10 Simple Terms to Understand Ship Construction Better

A seafarer may not be involved in the ship construction so much. But that does not mean that they need not know anything about it. For example if you see a crack in ballast tank, you should be able to tell exactly where the crack is.

It is only possible if you can identify the structures inside the ballast tank. Same applies to the other parts of the ship too.

We read about ship construction during our competency exams. Only problem is ship construction topics starts from the medium level and not from the basic level.

For example if we need to draw the construction of double bottom, learning just involves placing each component at its place without answering why.

The first step of studying the ship construction need to be knowing about the basic terms of ship constructions.

In this post I will discuss and explain 10 basic terms used in ship construction.

But before we do that, first lets discuss briefly what ship is made of.

What are ships made of

Ships are made of steel and we all know that. But what all forms of steel are used in building ships. I mean what shape of steel ?

Before learning anything about ship construction we need to know the shapes used in ship construction. Let us discuss few of these.

Plate

We all know what we mean by plate. It is a rectangular shape steel with certain thickness. The most common place you can see the plate in ship construction is the main deck and side of the ship.

Bar

There is only one principle difference between steel plate and steel bar. The steel bar has lesser cross sectional width than the steel plate.

This table gives a fair idea what cross section of a steel can be called a plate/sheet or a bar.

Beam

A beam is a structural element that increases load bearing capability and avoid bending of the structure. Beams can be categorized by many types but from point of ship construction, we need to categorize it through cross sectional shape.

For example, there are L-Shaped beams, C-shaped beams, I-shaped beams and SI-shaped beams. SI shaped beams are I-shaped beams with slight curved cross section.

Shape of the beams are not limited to these ones. It can be in many other shapes.

Now that we know the shapes and structure of the steel used in ship construction, we can discuss the ship construction terms. But we have to understand that all these structure will be a form of plate, bar or beam.

1. Keel

First thing first. When we say “Keel of the ship”, we are referring to a steel plate. Though it can also be in the form of steel bar but it is very rare these days.

Keel of the ship is the bottom most plate of the ship laid along the entire length of the ship.

During construction of the ship this is the first thing that is placed on the blocks. Because of this, laying the keep is considered to be the time when construction of the ship begins.

Keel laying also has the importance with respect to many IMO conventions as this is the date construction of the ship starts.

So keel is the steel plate with slightly more thickness than the other adjoining plates on its sides. This area need more strengthening because the ship sits on this area on the blocks, during construction as well as during dry docks.

We can consider the keel of the ship as the backbone or spine of the ship.

Years back keel laying involved actually laying the keel of the ship on the blocks. See below the keel of the titanic being laid.

Nowadays keel laying is considered to be the time when first piece of the ship structure is placed on the the wooden blocks. Like see below the keel laying of cruise ship “Anthem of the seas“.

2. Shell plating

Again when we say shell plating, we are referring to a steel plate. In this case it is too obvious from the name.

Shell plating is the steel plate used on the shell of the ship. The dictionary meaning of the shell is “hard outer covering”. So shell plating is whatever we see on outer side of the ship.

One of the main purpose of shell plating is to provide watertight skin of the ship. Apart from that it also adds to the longitudinal strength of the ship.

Ship construction requires us to be able to pin point each component. So when we say shell plating, it is a huge area. We can further divide this area in two broad parts, which is

§ Bottom shell plating

§ Side shell plating

I am so much sure that you already would know what these two terms mean.

Bottom shell plating is the shell plating on the bottom part of the ship. Or we can can that bottom shell plating form the watertight skin of the bottom of the ship. It is the plate laid in the horizontal direction.

To a certain extent we can say that even keel of the ship is also bottom shell plate.

Side shell plating is the shell plating on the sides of the ship that forms the watertight skin along the ship side. It is the plate laid in the vertical direction.

3. Deck plating

This is one is too obvious. Deck plating again is a plate. It is steel plate that forms the main deck of the ship.

As per the location, we can give a more specific name to the deck plating. For example accommodation deck plating or main deck plating.

4. Strake

To understand what strake is, we need to again understand the construction of shell plating.

I am dead sure you know that ship’s side plate (Shell plating) is not made of single metal sheet. It is made of smaller sections of steel plates. These smaller sections of the plate are welded together.

In old days these were riveted instead of welding.

Irrespective of with what method these plates are joined together, these are placed in a certain pattern.

Let us understand this with an example of box shaped ship. The smaller plates that we are talking about would look something like this for the box shaped ship.

In naval architecture and ship construction each of these plate has an identification. These are numbered in certain pattern. For example the plates on the side shell plating are numbered like this.

§ The plates are identified by a letter and a number

§ Each plate is given a letter in the vertical direction starting with A from the bottom.

§ Each plate is given a number in the horizontal direction starting from aft.

Now based on these numbering system the identification of plates for the box shaped ship would look something like this.

Each section of the plate placed in fore and aft direction is called a “strake”.

So in the above example, we have A-Strake, B-Strake and so on.

If we want to be more specific, We can say that we have starboard A-Strake , starboard B-strake and so on. Similar naming will be on the port side.

