Saturday, 15 September 2018

Relaxations  in Cabotage Law


Cabotage traditionally refers to the transportation of goods within the country via the coastal routes by the transporters of the same nation, thereby restricting the foreign flagged transporters to perform these “chores” in that territory.  Cabotage law is a set of guidelines set by a nation to ensure and guarantee that the entitled ones reap the maximum benefits of waters of one’s own land. Following the historical trail, like most of the other countries of the world, India would not allow the foreign flagged ships to carry goods around until now; things have taken a turn with India relaxing the cabotage laws on May 21, 2018 with a Ministry of Shipping notification, in what is said to be a “game changing decision” by benefitting Indian ports and helping India become a transhipping hub.
The relaxations were long awaited. It has been put that one of the major causes the relaxations were given was that the Indian shipping companies could not provide appropriate services as and when required.  They are also unlikely to have adequate number of vessels to cater to the ever-growing demand of the domestic industries. Water transportation being the cheapest, such a situation could not be allowed to be the Achilles heel.

Positive Aspects

 The relaxations will double up its benefits by forming conducive circumstances to help India become a major transhipment hub.  India can now attract cargo loaded on vessels that are originated or destined to foreign ports that can drop cargo at different Indian ports en-route.  More number of vessels would ensure less delay in cargo transportation. Indian ports would be more busy and gain some extra revenue.
 The freight rates would descent consequently. The persisting problem of empty containers being accumulated on the ports due to an imbalance between imports and exports would also be addressed. The foreign flagged vessels would be allowed to reposition these containers from the ports that have extra, unused containers to the ports facing a crunch of containers as opposed to the earlier cabotage law.

Requirement of a  Level Playing Field

While we look at what the relaxations in the cabotage law have to offer to the shipping industry in India and how the foreign players could change the existing scenario of the industry, emphasis must be laid on providing a level playing field to the Indian flagged ships that have invested over 68000 crores INR based on the existing policy. The Asian countries have relatively relaxed the law more than other part of the world. The relaxations in the cabotage law have already been declared as a “retrograde move” by the Indian National Shipowners’ CEO Mr. Arun Devli. The relaxations might also result in the loss of “Right of First Refusal” to the Indian ship owners.  The point is to put the Indian flagged ships at a level equivalent to the foreign flagged ships who have not relaxed cabotage laws at their land but would be enjoying the relaxations here at India ;  being benefitted doubly they could and would cite lower freight rates. Indian ship owners would not be able to match their rates and ultimately, lose the contract.  The difference could be as humongous as 41%, according to a 2015 data. With globalisation it has become integral for the government to protect its subjects not by eliminating foreign competitions but by pitting both the groups at a level playing field.

Creating a Level Playing Field

One way out could be taking away the burden of hefty taxes from the Indian ship owners; taxation is very consuming in India for the mariners. Other way out, the traditional way, higher taxation rates could be levied so as to create evenness as well as a source of income to India. The reverse charging scheme could be beneficial as well where the foreign flagged vessels were asked to pay service tax based on freight rates and commercial value of their goods at a rate of 15% whereas domestic vessels were somewhat relaxed (at a rate of 4.5%)    The Indian Controlled Tonnage scheme is an Indian ship owner friendly step but only when coupled with the Right of First Refusal. According to this scheme, Indian ship owners are allowed to flag their ships outside India if half of their crew are Indians. The foreign ship owners could also be asked to register their ships in India in order to operate here and pay taxes, which is the National Fleet policy, and enter a level playing field.


The overall impact of the relaxations in the cabotage law would be out in some time for everyone to critically analyse and assess the results. In future, it could as well lead to Indian ship owners  getting their vessels flagged at a foreign land.  If the government manages to create a level playing field for the Indian ship owners, who have been growing their fleet progressively, it could benefit the shipping industry to prosper as well as the end customers who would enjoy a cut in delivery cost and time. The ports will certainly attract more traffic and India is set to become a major transhipment hub.  The rail and road transportation will be relaxed and their rates may descend as well.
India has asked for a global exposure for its cargo movement and to improve the water transportation while other countries still haven’t relaxed their cabotage laws. It is up to it to ensure that the local industry does not succumb to the competition. In the 21st century, sooner or later, relaxations have to be made if the supply portion lags in delivering its service and India has put out a difficult yet bold step; the fruits of the seeds sowed....calculated yet awaited.

