What Is A Skeg In A Ship?
The skeg is an important but not essential component of a ship. It is a sharp vertical projection or external structural feature located at the stern near the centerline, directly attached to the hull plating. It is usually (but not necessarily) located forward of the rudder. Since it is not part of the main hull structure, it can be considered a functional appendage.
It can also be defined as the stern extension of the main hull’s external keel. (The keel is one of the primary internal longitudinal strength components contributing to the overall strength of the hull.)
Skeg in Ship
After a certain point at the stern (and bow), the hull plating rises above the baseline, based on the curvature of the hull in side view.
Thus, the keel plating and the central longitudinal lower keel, where the hull curvature is highest, rise to the height of the curvature.
Thus, the keel curvature extends linearly from the hull bottom curvature and can be considered an element that directly contacts and extends linearly (outward) from the flat hull plating.
Full-beam Rudder (Unbalanced)
In engineering, it has a tapering projection (wider at the top and narrower at the bottom) when viewed transversely; it has flat sides when viewed from the side or longitudinally. Structurally, it typically consists of two large, flat side plates connected longitudinally to the lower hull, connected by a series of intermediate transverse plates with the aforementioned tapered shape.
The lower stern consists of another flat, closed plate, which connects the two side plates and is welded to two overlapping transverse plates. Other structural, supporting, or reinforcing members may be required. At the forward end of the keel, there is typically a vertical, angled plate connected to the lower sheathing, called the stern plate or foreplate.
Semi-balanced Keel Rudder
The tapered transverse plates serve primarily for structural support of the keel. The keel edges are typically located on the upper side because they are not only external structures but also subject to high load factors from waves and currents, as well as the potential risk of contact with any surface in the event of a grounding or accidental damage.
Similarly, from an engineering perspective, the key parameters of a ship’s keel are best defined by A (the surface area or lateral area of the keel plate) and t (the average width of the cross-sectional area, which should be taken as the average width as the cross-sectional area decreases). In fact, due to the design and construction of the ship, its A/T ratio is very high, meaning the ratio of its area to its width is very large, as you can see. In other words, its hull is very slender.
Ship Applications and Some Details
1. Fluid Dynamics
The most important aspect of a ship is its fluid dynamics. Simply put, the design of a ship’s fin significantly affects the airflow pattern, which in turn affects the dynamics of the rudder (usually parallel to the fin) and the propeller. This improved fin design can reduce drag and improve propeller efficiency. Another important aspect to consider is air turbulence. Experimental and practical experience have shown that fins effectively reduce air turbulence by transforming the turbulent nature of the airflow into laminar flow at the stern.
2. Directionality and Sea Trim
This is also a very important aspect. In this respect, a ship’s tail has an advantage over an aircraft’s tail, the difference being that the latter is mandatory. In contrast, ships have a relatively slow speed and stable buoyancy, so disorientation, if any, is less sudden and poses no danger or risk.
In older ships, especially smaller ones, the keel was often used to maintain course and prevent yaw. However, in modern, larger ships, thanks to improved rudders and advanced navigation and stabilization technologies such as propulsion systems or dynamic pitch control systems (DPS), the keel’s importance in maintaining course is less pronounced.
However, sterns have proven to be very effective in controlling yaw and reducing roll. They also play an important role in maintaining the ship’s seaward balance, improving stability and vertical balance, for example by increasing roll damping. Consequently, sterns have been incorporated into many ship designs, particularly those with thinner profiles and smaller displacements.
3. Strength, Protection, and Support
While they are not integral to the ship’s structural components, they undoubtedly contribute to longitudinal strength. They not only provide important support for the hull but also support the rudder and propeller during impacts, absorbing loads. They also provide support during routine operations such as dry docking, when the hull is resting on a hard surface.
Skegs can sometimes be equipped with ballast. In some designs, they also support the stern tube for the rear propeller shaft. Skeg are typically welded to the hull later in construction.