For the bottom shell plating, similar numbering is done for the plates forming it. For the bottom shell plating

§ Plates are given a letter starting with “A” to the plate adjacent to the keel plate.

§ Plates and given a number starting from the aft

So the plate adjacent to the keel plate will have a identification of A1, A2 and so on starting with A1 from the after most plate.

Now there are three type of strakes that you would hear in ship construction. Let us see what these are.

Shear Strake

Shear strake is the top most strake of the Side shell plating. So if side shell plating has strakes from A to K, then K-strake is the shear strake. This is the strake that connects with the deck plating.

Can you identify the shear strake in the box shape ship example that i gave above ? Yes, you got it right. I-Strake is the shear strake in this case.

Shear strake is usually of higher thickness and strength than other strakes. This is because this strake can have lot of physical damages during the life of ship.

Keel Strake

I earlier said that keel is a plate that is back bone of the ship. But keel plate is also made up of smaller sections of the steel plates. These strake of plates that form the keel of the ship is called Keel Strake.

The plates in the keel strake are number from aft to forward starting from the aft one which has the

number “1”.

Unlike other strakes, keel strakes do not have letter associated with its naming.

Garboard Strake

Garboard strake is the first strake on each side of the keel strake. As we discussed in the bottom strakes, the first stake from the keel is named A-strake.

So we can also say that Bottom A-strake is called Garboard strake.

Now for the box shape ship, the naming of strakes might look simple but for actual ship with curved sides and flared bottom, it might not be that simple.

The naming and construction of side and bottom shell plating is provided in the “Shell expansion plan”.

Next time you get your hands on a shell expansion plan of your ship, try to read it to identify shear strake, keel strake and garboard strake.

So below picture shows how shell plating and Strakes are connected with each other.

5. Stiffners

A steel plate of 2mm thickness can be easily bent. If you lift this plate with the crane from its ends, it will most likely bend at the center. Now what can you do to avoid it from bending ?

One easy solution is to increase the thickness of the plate by welding two or more plates together. But in ship construction this is not a good idea. One among many reasons for this is that we do not want to increase the light weight of the ship.

The better option is to weld two of more flat bars either in fore and aft direction or athwartship direction.

In this case these flat bars will be called stiffners. Anything that is used solely to provide strength to a ship structure can be called a stiffner.

A stiffner welded in longitudinal direction is called “Longitudinal stiffner”. Same way a stiffners welded in transverse direction is simply called “transverse stiffner”.

Longitudinal stiffners and transverse stiffners can further will named according to various factors including the amount of strength these provide to the structure.

Let us discuss few of these.

6. Girder

Girders are one form of longitudinal stiffners. These are

§ Big size steel plate (or sometimes beams) connected to bottom of the ship

§ Runs in fore and aft direction

§ strengthen the ship against bending moment

Girders are also named according to its location. The girder at the center of the ship is called center girder.

In double bottom tanks, the thick solid metal plate that divides the ballast tanks into port and starboard tank is the center girder.

Apart from the center girder, there can be few girders on each side of the ship or double bottom.

Depending upon the breadth of the ship, there can be more than one side girder on each side.

While center girder runs in full depth of the double bottom tank, side girders may or may not run in full depth.

Though below photo is not that of a ship, it is good representation of a ship’s strengthening components. If you see in this case girder is further strengthened with something we may call “Vertical Stiffners” as there is no other specific name for it.

7. Longitudinals

Longitudinal is short name for longitudinal stiffners.

In most cases the longitudinals are beams (mostly L-beams) and runs along the length of the ship. Double bottom space is the most common space where you can easliy identify longitudinals.

Longitudinals are further named depending upon what section these longitudinals are strengthening.

In double bottom ballast tank, longitudinals strengthening the bottom are called “bottom longitudinals”. Longitudinals strenghtening the top of the ballast tank are called “top longitudinals”.

If you have been to a double bottom tank, these longitudinals are difficult to miss. The bottom longitudinals are the one we sometime step on to avoid stepping in the left over water (or mud) of the ballast tank.

I am sure you can locate the longitudinals in above picture that I shared. If you cannot, I share it again with marking of longitudinals.

8. Frames

Frames act as major transverse stiffners. Frames are

§ Big size steel plate connected to bottom of the ship

§ Runs in athwartship direction

§ strengthen the ship against buckling and bending.

We can consider the frames to be similar to the girders but with only one difference. Frames runs in transverse (Athwartship) direction.

If keel is the backbone of the ship, the frames are considered to be the rib cage.

Frames are numbered starting from aft perpendiculars.

9. Transverse

Transverse is the short name for “transverse stiffners”. These are different from frames in two ways. First unlike frames these do not extend to full height of the compartment. Second transverse are of lesser strength and thickness than frames.

Based on the location of transverse, these may be called “bottom transverse”, “inner bottom transverse” or even “deck transverse”.

10. Stringers

In the above photo, notice how the girder is stiffned. The stiffner is welded in vertical direction. If same stiffner is required to be welded in horizontal direction, it will be called stringer.

One of the location where stringer is mostly used is the forward part of the ship. Bow of the ship has to bear lots of stress and so requires extra strengthening.