Article By- Rajiv Ratna Jha

Monday, 10 September 2018

Advanced Marine Vehicles

Many of us are familiar with the principle behind aircrafts - The Bernoulli's Principle. The difference in pressures above and below the aerofoil section of the wing creates lift . But I doubt whether many are apprised of the fact  that the same principle has been applied to ships also over the years! In fact some of the fastest warships in history were able to achieve such speeds by making some specialized sections attached to their hulls "fly" inside the water just like the wings of an aircraft.Racing fascinates us in every medium be it be on land , air or water. Ever wondered what makes the racing boats capable enough to cross speeds of 300km/hr?. Read the article to gain valuable insight about more such developments in making of Advanced Marine Vehicles.

The ocean environment is dynamic in nature and the conventional displacement hull forms are not best suited for all circumstances. Many new hull forms have been developed over the years to serve different purposes. 

High Performance Mono Hulls


A Monohull is basically a boat or a ship with a single hull making it the most extremely used, efficient, versatile hull up to a Froude no. of (1).
Most high speed small monohulls have recently been converted to hard chine structures. They are now provided with greater beam and reduced length, thus giving improved performance in calm water and leading to high vertical accelerations in a seaway. Their ride has been improved by using higher deadrise angles leading to a deep V form.
Ø  For getting a higher Froude no. we need to lift the hull out of the water, by planing the hull.

Ø  Destriero and Jupiter are the names of two famous planing monohulls. Planing is the method of operation for a naval vessel in which its weight is predominantly supported by hydrodynamic lift, instead of hydrostatic lift (buoyancy).
Ø  We can make a hull go faster by making it extremely slender and using a very narrow beam. But if we take it to extreme, it results in an unstable ship, hence some sort of change has to be done to get stability.
Ø  Trimarans belong to a class of vessels properly called Stabilized Monohulls. They are characterized by the extreme slenderness of the main hull.


catamaran is a multi-hulled watercraft which has two parallel hulls of equal size. It is a geometrically-stable craft, which derives its stability from its wide beam, instead from a ballasted keel but from a monohulled sailboat. “Catamaran” is abstracted from a Tamil word "Katt-umaram" which basically means logs tied together.
Catamarans typically have less hull volume, higher displacement, and shallower draft (draught) than monohulls of comparable length. It provides large upper deck areas for passenger facilities in ferries for helicopter operations.
The hull separation in a catamaran is about 1.25 times the beam of each hull which helps in its stability and helps to take the manoeuvrability of these vessels to a greater extent. Catamarans are available in all sizes from small (sailing or rowing vessels) to large (naval ships and cargo ferries). 


Catamarans have faced many problems in the past mainly related to ride facilities. So as to make the ride quality smooth, “PHILIP CHRISTIAN HERCUS” the famous Australian naval architect came up with the wave piercing hull form.This new hull form design takes the help of a narrow protruding beam piercing the waves instead of rising above one another.
Incat Tasmania ,is an international manufacturer of  high speed crafts, mainly wave piercing catamarans found by “BOB CLIFFORD” and “PHILIP HERCUS”. The company excels in building large vessels using aluminium as a construction material, wave piercing and water-jet propulsion technology. The vessels have been constructed up to a length of 112 metres, with a size of 10,800 Gross Tons and having  cruising speeds up to 58 knots (107 km/h).
Image courtesy:
Built at Incat's shipyards in Hobart, Tasmania, “FRANCISCO”, this is the world's fastest catamaran ship. With a cargo of over 1,000 passengers and 150 cars, the 99-meter catamaran can travel at speeds of up to 58 knots or 107 km per hour.


The term "hydrofoil" may be sub-divided into two meanings:

> A hydrofoil is a foil which can operate in water. They are same in appearance and purpose when related to aerofoils.
> The term "hydrofoil" is often used to refer the floating vessels that use  the hydrofoil technology.

