The Most Popular Types of Propellers Copy

We can understand some propeller characteristics from the diameter and number of blades. Power propellers, for example, can be found on Tugs, have a large diameter, 4 or 5 blades and a low rotational speed. Propellers intended for fast ships have a small diameter and high rotational speed. Silent propellers for Military or Submarine ships, with specific shapes designed to reduce noise, make it difficult for the enemy to locate.

But that is not all. From the early 1800s, when the propeller replaced the blade propulsion, many new thrust systems appeared, and many are now in everyday use.

Ducted Propeller or Kort Nozzle

The Kort is a propeller placed in a cylinder duct to accelerate the inlet stream and obtain greater thrust with the same power. It can be fixed or rotatable to perform propulsion and steering functions. It is generally used on small propellers for small tonnage boats and is widely used on traditional tugs as this system increases the bollard pull by up to 30%.

The advantages of this type of propeller are felt sensitively at low speeds, better course stability and protection from debris and wrecks. At the same time, it loses efficiency at high speed, generally over 10 knots, as the mantle’s resistance is often more significant than the increase in thrust it can produce.

The paddle-wheel effects, whether right-handed or left-handed, are significantly reduced. Some naval units operating in shallow water also appreciate the absence of the ‘water-suction effect’ near the bottom. Usually, these propellers are called “Kort Nozzle”, which, as often happens, can be the original manufacturer’s name or, in this case, the one who most developed their shapes and characteristics.

Ducted propellers with a “decelerating duct” allow maintaining a determined pressure level to delay the cavitation phenomenon and reduce noise. They are for larger diameter propellers and particular or military uses.

You can find more information here: Kort Nozzle.

INSERIRE VIDEO

Voith Schneider

We will improperly associate this particular naval system with a propeller because the characteristics and application are distinctly different. It comprises a sizeable horizontal disc with a vertical rotation axis placed under the keel and five articulated blades that extend towards the bottom. The rotation of the disc provides the thrust, then directed with the proper movement of the blades.

This type of thruster eliminates the need for an additional rudder or manoeuvring thrusters, as the power force produced can be directed in any direction. It is widespread on Tugboats, Auxiliary vessels and Ferries operating in confined sea areas. The Voith Schneider engine – also called Rotor or Cycloid – is an excellent alternative propulsion system to the propeller.

The Voith can be only in pairs, one in the bow and one in the stern, as in bidirectional Ferries, or side by side, as is typically the case in harbour tugs. We can have a flat shape towards the hull ends; or, in the alternative, a long fin (skeg) aft to house the disc. It will ensure better course stability and control over involuntary rotations and lateral resistance during angled tugboat manoeuvring.

Rotors allow the ship to rotate even 360° on the spot, veer and turn freely. The articulation of the blades permits a quick transition from ahead to astern and vice versa, an equally quick start from a standstill and good manoeuvrability when docking or working at low speeds.

The limits of Voith systems are by the vulnerability of the blades, which, protruding from the profile of the hull, can touch the bottom. For this reason, they are all equipped with protection formed by a metal cage anchored above the hull and below a thick plate, which has the task of absorbing and limiting any contact damage. Reduction of the hull draft to compensate for the bulk of the vertical blades and the flat bottom amplifies any rolling movements.

You can find more information here: Voith Schneider.

The Hydro-Jet

The sophisticated propeller system of New Zealand origin, the waterjet, adopted by many fast planing hull ships, consists of a sea inlet valve, a hydraulic pump, and an adjustable nozzle that acts as a rudder and a deflector for reverse. The pump, powered by a diesel, electric or gas turbine engine, sucks water from the sea and directs it at high speed towards the nozzle it comes out, providing the thrust to the boat. The pump can be centrifugal or axial when the impeller (the propeller) is on the axis with the fluid.

The hydro-jet propulsion system found its first application in pleasure boating, an alternative to the outboard motor. Due to its intrinsic safety, not having the propeller exposed, it was adopted as the only propeller on jet skis and all those vessels that operate with people at sea (e.g. Coast Guard SAR units). In good weather conditions, cruising speed is very high and allows good manoeuvrability in shallow water harbours. Thanks to all these qualities, the excellent power and ease of handling have finally begun to spread on HSV (High-Speed ​​Vessel), fast units, and passenger catamarans. The absence of vibrations and cavitation and the advantage of working in shallow water make it suitable for various military patrol units.

The latest known development of this engine is the DeepSpeed, an electric Hydro-Jet that exploits the thrust of the jet by moving it outside the boat. This outboard motor is dedicated to pleasure boats and is still under development.

You can find more information here: Hydro-Jet.

 

INSERIRE VIDEO

Hovercraft (ACV Air Cushion Vehicle)

When the propellers move masses of air surrounding the hull and support its weight at the water surface, we are in the presence of the Hovercraft! This particular boat can advance on different flat surfaces and reach an incredible speed of 150 Km/h at sea without producing wake or wave motion. Unlike many thinks, the Hovercraft does not hover in the air but floats on a thin lubricating layer of air produced by the propellers, which have the dual task of lifting and propelling.

We saw them ferrying men and vehicles in the Channel Strait between the 1970s and the early 2000s. Today they are less common in this capacity. Since, in addition to the opening of the English Channel, new and lightfast ships (SES – Surface Effect Ships) have developed, which, unlike Hovercraft, do not require sizeable dedicated docking ramps. Equipped with aeronautical fixed or variable pitch propellers, they have the characteristic of exceptional manoeuvrability. They can rotate 360 ​​degrees on the spot at full speed and maintain their position even in strong currents and debris.

These qualities, combined with the excellent load capacity and the ability to operate on surfaces such as ice, snow, sand, mud, rivers and inland waters with a limited draft, make them excellent for search and rescue operations. Military and paramilitary apparatuses also take advantage of the inherent feature of sonar and radar jamming.

You can find more information here: The Hovercraft Museum.

CRP Counter Rotation Propeller

The principle of counter-rotation finds its first application in the propulsion of torpedoes since, together with the gyroscope, it stabilises the roll to maintain the course. Today, sophisticated counter-rotating propellers have spread to various ships, as they enable the efficiency of 10-12% higher than traditional single propellers and a consequent energy saving.

The system consists of two coaxial propellers that rotate in opposite directions. The forward propeller is larger than the aft one – the distance between the two is 20-25% of the diameter of the larger one. Generally, the bow propeller has four blades and the aft propeller five to avoid overlapping and vibrations. The purpose of the counter-rotating propellers is to zero the rotational energy that develops in the wake. In practice, the aft propeller intercepts the forward propeller’s slipstream, amplifying its thrust and cancelling its rotational motion.

The CRP (Counter-Rotating Propellers) are less common than traditional propellers for reasons related to the delicacy and complexity of the shaft lines and the indispensable gearbox that distribute power. All this affects the initial costs and maintenance, discouraging many shipowners. However, the CRP equips some VLCCs and some fast merchant ships, several special ships, and, as we will see later, some azipod propulsion ships.

Azimuth Propulsion Systems

This category includes propellers with the hub capable of rotating 360° around a vertical axis. This rotation allows steering with direct thrust, transverse thrust and inversion of thrust.

The engines can be located inside the hull or in unique pods. In the first case, we will have a remote mechanical connection, while in the second, it will be direct electrical. The first variant is suitable for powers up to 2 MW; the second equips modern pods with greater than 25 MW powers.

We will see the system in detail in the following chapters.