Tuesday, 17 April 2018

Types of Propellers

Introduction

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

SCREW PROPELLER- PART 2

INTRODUCTION


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.

SCREW PROPELLER- THE REFERENCE SYSTEM


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.

SCREW PROPELLER- THE GEOMETRY


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.

        PROPELLER VIEWS


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.

PROJECTED VIEW

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.

DEVELOPED VIEW


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.

EXPANDED VIEW


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.

CONCLUSION


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.





Monday, 26 March 2018

SCREW PROPELLER- PART 1

INTRODUCTION


Screw propeller is the hydrodynamic device that rotates in a hydrodynamic medium due to a torque (Q) provided by the main engine of the ship. Due to this rotation, a thrust (T) is produced by the propeller and this thrust is transmitted from the propeller blades, through the propeller shaft, finally to the thrust block of the ship that transfers the load to the entire hull of the ship, which pushes the ship forward.

Image Courtesy: Google Images.

In spite of several advancements in the marine propulsion system in the recent past, screw propellers are still the most widely used propulsion system, generally fitted to almost every displacement vessels. In this article, I am going to explain the geometry of screw propeller and various terminologies associated with screw propellers.


SCREW PROPELLER- VARIOUS PARTS

Screw propeller consists of a number of blades that are attached to a paraboloid structure called the boss. Generally, the connection is done by welding the blades with the boss for a fixed pitch propeller. Whereas for a controllable pitch propeller where the blade can change its angle of orientation, a special mechanical arrangement is provided that assists smooth rotation of the propeller blades along with maintaining the water tightness of the whole system.
          The side of the propeller blade seen when one observes the propeller from behind the ship is called the face of the propeller. The other side of the propeller blade is called the back of the propeller.
          The narrow end of the boss creates a lot of disturbance as it rotates in the hydrodynamic medium leading to flow turbulence, separations and even hub vortex cavitations. Thus in order to smoothen the flow around the hub tip, a smoothly streamlined cap is provided called the boss cap.
          The axis passing through the centreline of the propeller shaft is called the propeller axis. The part of the propeller blade farthest from the axis is termed as the propeller tip. In the propeller blade, the side that pierces the water first as per the direction of rotation is called the leading edge of the propeller, and the other side that follows the leading edge is called the trailing edge of the propeller.
Image Courtesy: Team LSD.

SCREW PROPELLER- RIGHT AND LEFT-HAND SCREW

Screw propeller whose direction of rotation if is represented by curling of our fingers of our right hand produces thrust along the direction of the thumb finger, then it is called right-hand screw propeller. If a propeller exhibits the same phenomenon for the left hand, then it is called left-hand screw propeller.
          Generally, in most of the ship where single screw propeller is provided, a right-hand screw propeller is used. There is no such reason behind this. It may be considered as a conventional practice. On the other hand for twin screw propellers one of the propellers is right hand and another is the left-hand screw. This is done to negate the circulation produced by one propeller to disturb the flow onto another.

Image Courtesy: Google Images.

SCREW PROPELLER- THE SLIP PHENOMENON

Screw propeller works with the basic principle of a screw, such that with every complete rotation of the screw, it advances by 1 pitch. The only difference being that in a screw and nut pair due to the presence of a solid thread, the nut exactly advances by 1 pitch on one complete rotation of the screw. In case of propellers, the situation is analogous but as the propeller works in a hydrodynamic medium where there is no thread, so the propeller slips. Thus on every rotation, the propeller advances by a distance less than the pitch of the propeller.
Image Courtesy: Google Images.

SCREW PROPELLER- TERMINOLOGIES

To understand screw propellers we must understand some of the characteristic terminologies associated with the geometry of the propeller.

RAKE

We know that propeller is made up of different radial sections joined together and the face of the radial section lies on a helicoidal surface. If the line generating this helicoidal surface is perpendicular to the propeller axis then the propeller blade is said to have no rake.
           Rake is provided to increase the clearance between the stern frame and the propeller blade tip such that the pressure pulses emitted by the propeller blade due to the rotation in the hydrodynamic medium don’t impart vibrations in the hull.
Image Courtesy: Google Images.