The term "hydrofoil" is commonly referred to the wing-like structure which is clamped onto the struts below the hull, or across the keels of a catamaran in a variety of boats, which helps to lift the boat out of the water during forward motion, thus resulting in reducing the drag force on hull. Hydrofoils can be artificial, for example the rudder or keel of a boat, the diving planes on a submarine, a surfboard fin. As soon as a hydrofoil-equipped marine craft increases its speed, the hydrofoil elements below the hull develop or produce enough lift force to raise the hull up, out of the water. This results in a great reduction in drag force of the hull, along with a corresponding increase in speed and efficiency in operation with respect to fuel consumption.

Image courtesy:


Surface Effect Ship (SES) or Sidewall Hovercraft is a naval vessel which comprises of an air cushion or pillow, just like a hovercraft, and also twin hulls, as a catamaran. When the air cushion is in practice, a small part of the twin hulls lie in the water. When the air cushion is turned off, the full weight of the vessel is balanced by the buoyancy of the hull pair.
The “SES” has acquired two advantages over a normal hovercraft in case of open sea operation. It encounters more resistance when it slips sideways and it operates in air or sea, and it can use water jets for propulsion since the inlet nozzles are always covered by water.
Surface effect ships are a special type of vessels which are a combination of hovercrafts and catamarans.  They are so designed that they can use both air cushion and conventional propulsion as and when required. This helps them to attain a greater speed on sea water. The main advantage of the surface effect ships is that they retain their stability in any kind of water conditions on the basis of being equipped with both – dual hulls and air springs. The maximum speed that can be achieved is around 80 knots.

Video Courtesy : YouTube


The ACV or Air Cushion Vehicle is a vessel, which is capable of moving in land, sea, snow, mud or other surfaces. It does not have the side hulls of the SES or catamaran. It has got an air cushion like structure, which makes it an amphibious watercraft. It allows very low drag force, thus ensuring speeds to establish high Froude Number. (Froude Number=4)
ACVs are generally noisy, making it a bit uncomfortable, and mechanically complex, but they do possess unique features, such as the ability to fly up over the beach. Large hovercraft have successfully served on the English Channel for over 25 years.
According to my knowledge, I have seen some written materials which state that the hovercraft was also used for airport‐to‐airport service across San Francisco Bay.


When a normal aerofoil is operationally active in a region close to the ground, its normal pressure distribution is disturbed. Pressure tends to develop to a higher level under the wing and adds to the normal dynamic lift of the aerofoil. This enhanced lift force is popularly known as the ground effect. Wing in Ground-effect (WIG) aircraft, can be referred as "flying boats" made in purpose to operate just above the wave crests in order to avoid occasional water contact during flight. WIG aircraft possess one or more wings which possess three orders of magnitude larger than the foils of hydrofoil craft. When a WIG aircraft has accelerated to a sufficient high velocity through the water,then the aerodynamic lift generated by the wings helps to lift the aircraft entirely out of the water. By remaining close to the water's surface, WIG aircraft faces significantly less resistance than it would face at higher altitudes because of the aerodynamic lift.


The "sustention triangle" is a very popularly used device for categorizing different ship types. It is an imaginary or conceptual device used for the better understanding of the floatation process of boat. Traditional or normal ships float because they are immersed in water and get an up-thrust or buoyant force due to Archimedes' principle. It occupies the lower left corner of the triangle. There are other ways to hold ships up. As in case of hovercraft, for example, where the ship is lifted on a bubble of air.
Hydrofoils and hydroplanes are both dynamic lift marine vessels. It thus occupies the topmost corner of the triangle.
Some vessels occupy the intermediate positions on the edges of the triangle. For example, an SES (mixture of catamaran and hovercraft).


