Naval Architecture

“It wasn’t just a ship. It was a ship of dreams.”  That’s how the people defined the “Titanic” before her maiden voyage. That proves   how they wondered by seeing a great ship, that great vessel. Even though people talk about the Great ships like titanic, many of them haven’t much knowledge about ship designing. However the way of designing ships is called “Naval Architecture”.  It is a very interesting field to study. But it’s not very popular in Sri Lanka.

The difference between “marine engineering” and “naval architecture”, “naval architecture” means designing of the vessel. Naval architect is a professional engineer who is responsible for the design, construction, building and repair of the ship  ”Marine engineering” means integration of the all components of the ship. Marine engineers ensure that ship systems function as expected.  Colombo dockyard is the best place to gain knowledge about naval architecting & marine engineering. Naval architects should have knowledge about following subjects such as thermodynamics, hydrodynamics, material science, Resistance and propulsion in ship design, ship hull strength, floating and stability etc.

Type of ships

Designing this kind of a huge vessel is not an easy task. Naval architects have to apply the every engineering, scientific and ergonomically related factors to make a successful great vessel as an output. Followings are the preliminary factors considering in the design or build a ship.

·         Dimensions

·         Displacement

·         Stability of the structure

·         Propulsion characteristics

·         Hull formation

·         General arrangement

·         Principal structural details

Also Ships are classified to different types of categories according to their purpose, size, and type. These are some of them.

Ø  Container Ships

These are used for carrying huge load of cargo containers overseas

Ø  Bulk Carrier Ships

These ships are used for transport unpackaged bulk cargo such as coal, food, cement, ore, steel coils etc.

Ø  RoRo / Roll on Roll off  Ships

Special type of ships which are used for the transportation of automobile vehicles. Entrance to the vessel is at front (bow) or back (aft) or both sides

Ø  Tanker Ships

Tanker ships are specialized for carrying large amount of liquid cargo.

Ø  Passenger ships -The primary function of this ship is transporting passengers on the sea. Also known as cruise ships. In modern day these ships are similarly to ‘floating cities’

Ø  Offshore Vessels

Use for construction jobs in the sea, scientific researches and oil explorations

Ship Constructions

There are uncountable components casting up together to build a gigantic ship. In the construction sites each of the block parts of the ship are built and assembled Most of the parts are assembled and joined by welding. However there are lots of technical processes related to the ship construction.

The hull of the ship usually has curvature shapes. But it  is not easy to produce as it is. There are some special methods use to bend the metal sheets in ship yard. This process is called “plate bending” .In this process curved surfaces can be divided in to two groups. “None developable surfaces” and “Developable surfaces”.

·        None developable surfaces- this shape cannot be produced by direct rolling or hydraulic Bending of straight plates. Producing method of this product is a kind of a example for reverse engineering.

                        

·        Developable surfaces- this shape can be produced by direct rolling or hydraulic Bending of straight plates.

 

         

· Hydraulic press is another essential machine used in ship construction sites. This is used for purposes like plate bending, straitening of pre-bent plates etc. Hydraulic pressing is a beneficial process for ship construction as well as other industries. This is a cold work process. Hence there is no residual stress generating on the metal plates. It is an advantage. But the main disadvantage of above process is “Elastic spring back”.

                      In that case when the hydraulic ram gives a stress on the plate more than the proportional limit the plate enters a plastic region. After when the machine is unloaded the unloading takes place along a line that is parallel to linear part of the stress strain curve up to the proportional limit. Therefore the actual deformation obtained is less than what was actually required. (Shown in below)

Furthermore shell plates are rolled by using “plate rolling” method. This is used to correct the curvature obtained by hydraulic press. Two lower rollers are driven and the plate is fed above them. The top roller, larger in diameter than the other lower two rollers, rotates along with the motion of the plate. A hydraulic beam is used above the top roller to apply downward load on the plate.

                               

Before welding came into popular use in the early 1900s, every ship was constructed on the building berth. The each part and framed structure was held together by long pieces of wood called ribbands. Plating was then added and all the parts of the structure were riveted together. In modern ship building each of parts is built up from subassemblies or component parts, which are then welded together to form the complete section.

Main parts of a ship

v Stern

The back part of the ship. Some scientific factors such as Hydrodynamic efficiency, construction simplicity, flow patterns are considered for designing a ship’s stern. It should induce to avoid vibrations. The stern should be able to keep uniform inflow of water to propeller. Hence it cause to high propulsive efficiency. There are main 3 types of sterns. Elliptical Stern, Cruiser stern, Transom stern

The stern and in particular its underside effects the propulsion efficiency. Reduce the turbulence in the area between propeller and outer surface above the propeller accrete its efficiency. (shown in the Figure 08)

Ø Important facts

·       Benefits of having elliptical stern includes larger deck space in the back-word (aft) deck. Massive amount of excess buoyancy is provided.