SKEW

In the projection of the propeller blade on a plane perpendicular to the propeller axis (projected view) a line is drawn joining the midpoints of the projected sections. If this line is perpendicular to the propeller axis then the propeller is said to have no skew.
           Skew is provided in a propeller to increase its expanded area so that loading on the propeller (Burrill Cavitation Criteria) is less and thus cavitation is reduced. Also due to skew the different radial sections of the propeller don't pass simultaneously along the stern counter of the hull where the disturbances in the wake are high. thus pressure fluctuations are not over a large area and the peak is reduced. Thus skewed propeller performs better in a hydrodynamic medium. 
Image Courtesy: Google Images.

 
PITCH AND PITCH ANGLE

Pitch is defined as the distance the propeller would advance by one complete rotation of the propeller. For a radial, section pitch may be same or different.
Pitch angle () is defined as

            Where,
            P= Pitch.
            r= radial section radius.


CONCLUSION

In this article, I have tried to introduce the readers to the basic terminologies of a screw propeller. The geometrical parameters of a screw propeller will be discussed in the upcoming article. 

This is an informative video of various Marine Propellers prepared by Team LearnShipDesign. Hope you all like it.



Article by- Rijay Majee.

Sunday, 25 February 2018

Ship Launching

INTRODUCTION


Launching of any structure is quite an anticipated occasion depicting the hard work paying off and victory after years of scrupulous construction and numerous man hours deployed. And when it comes to a near self-sustained vessel sailing on the notorious ocean or river waters, the celebration is unimaginable. It is one of the most important procedures in the entire chain of ship construction processes.
               But even a subtle miscalculation or error may compromise the ship’s launching and mourn the spectators’ faces as they witness something going horribly wrong. To obviate any such risk ship launching is meticulously planned and everyone directly involved is quite punctilious.

TYPES OF SHIP LAUNCHING


Various types of Ship Launching Methods are used depending upon the feasibility of ship and geographical parameters.-

Stern First

This method dates back to ancient times and is one of the most familiar methods for ship launch. The prerequisites involve arranging the slipways to be nearly perpendicular to the shoreline. Nowadays reinforced concrete mats are used as slipways to provide sufficient strength.Construction is done on temporary cribbing so as to give access to the hull’s outer bottom.As preparation for launching, a pair of standing ways with greased surfaces is erected under the hull and out into the barricades.A pair of sliding ways is placed on top, under the hull, and a launch cradle with the bow and stern poppets is erected on these sliding ways. Common mechanisms for releasing the vessel in launching ceremony include weak links designed to be cut at a signal.On launching, the vessel slides back, down the slipway until it floats by itself.

Image courtesy: Google Images.


Sideways

This type of launching is used where the water channel is not feasible for lengthwise launching. The slipways are built so that the vessel is side-on to the water and is launched sideways. However more sets of launching assisting construction are required to support the ship and this act as a downside. Rigorous calculations have to be made to check the stability of the vessel as it touches the water surface to avoid capsizing of the vessel due to lateral moment created during launching process.

 Image courtesy: Google Images.


Air Bags

This method has the upper hand of requiring less permanent infrastructure, risk and cost and is quite safe and innovative. A series of inflated tubes placed under the hull deflate to cause a downward slope into the water. The airbags made of reinforced rubber layers have high load capacity and are usually cylindrical in shape with hemispherical heads at both ends. The Xiao Qinghe shipyard was the first to manifest this type of launching in 1981.


 Image courtesy: Google Images.


Float Out

Though not technically recognised as a ship launching method, this method is most widely used one among the shipyards. Ships built in drydocks are launched simply by filling the dock with water and the vessel is 'Floated out'. Thus it is a simple, effective and safe procedure. though the initial investment is high.

 Image courtesy: Google Images.