It is also an imaginary, conceptual “design space” which consist of mutually orthogonal axes. The result of this consideration leads directly to the sustention cube.
The Contents of the Sustention Cube
The corners are defined by combing the following pairs, to produce eight points:
> Passive or Active
> Hydro or Aero
> Static or Dynamic

Thus the eight corners are:
> Passive Hydrostatics- Conventional ships and barges.
> Passive Hydrodynamics- Hydrofoils.
> Passive Aerostatics- Blimps
> Passive Aerodynamics- WIGs
> Active Hydrostatics- Hovercraft
> Active Hydrodynamics
> Active Aerostatics
> Active Aerodynamics

Article By- Suryadip Ghosh

Monday, 3 September 2018

Cavitation - The constraining factor

Transportation across the oceans has attracted the attention of mankind since the dawn of history. What started with a simple bundle of logs has evolved into a huge sophisticated vessel of today. Earlier, wind (sails) and human power (by oars) were the only means of propulsion but since the advent of 19th century Mechanical Propulsion began to be used in ships. Most of the modern ships have quite similar propulsion arrangements consisting of a wheel rotating about a transverse axis with radial blades or paddles to impart astern momentum to the water around, giving it forward thrust . 

As the size of vessels grew, the thrust required from these screw propellers also increased. Consequently, the rpm of these propellers escalated. When the  propellers’ diameter is restricted and they are required to produce very large thrusts at high rpm’s  the propellers are likely to experience a phenomenon called “Cavitation”.


Water begins to vaporise when its vapour pressure equalises the saturation vapour pressure. The vapour pressure of water is 1.704 kN/m^2 at 15 °celsius and 101.325 kN/m^2 (atmospheric pressure) at 100 °celsius. If the pressure inside water falls to the vapour pressure the water at that point begins to vaporise forming voids filled with water vapour. The formation of such vapour filled cavities is termed as cavitation.Cavitation also depends upon the purity of water. The impurities in sea water such as suspended solid particles and dissolved gases act as nuclei for the formation of cavities.Vapour cavities are formed locally but when pressure increases clear of this region the vapour cavities collapse or “implode”. This implosion occurs with a high impact force with stresses nearing 2800 N/mm^2. 

A propeller generates thrust by creating a pressure difference between the face and back of its blades. The pressure on the back of the blade falls below the ambient pressure and the pressure on the face rises above it. Hence, the back face of the blade becomes prone to cavitation.

Types of Propeller Cavitation

The type of cavitation occurring has two bases for classification :

   1.Classification on the basis of the region where cavitation occurs:
  • Tip Cavitation
  • Root Cavitation
  • Boss or Hub Cavitation
  • Leading Edge Cavitation
  • Trailing Edge Cavitation
  • Face Cavitation
  • Back Cavitation
    2.Classification on the basis of the nature of the cavities or appearance:
  • Sheet cavitation
  • Spot cavitation
  • Streak cavitation
  • Cloud cavitation
  • Bubble cavitation
  • Vortex cavitation
Cavitation exhibited in a cavitation tunnel in a stroboscopic arrangement
Hub Vortex Cavitation
Leading  Edge Cavitation
Tip Vortex Cavitation


  1. Cavitation in marine propellers results in the increase of blade rpm without any appreciable increase in speed of the ship hence reducing the efficiency and thrust of the propeller. 
  2. When the bubbles implode they generate very strong local shock waves in water which generate a lot of noise and do mechanical erosion of the blades in form of pitting. The frequency of collapsing is generally around 15,000 Hz to 20,000 Hz. The collision intensity depends upon a number of factors-: 
  • Life span of cavitating bubble which is about three milliseconds which makes the event rapid. The faster the surrounding water collides, the greater the energy it possesses.
  • The size of the cavitating vapour bubble. Generally, the vapour bubble formed by cavitation at 68° Fahrenheit is about 35 times larger than the one produced at 212° Fahrenheit. A larger bubble corresponds to a greater amount of water colliding.
  • The quickness and amount of water together represent the total kinetic energy with which the imploding bubble attacks the metal.
  • It also accelerates corrosion in blades due to ‘Cavitation stain’. Cavitation Stain is a phenomenon where the oxidised layer that protects the metal is removed by cavitating bubbles hence making the metal vulnerable to corrosion. Corrosion damage is visually detectable and can eventually destroy the propeller.  
  • In the given figure a series of 18 shots show progressive events of flattening of a spherical bubble and finally imploding with the release of enormous energy.
Series of 18 shots show progressive events of cavitation

Pitting Due to Cavitation
Propeller Pitting due to cavitation

Prevention of Cavitation

Due to the detrimental effects of cavitation propellers are designed so as to restrict its level to a value so that its effects are negligible. It is achieved by following three methods:

  • Increasing the cavitation number.
  • Decreasing the loading on the propeller
  • Designing the propeller for uniform loading according to Keller criteria, which is related to the number of blades of the propeller, diameter, thrust, and the depth of shaft.