·       The deck of the ships which has transom stern can easily get wet during reversing operations and in a heavy sea.

·       A lot of research is still carried out to find even better & effective stern structure.

The Naval Architects of “Queen Mary 2” designed a hybrid stern by a rounded fixed to the square shape as a response to the rough seas that the ship would experience in a North Atlantic. This is called as a “Constanzi Stern”. A constanzi stern provides the transom which required for azimuthal pod propulsions, and provides better sea holding characteristics.

v   Propeller

        Propeller is a rotating fan. The function of the propeller is propelling the ship by using the power generated and transmitted by the main engine of the ship. Actually the propeller works by turning torque into a thrust. There are forces created by moving the water away from the blades of the propeller and these forces are what helps move the ship forward. The water, coming across the propeller, gets a cone shape, widening when it leaves the propeller. Ship propellers on the basis of Bernoulli’s principle and Newton’s third law. Normally a ship can be made with a single propeller or double propellers or specially with three propeller. It depends on the requirements of the ship. Sea water is a corrosion accelerator. So the manufacturing material of the propeller must be a corrosion resistant material. Alloy of aluminum and stainless steel is mostly used for this. Stainless steel is made up of iron, chromium, nickel, manganese and copper.  The chromium is added as an agent to provide corrosion

v   Hull

Hull is the most important part of the ship. If there would be a failure in the hull, since all the other functionalities in the ship doesn’t even matter. Because this is the outer cover of the ship. Mostly mild steel use as the raw material. The first step in designing a hull of a ship is designing its shape and form. The form of the ship’s hull is measured by using some special various coefficients

·        Block Coefficient - ratio of the ship’s underwater volume to the volume of the imaginary rectangle surrounding the underwater portion of the hull. Value of this coefficient should be less than 1.

·        Midship coefficient - This coefficient is the ratio of the submerged area of the midship section to the surrounding rectangle

        These are the main steps of the hull construction.

1. Calculation of Loads on the Hull
2. Scantling Calculations for Midship(for girders, beams, plates)
3. Midship Section Modulus( bending stress at the deck and the keel,safety factors are calculated in this step)
4. Frame-wise Scantling Calculation
5. Calculation of Steel Weight
6. Development of 3D Structural Model and FEA Analyses.

References

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2.Marine insight. 2020. [online] Available at: <https://www.marineinsight.com/naval-architecture/hull-ship-understanding-design-characteristics/> [Accessed 27 April 2020].

3.Ship Propellers - An Overview | Sciencedirect Topics. [online] Science direct. Available at:  <https://www.sciencedirect.com/topics/engineering/ship-propellers> [Accessed 04 May 2020].

4.2020. Propeller, Types Of Propellers And Construction Of Propellers. Marine Insight.  [online] Available at: <https://www.marineinsight.com/naval-architecture/propeller-types-of-propellers-and-construction-of-propellers/> [Accessed 02 May 2020].

5.Naval Architecture - Strength of Ships. [online] Britannica. Available at: <https://www.britannica.com/technology/naval-architecture/Strength-of-ships> [Accessed 01 May 2020].

6.Sinha, T., 2020. Different Types of Sterns Used For Ships. [online] Marine Insight. Available at: <https://www.marineinsight.com/naval-architecture/different-types-ships-sterns/> [Accessed 03 May 2020].

7.Encyclopedia Britannica. Ship | Watercraft. [online]. Available at: <https://www.britannica.com/technology/ship> [Accessed 03 May 2020].

8. Chakraborty,S,2013. Ship Construction: Plate Machining, Assembly of Hull Units And Block Erection. [online] Marine Insight. Available at: <:https://www.marineinsight.com/naval-architecture/ship-construction-plate-machining-assembly-hull-units-block-erection/> [Accessed 26 April 2020].

9.Fracking Boom Sends Chemical Tankers to Five-Year High – gCaptain. Science direct. Available at: <https://www.sciencedirect.com/topics/engineering/ship-propellers>. [Accessed 26th April 2020].

10.What is Block Coefficient of a ship?. Available at: http://marinegyaan.com/what-is-block-coefficient-cb-of-a-ship/> [Accessed 28 April 2020].

11.Marine Insight. Available at: < https://www.marineinsight.com/naval-architecture/types-of-bow-designs-used-for-ships/> [Accessed 29 April 2020].

Heshan Dharmapala

Department of Manufacturing and Industrial Engineering

University of Peradeniya