LAUNCHING CURVES


Predictions of the movement are vital to ship’s safe control. A set of six curves is prepared to predict the behaviour of the ship during launch. They are curves plotted against the distance of travel down the slipway for end launching process

1.       Weight
2.       Buoyancy
3.       Moment of Weight about fore poppet
4.       Moment of Buoyancy about fore poppet
5.       Moment of Weight about after end of groundways
6.       Moment of Buoyancy about aft end of groundways



 Image courtesy: Basic Ship Theory (K.J. Rawson & E.C. Tupper).


The Important features of these curves are as follows-

·         At the point at which the moment of buoyancy about the fore poppet equals the moment of weight about the fore poppet, the stern lifts.
·         The difference between the weight and buoyancy curves at the position of stern lift is the maximum force on the fore poppet.
·         The curve of the moment of buoyancy about the aft end of the ways must lie wholly above the curve of the moment of weight; the least distance between the two curves of the moment about the aft end of ways; gives the least moment against tipping about the end of ways.
·         Crossing of the weight and buoyancy curves before the after end of ways, indicates that the fore poppet will not drop off the end of the ways


           Thus launching, though a celebratory and ceremonial event requires a lot of background calculations with minimal errors so as to be able to predict the vessel’s motion in advance. Each parameter needs to be assessed with utmost accuracy to prevent stability loss or any undesirable accidents on the launching day.
           Here is a video of various ship launches recorded. They are fascinating to watch, but at the same time, they involve a long calculation and thinking process and a huge effort of engineers to accomplish the task.

Video courtesy: wapindia.in


Article by: Shivansh Singh

Sunday, 11 February 2018

HULL VANE- THE CONCEPT

INTRODUCTION


The history of Hull Vane can be traced back to 1992 when it was first used in full-scale trials of a catamaran. Surprisingly, the results of the test showed that the vessel had reduced bow-up trim and resistance and, this had driven interest among the engineers to further carry out research on this device.
At first, the questions that strike our mind are what actually is a Hull Vane?
How does it look like?
What is the purpose of using it in ships?
And where actually is it used on ships?

 All these questions are answered in this article to give a general idea about Hull vane, its geometry and purpose.
Model used for hull vane model test 
Image courtesy- Google Images

Hull Vane is a fixed, resistance reducing foil attached to the hull below the water line near the stern of the ship.
In order to increase the fuel efficiency of the ships, the hull resistance must decrease. The concept of hull vane first struck the mind of Dr IR. Pieter Van Oossanen of the Netherlands and the first patent was filed by him in the year 2002. Since then a number of tests, for the optimization of this device, have been carried out using model tests, CFD and full-scale trials. The results were remarkable and showed fuel reduction in excess of 20% for yachts and 5% to 10% in other vessels mostly naval vessels, merchant ships, cruise ships, etc.
Hull vane animated concept
Image courtesy- Google Images

During the trial of the catamaran in 1992, it was found that the vessel wasn’t acquiring its required speed due to excessive trim and wave generation. By placing a foil in the steepest part of the interacting wave system aft of the midship, reduced the bow-up trim and the resistance significantly. Since then a number of tests, trials were carried out for a range of vessels like container ships, Ro-Ro vessels, Supply vessels, cruise ships, etc. and the results show a decrease in resistance to 26.5% to an increase of 9.5% which clearly indicates that the device is not suitable for all kinds of vessels. 
The second application of the Hull Vane, on the 2003 IACC yacht Le Defi Areva.
Image courtesy- Google Images

In 2014, two vessels equipped with Hull Vane were launched. A 55 meter supply vessel Karina manufactured by Shipyard De Hoop in the Netherlands and 42m yacht built by Heesen Yachts. The required engine power was reduced by
15% in the former and in the latter vessel a resistance reduction of 23% was significantly observed.

HULL VANE - WORKING PRINCIPLE


In this section, we will discuss how the Hull Vane actually does what it has been designed to do. We can observe four prominent effects of hull vane on the vessel dynamics.