Supercavitation and Supercavitating Propellers

Generally it has been observed that it is difficult to avoid cavitation completely in high speed crafts where the propeller rpm crosses 1000 rpm. So instead of avoiding it some propellers are being designed such that they use this property to create a sheet of vapour around the propeller blades which increases the effective diameter of the propeller and thereby increases the thrust as well as the efficiency of the propeller. Cavitation is not a problem for these propellers as they are designed to work in cavitating zone. Although before achieving the design speed (when cavitation occurs) these types of propellers demonstrate very low efficiency due to their non-streamlined (wedge shaped) sections.

Super cavitating propeller with wedge shaped training edges

Supercavitation is even used for torpedoes and other high speed objects in marine environment. The vapour sheet formed due to cavitation corresponding to the high speed of torpedo encompasses the object thereby greatly reducing the drag and hence enabling higher speeds. They are being developed by defence forces in fast supercavitating torpedoes and war projectiles. For instance, the soviet torpedo named VA-111 Shkval, German torpedo Barracuda, weapons like SPP-1 underwater pistol, APS amphibious rifle.

VA-111 Shkval using the phenomenon of cavitation

Thus cavitation though has some serious detrimental effects, it can be used as a beneficial tool in particular cases. It is one of the most important factors determining the diameter and rpm of the propeller blade by placing major constraints. An optimum design and balance needs to be maintained between the blade diameter and rpm to obviate the risk of cavitation.

Article By - Soumya Sameer

Tuesday, 17 April 2018

Types of Propellers


A propeller is a fan like rotating structure generally at the aft of a ship imparting thrust to propel the ship. Most ships use the conventional screw propeller. But in some ships depending upon the need and requirement, different types of propellers are used.

The different types of propellers with varying characteristics are listed below.

Paddle Wheel

Image Courtesy- Google Images.

 It is a very simple type of propulsion system. It consists of wheels with paddles attached at its periphery. It has its axis of rotation about the transverse direction of the ship above the waterline. The paddles get immersed in water only when they are at the bottom of the wheel. As the paddles rotate in water, they accelerate it, experiencing a reactive thrust from the water which is transmitted to the ship.
 It is of two types

  • FIXED: These are simple and easy to construct. In this type, the paddles are firmly attached to the wheel. This system requires wheels of large diameter so that they can enter the water at large angles. Large diameter means low rpm and more weight. This is the main drawback of this system.

  • FEATHERING: In this system, the paddles are attached at the ends of the wheel in such a manner that the angle of entrance is much larger than that of fixed type.By this principle, the diameter of the wheel can be reduced.

Tandem propeller

Image Courtesy- Google Images.

In this arrangement, two propellers are mounted on a single shaft turning in the same direction. Tandem propellers are fixed so as to reduce loading on a single propeller as it can lead to cavitation. Here the thrust is divided between the two propellers.
In normal loading, they are not of much use but in heavy loading, they produce better loading than a single propeller.
Generally, size and number of blades are kept the same

Overlapping propeller

Image Courtesy- Google Images.

It has the same advantage as a tandem propeller as the load is divided between two propellers. There are two propellers with their shafts placed at a horizontal distance less than the diameter of either propeller.
They have higher hull efficiency because they work in a region of a higher wake. The advantage over twin screw is that no extra appendage is required to support and hence the resistance is reduced.
Sometimes the mutual interaction between the propellers may, however, result in more cavitation.

Controllable Pitch Propeller

Image Courtesy- Google Images.

In this type of propeller, the blades are not directly fixed to the boss but attached to separate spindles. The spindles can be turned about the axis and so the pitch of the propeller can be altered. These are mainly used in ships requiring full power at varying speeds and resistances.These are mainly used in tugs, ferries, icebreakers etc.

