadrant can be mounted.

The advantage of outboard rudders, such as those hung on a transom, is their ease of removal for service or repair while the vessel remains afloat.

The two rudders shown here are quite different examples of outboard rudders.

Fig 2 - An outboard rudder hung on the transom of a light-displacement sailboat.

For example, Fig. 1 shows a keel-hung outboard rudder, which is seldom seen on today’s cruising boats.

Fig. 2 illustrates a more modern, lightweight outboard rudder, hung on the transom of a nimble sailing vessel. Both designs highlight the practical differences in rudder placement and purpose.

Inboard rudders, on the other hand, are favored for their protection and streamlined design, as demonstrated in Figs. 3 through 9. They integrate neatly into the hull and often support more complex steering systems.

Unbalanced Rudders

Fig 3 - An unbalanced sailboat rudder.

This unbalanced rudder is supported by a full-length skeg.

It is unbalanced because the entirety of the rudder is aft of its axis, the axis being on the centreline of the rudderstock.

When turned, the full force of the water flowing past the skeg acts on one side of the rudder - a fact that will be very much apparent to the helmsman, particularly on a tiller-steered boat.

Balanced Rudders

Fig 4 - A Balanced Sailboat Rudder.

The rudder shown here is said to be balanced because part of it is forward of its axis.

As the rudder is turned, the force exerted by the water flow acting on the forward part partially counteracts the force applied to the area of the rudder that is aft of the axis.

This reduces the load on the tiller and makes life easier for the helmsman.

Semi-Balanced Rudders

Fig 5 - A Semi-Balanced Sailboat Rudder

The rudder shown here is supported by a half-length skeg.

The upper part of the rudder (aft of the skeg) is unbalanced.

However, the section of the rudder below the skeg is balanced by the part of the rudder projecting forward of its axis.

The helm loads will be lessened as a result.

Hence this type of rudder is said to be semi-balanced.

Keel-Hung Rudders

Fig 6 - A Keel-Hung Sailboat Rudder

Keel-hung rudders are often seen on heavy-displacement, long-keel boats of yesteryear.

This one's on a Nicholson 32 from the 1960's.

Whilst the propellor is well protected, the propwash from it acts on only a small area of the rudder which, combined with the massive displacement and full-length keel, makes close-quarters manoeuvring under power something of a hit-and-miss affair.

Skeg-Hung Rudders

Fig 7 - A Skeg-Hung Sailboat Rudder

The skeg is a structure built into the hull of a sailboat solely for the purpose of supporting the rudder.

Full length skegs provide a high degree of rudder security but can be generate high helm loads, requiring a long cockpit-sweeping tiller or wheel steering.

Half-skegs can solve this issue as they allow for a semi-balanced rudder.

Transom-Hung Rudders

A transom-hung rudder with additional support provided by a full-length skeg.

Fig 9 - A Skeg-Hung sailboat Rudder

Transom-hung rudders are attached to the boat by hinging mechanisms known as Pintles and Gudgeons.

Pintles always incorporate a pin, whereas Gudgeons always have a hole for a pin. Usually, Pintles are attached to the rudder and Gudgeons are attached to the transom - but not always.

The example shown here (on a Sadler 25) is something of a hybrid, incorporating a full length skeg to provide additional security.

Spade Rudders

Fig 9 - A Spade Rudder

From a hydrodynamic point of view, the Spade Rudder is most efficient and is becoming the norm on modern sailboats.

The smaller the gap between the rudder and the hull, the greater the end-plate effect provided by the hull.

But being a cantilever with no support along its leading edge, unless it's properly designed and engineered, robustness may be an issue.

Rudder Materials & Construction

Most modern rudders are constructed from fiberglass shells encasing a core material. The choice of core material depends on the rudder’s intended use:

Foam cores, often PVC or polyurethane, are lightweight and buoyant, making them ideal for cruising designs.

Balsa wood cores are a traditional option, offering strength and reduced weight but requiring careful maintenance to avoid moisture damage.

Carbon fibre cores are cutting-edge, providing exceptional strength-to-weight ratios for high-performance racing yachts.

These cores are enclosed in fiberglass reinforced with epoxy resin, creating a strong yet hydrodynamic structure.

Hydrodynamic Principles

A rudder’s shape plays a crucial role in its efficiency.

High-aspect ratio rudders minimize drag and maximize lift, which is why they’re commonly found on racing sailboats. However, their sleek profile makes them less forgiving in heavy seas.

Conversely, low-aspect ratio rudders, more common on cruising yachts, offer stability and resilience at the expense of increased drag.

The tightness of the rudder-to-hull gap enhances the end-plate effect, which reduces turbulence and increases responsiveness.

This balance of design and function allows experienced sailors to optimize handling for different conditions.

Steering Systems

Rudder compatibility with steering systems is another key consideration. Tiller steering, often paired with outboard or unbalanced rudders, provides direct feedback but can be physically demanding.

Wheel steering, more common on inboard rudders, reduces helm effort but requires additional mechanical components.