THE THRUST FORCE


It is based on the basic foil theory. A schematic overview of the forces on the Hull Vane is given in the below figure.
Schematic overview of the forces on the Hull Vane in a section view of the aft ship.
Image courtesy- Google Images

Ξ± is defined as the hull vane inflow angle ( the angle between the inflow and the chord line), Ξ² is defined as the hull vane angle ( the angle between the chord and the body fixed x-axis). The vessel displayed in the figure is at zero trim.
The foil creates a lift force vector LHV which is by definition perpendicular to the direction of flow of water, and a drag force vector DHV in the direction of the flow. The sum of these vectors FHV can be decomposed into an x-component and a z-component:
LHV + DHV = FHV = Fx,HV + Fz,HV
If the x-component of the lift vector is larger than the x-component of drag vector, the resulting force in x-direction provides thrust. The lift and Drag Forces can be estimated using:
LHV = CL * ½πœŒV2A
DHV = CD * ½πœŒV2A
If ΞΈ is defined as the trim angle (the angle between the body fixed x-axis and the inertial x-axis) the thrust force that is generated by the Hull Vane can be derived by the equation:
F x, HV = sin (𝛼+𝛽+πœƒ) LHV – cos (𝛼+𝛽+πœƒ) DHV

TRIM CORRECTION


The force in the z-direction affects the trim, and especially at higher speeds, this trim reduction proves to have a large influence on the total resistance of the vessel. This effect can also be achieved with interceptors, trim tabs, trim wedges or ballasting. Similarly, to the force in the x-direction, the force in the z-direction can be estimated using:
 F z, HV = cos (𝛼+𝛽+πœƒ) LHV + sin (𝛼+𝛽+πœƒ) DHV
With this, the influence of the hull vane on the running trim can be derived using:
π›Ώπœƒ = tπ‘Ÿπ‘–π‘šπ‘šπ‘–π‘›π‘” π‘šπ‘œπ‘šπ‘’π‘›π‘‘ / π‘Ÿπ‘–π‘”β„Žπ‘‘π‘–π‘›π‘” π‘šπ‘œπ‘šπ‘’π‘›π‘‘ π‘π‘’π‘Ÿ π‘‘π‘’π‘”π‘Ÿπ‘’π‘’ π‘œπ‘“ π‘‘π‘Ÿπ‘–π‘š
≈ FZ π‘Žπ‘Ÿπ‘š / 𝐺𝑀L π›₯ 𝑔 sin (1°)
Not only the trim reduction itself has a positive influence on the hull’s performance, but the trim also affects the angle of attack of the water flow on the hull vane.

REDUCTION IN WAVES FORMATION


The flow along the hull vane creates a low-pressure region on the top surface of the hull vane. This low-pressure region interferes favourably with the transom wave, resulting in a significantly lower wave profile. The wave reduction is so significant that it can be observed by eye. The reduction of waves not only leads to a more beneficial resistance, it also leads to less noise on the aft deck, and to a lower wake. The former is mainly beneficial for yachts and the latter for inland shipping, where wake restrictions limit ship speeds in ports or other enclosed areas.
 Wave pattern on the 55 meter supply vessel without Hull Vane (top) and with Hull Vane (bottom) at 20 knots
Image courtesy- Google Images

 Comparison of the wave profile of the 55 meter supply vessel without Hull Vane (left) and with Hull Vane (right) at 13 knots.
Image courtesy- Google Images

Image courtesy- Google Images

REDUCTION OF VESSEL MOTIONS IN WAVES


Another significant effect of the hull vane is that it dampens the heave and pitch motions of the vessel. When the vessel is pitching bow-down the stern of the vessel is lifted and the vertical lift on the hull vane is reduced by the reduced angle of attack of the flow. This counteracts the pitching motion. Similarly, during the part of pitching motion in which the stern is depressed into the water, the vertical lift on the hull vane is increased. This again counteracts the pitching motions and similarly, it also dampens the heave motions.
 Image courtesy- Google Images


The advantage of reduction in motions is that the added resistance due to waves is reduced, which makes the hull vane even more effective in rough waters. As the motions are reduced, it increases the comfort levels onboard the vessel, safety and range of operability in various sea states.