Some advantages over the conventional fixed propeller are
  • They provide better acceleration, stopping and manoeuvring properties.
  • Non reversing propulsion machinery may be used thereby reducing its cost, weight and space occupied.
  • At all loading conditions, the full power of machinery can be used.
  • The speed of the ship can be varied without altering the speed of the main engine.
  • Speed can be directly controlled from navigation bridge.
  • It is easy to replace damaged blades.
Some disadvantages are:
  • The control mechanism is very complex.
  • It has high initial cost.
  • Maintainance costs are also high

Ducted Propeller

Image Courtesy- Google Images.

In this type a non -rotating duct surrounds a screw propeller.
The gap between the propeller and duct is very minute.
These are mainly of two types:

  • ACCELERATING TYPE: They increase the velocity of flow of water to the propeller.

  • DECELERATING TYPE: They decrease the velocity of flow of water to the propeller.

Some advantages of ducted propeller over normal propeller are:
  • Better course stability
  • Less effect of load and speed variation on efficiency
  • Fewer chances of damage to the propeller
  • Improved efficiency at high loading
Some disadvantages of the ducted propeller are
  • More chances of cavitation
  • Poor astern performance

Supercavitating Propeller

Image Courtesy- Google Images.

Supercavitating propellers are used when the design criteria of the propeller are such that cavitation cannot be mitigated. It can produce very high thrust at same efficiency without cavitation, corrosion and noise. Ships with high engine power, speed and rpm deploy such propellers.The back of the propeller blade is covered by vapour filled cavity.There is a separation of flow on the back at the leading and trailing edge.The main objective is that the back of the blade should not be in contact.
 These type of propellers have however less blade strength owing to a thin leading edge of the blade.
They also don’t work properly at low speeds.

Surface Piercing Propeller

Image Courtesy- Google Images.

These type of propellers are partly submerged in water.
These are fitted just at the end of the ship rather than under it. The propeller shaft is just above the water surface. Since no extra appendages are required the drag resistance is considerably reduced.

Some advantages of the surface piercing propeller are:

  • It requires less power to achieve the same speed as compared to the fully submerged propeller.
  • Cavitation is considerably reduced.
  • Since it is not vulnerable to cavitation, they can have low blade area

Some disadvantages of the surface piercing propeller are:
  • Since the blades enter and leave the surface at each revolution, it is subjected to periodic loading and can lead to fatigue.
  • They have very poor astern performance.
  • Difficult to operate at low speeds

Contra-rotating propeller

Image Courtesy- Google Images.

It uses two propellers placed on two coaxial shafts.The propellers rotate in opposite directions.It helps to reduce the rotational energy losses caused in the slipstream.
Some advantages of contra-rotating propellers are
  • Loading is shared between two propellers and hence cavitation is reduced.
  • Efficiency is higher than a single propeller.
  • Less pressure fluctuations and noise

Some disadvantages are:
  • Greater weight of machinery at the aft
  • Complexity of gearing and coaxial shafts
  • Sealing of the shafting system is difficult

Azimuth propeller

Image Courtesy- Google Images.

An azimuth propeller is a configuration of marine propellers placed in pods which can rotate at any horizontal angle.
Some advantages of azimuth propellers are:
  • Rudder use is not required
  • Excellent manoeuvrability
  • Good astern performance
  • Good speed control
  • Vibrations are less

This is a video made by Team LearnShipDesign on some basic type of propellers. Hope you all like it.

Article By- Anil Kumar Singh

Sunday, 1 April 2018



In the previous article on screw propellers, I acquainted the readers with the basic terminologies related to screw propeller geometry and its slip phenomenon. In this article, I will impart an insight into the core helicoidal geometry of screw propeller emphasising on the different views of the propeller.


To describe the propeller geometry we generally take reference of the cylindrical coordinate system. This is because in propeller we talk about radial sections and helix formed on a cylinder. Thus a cylindrical coordinate system is appropriate. The coordinate system is as shown.
The major planes in the coordinate system are z=0, Θ=0 and r=0. Any offset of the propeller blade is taken in 3-dimensional space with reference to these planes.