 INFLUENCE OF POSITION OF THE HULL VANE


According to Moerke, if the Hull vane is fitted too close to the hull, it might lie in the boundary layer thus reducing the lift it generates. In addition to it, the low-pressure region on the upper side of the hull vane is reflected on to the hull and additional pressure resistance is created on the hull. Hence, the resistance of the combination of the hull and hull vane increases. After carrying a number of CFD analyses, it was found out that if the hull vane is placed behind the transom of the vessel, the pressure reflection can be reduced along with a slight reduction in the thrust generated by the hull vane.
Another consideration in the positioning of the hull vane is the angle of water flow near the stern of the vessel. The largest angle of attack can be achieved by placing it in the steepest part of the transom wave. But at high speeds, this location is found to be too far aft of the hull. An Additional complication is that this optimal location is very dependent on the wavelength and thus on the ship speed. In the vertical direction, a higher angle of attack can be achieved by placing the hull vane closer to the hull which is restricted by the free surface effect on the lift generated by hull vane, slamming by waves and pitching motions if it is placed too close to the water surface.
Hull Vane fitted behind the transom
Image courtesy- Google Images

INFLUENCE OF SHIP SPEED AND SHAPE OF THE HULL


According to Moerke, the hull vane is more effective at higher speeds. This statement was also supported by MARIN as they observed a power reduction of 3.3% at 17 knots (Fn 0.21) and up to 10.2% at 21 knots (Fn 0.27) for a 169m container vessel during its model tests. For high Froude numbers, the results in saving are much better. Also from tests and trials, it has been found out that hull vane is most favourable for Froude numbers in the non-planing region, between 0.2 to 0.7. 
Comparison of Resistance for a 42m, 47m and 55m motor yacht and 300m container vessel fitted with and without hull vane.
Image courtesy- Google Images

The addition of hull vane adds to the wetted surface area, the friction resistance thus increases in comparison to a vessel without hull vane. Above Fn 0.2, pressure resistance becomes more dominant. Therefore, best results are obtained for a range of 0.2 to 0.7 Fn. At higher Fn, the force generates by the hull vane creates an unbeneficial bow-down trim.
According to Moerke and Zaaijer, if the buttock angle is increased, the angle of attack of the flow to the hull vane increases and the lift vector is directed more forward increasing the resulting decomposed force in the x-direction. Also, the effect of pressure is minimized if the water column near the transom is maintained as much as possible. The leading edge of the hull vane experiences a lower hydrostatic pressure than the trailing edge when it is positioned below the front of the transom wave. The shape of the stern of the ship also a major role. Flat buttocks are considered ideal as they ensure a uniform flow.                                             

HULL VANE AND SHIP TYPES


The effectiveness of the hull vane is also dependent on the ship type as stated earlier. It is not very effective for bulk and crude oil carriers. For vessels less than 30m LOA, the investment costs are high as compared to savings using a hull vane.
Ideally, hull vane is best suited for medium and large-sized vessels operating at moderate or high non-planing speeds like the ferries, supply vessels, cruise ships, patrol and naval vessels, motor yachts, reefer ships, Ro-Ro vessels, car carriers and container vessels.
Hull Vane
Image courtesy- Google Images

The hull vane is a fuel saving device aimed to lower the pressure resistance which is the dominant component at higher speeds. CFD computations, model tests and sea trials have shown potential resistance reductions of more than 20% depending on the ship speed and hull shape, especially on merchant ships with resistance reduction between 5% and 10%.


This is a hull vane documentary video. It will give better insight about the concept of hull vane.


                    Video courtesy- Hull Vane Bv (YouTube channel)



Acknowledgements:
Hull Vane (Van Oossanen Naval Architects, The Netherlands)

Article by: Kushagra Gupta