To be specific the geometry of the propeller is a bit complex. I have tried to simplify the geometry as much as possible.
If we take a propeller and cut a radial section, then it will look as shown.
The face line of the radial section is a part of a helicoidal surface with some offset at the leading and trailing edge. What does this mean?
This means that suppose I have 2 sticks orthogonally arranged such that one stick is rotated about the second stick as an axis and the first stick advances along with rotation. Thus the loci of the tip of the stick as shown will trace out a helix on the imaginary cylinder as shown. The face of the propeller at a radial section is a part of the helix where the radius of the imaginary cylinder is the radial section. Also, the face offsets from the helix at the leading and trailing edge. The back surface of the propeller blade depends on the aerofoil section profile that each radial section of the propeller blade is made up of.

Image courtesy: Google Images.

Now if we cut the imaginary cylinder longitudinally and open the cylinder to form a rectangle, the helix forms the diagonal of the rectangle such that the face of the radial section lies on the diagonal as shown. 

Image courtesy: Google Images.

This denoted the actual section of the propeller. It can be thought that this actual section of the propeller is bent and giver a definite curvature and this is done for all radial sections. Then these sections are joined to form the complete propeller blade. This brings us to different views how we look at the propeller which will give us a firm idea about the various curvatures that are imparted to a plane aerofoil section to form a propeller radial section.


Before we jump onto propeller views it must be noted that the propeller blade radial section has curvature in 2 planes. If we see the propeller blade face and just concentrate on the radial section then we can see one curvature (the radial curvature) as the section is a part of the circular arc cut off from the total blade. Also as the face is a part of the helix, so looking at the propeller blade from the top view and concentrating on the radial section cut off from the blade, the section has a second curvature due to the helicoidal surface as shown.
Thus if we take a radial section of the propeller and straighten the 2 curvature one by one then the different views of the propeller are formed.


Suppose we have a propeller blade. This is made by various radial sections as told earlier. If we take the orthogonal projection of the blade radial sections on a plane perpendicular to the z-axis (z=0 plane), the view formed is called the projected view. These projections taken for all radial sections form the projected outline of the propeller blade. This can be understood by taking a propeller blade and projecting a light on the blade along the z-axis. The shadow cast on the wall which is perpendicular to the z-axis is called the projected view of the propeller. The area within the projected outline of the blade is called the projected area AP.

Image Courtesy: Team Learn Ship Design.


This view is a bit difficult to understand and has to be concentrated. Now again we take a particular radial section of the propeller blade which has curvature in 2 planes. We have learned the definition of pitch and considering a propeller having uniform pitch distribution along the radius of the blades, we can infer that the pitch angles of all radial sections will be different that comes from simple calculations. We know that the face of each radial section is a part of the helix chord and thus it will have a midpoint (C say). By definition developed view of a propeller is the projection of the radial section on a plane which is through the point c and makes an angle equal to the pitch angle (∅ for that radial section) with z=0 plane.
This is similar to that of projected view but difference being, the light is projected at an angle equal to pitch angle for that radial section with the z-axis.
The physical significance of this view is that the helicoidal curvature of the radial section is straightened in this view. Thus unlike projected view, this view is not like a circular arc but has offsets laterally forming an ellipse due to the straightening of the helicoidal curvature.
This process done for all radial section gives the developed outline and the area within the developed outline is called the developed area AD.

Image Courtesy: Google Images.


This view is easy to understand. If we open up the imaginary cylinder for a particular radial section then actually both the helicoidal and radial curvature of the section is straightened and it gives a proper aerofoil section with no curvature along the diagonal of the rectangle as shown.

Image Courtesy: Google Images.

This aerofoil section makes an angle of pitch angle ( for that radial section) with Θ=0 plane. If we rotate it by the same pitch angle, the expanded section is obtained. This view done for all radial sections give us the expanded outline and the area within the expanded outline is called expanded area AE
Image Courtesy: Team Learn Ship Design

It must be mentioned that these different views and areas are characteristics of a propeller and thus are very important to understand.


This article wraps up the screw propeller basics, giving useful insights into the reference coordinate system used to define a propeller, the helix concept and the various propeller views and how they characterise the geometry of a screw propeller. 

Article by: Rijay Majee.