As a child I have always wondered why do boats have round windows. Many of us may have noted while travelling that all vessels, with the exception of cruise ships, have circular windows.

These windows are frequently referred to as portholes; a contraction of the term port-hole window. And portholes are not only found on ships; they are also present on submarines and spacecraft.

 Now that I am a Boat lover , i did some research to find out the reason. Here is what I Found.

First of all, circular openings on ships are called port holes and the glass covers are called port lights. Technically it is incorrect to call them windows.

Round windows (or portholes/side scuttles.) are easier to seal and more resistant to wind and water than square windows. Circular shapes provide substantial support. Portholes are also circular in shape, as they serve as windows through which to poke old-fashioned artillery turrets.

A ship’s round windows, or portholes as they are more widely called, are one of its distinguishing features. Portholes refer to the circular openings, whereas port lights refer to the glass covers.

Porthole is the abbreviation for the term “port-hole window.” Portholes are seen not only on ships, but also on submarines and spaceships.

Generally, vessels with pressure applied to the hulls (whether ships, boats  or aeroplanes) have rounder windows to avoid stress concentrating on the corners. Round windows uniformly distribute stress across the building, making it less prone to failure.

A wave striking the side of a ship might exert pressure on it. For instance, you will observe that the windows closest to the waterline are rounder, whilst the windows higher above (which will not be subject to water stress) can be any form the designer desires!

What is always true about “windows” on ships, particularly those let into the hull proper, is that the corners are always rounded. 

The purpose here is to avoid what is known generally as corner or notch stress. In the video below the stress on the structure comes largely from pressure in the cabin of aircraft. In ships the stress is produced because the hull girder on large vessels is subject to bending and twisting stress as well as vibration. 

To avoid stress concentration, the corners of all windows in the hull girder and other loaded structures are rounded. Some isolated structures on ships may have squared corners at openings, but they are rare.

Portholes have been an intrinsic element of the ship building since the beginning. Portholes have always stood out in comparison to other types of transportation’s windows, owing to their circular structure. Originally, they were intended to provide the best possible view from the ship during a voyage.

As a result, it became critical to strategically modify the porthole’s height, with one end hinged.
It was originally intended to serve as a window in regions of the vessel with significant ventilation problems, allowing for the admission of both fresh air and light.

This was especially beneficial for retaining personnel, as the staffing requirements in those days were quite high.
Portholes were afterwards added to every nook and cranny of the ship.

So, why are there no square or rectangular windows on ships?

This is related primarily to structural integrity. The water exerts considerable pressure on the ship’s body, and square windows are particularly vulnerable to stress. 

Rectangular or square windows have a tendency to be weaker in some locations than others. Round shapes are logically more robust and easier to strengthen. The ship portholes’ unusual round shape provides protection to sunlight, as well as to sea and rain water.

Brass and bronze are frequently used for portholes because, unlike iron and steel, brass and bronze do not rust when exposed to sea water. A downside of steel is that it will eventually bend.

Not all ships are equipped with portholes. The structures of large cruise liners are substantially higher, and their upper deck suites are extremely high, with large windows and even balconies.

Why are boats equipped with circular windows?

As you can see, there are two primary reasons for round windows on boats. The cannon design is how it all began in the past, but modern windows remain round due to the robustness of design it provides for ships at sea.

Originally intended to secure guns to the side of Tudor warships, portholes, like all other forms of windows, are today primarily employed to let light and ventilation into lower-deck areas.
Despite their age of almost 500 years, portholes are still widely utilised on a variety of boats, ships, and even residences today.

However, why are they still used by so many ships and boats today? Continue reading for a more detailed explanation of why boats have circular windows.

What are Portholes and why are they called Portholes?

A porthole is a distinguishing feature of both boats and larger nautical vessels. Originally built for bigger ships during the Tudor period, they are currently found on a variety of seacraft and residential structures for both functional and aesthetic purposes.

Portholes, alternatively referred to as a ‘Bull’s-eye window’, as well as a side scuttle or side hole (as defined in the International Convention for the Safety of Life at Sea and the United States Code of Federal Regulations), are iconic in their appearance.

Port holes were established during the reign of Henry VII of England by French shipbuilder James Baker as a method of fastening huge cannons to the forecastle and aft castle parts of the ship when they were too enormous to be secured using normal ways.

To do this, he bore spherical pieces into the ship’s hull, through which the cannons’ firing ends could be placed. When not in use, these holes were concealed by doors, or ‘portes’ in French, which became perverted into the term ‘porthole’.

However, cannons are not the primary reason for round portholes. Indeed, this has a strong correlation with stress concentration.

What are the advantages of Porthole?

If you own a ship or boat and are considering replacing some windows, a porthole provides far more benefits than a classic nautical appearance.

Portholes are hinged on one side to allow for ventilation in below-deck quarters during calm weather. When closed, they are designed to be waterproof and weatherproof, which means that even in the most severe sea conditions, no water will be able to enter the ship.

Ships with circular windows are more durable.

Ships and sea-fairing vessels experience a great deal, from strong seas to the ship’s basic tossing and twisting. Normal, square windows have weak areas, particularly at their corners, which make them susceptible to shattering or leaking.

Round windows, on the other hand, distribute the load and strain evenly across the frame, as there are no weak corners. This is why aeroplane windows have rounded edges, as the increased pressure on the exterior of the ship results in rounded corners.

Portholes, like all windows, let in light, allowing those below deck to work for longer periods of time without the use of lights, and they also provide ventilation during calm weather.

Portholes, which are typically around two feet in diameter, can weigh up to 100 pounds, which adds to the window’s strength.

They are extremely durable and are available in a range of metals, including steel, aluminium, brass, bronze, and iron.

However, if you’re looking for a long-lasting, durable porthole that isn’t only for cosmetic purposes, bronze and brass are your best bet, as aluminium and steel windows have a propensity to rust and deform when exposed to sea water over an extended period of time.

Rooms on cruise ships with portholes

Not all cruise ship cabins have windows, and these ‘ocean view’ suites can cost far more than a regular interior cabin further within the ship.

However, cruise ship ‘porthole cabins’ can be a less expensive choice that still allows you to see the sea, as long as you are happy with a room on the ship’s lower decks.

Certain cruise ships designate certain types of rooms as ‘porthole cabins,’ which are typically located on the lower decks, closer to the water.

While porthole cabins may not offer the same panoramic views as rooms with larger, more modern windows and do not include a balcony or veranda, if you’re searching for a more vintage experience, booking a porthole cabin can be rather thrilling.

1 international knot =1 nautical mile per hour
1852 m is the length of the internationally agreed nautical mile.
The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m).
The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft [1853.184 m]).

Is it necessary to get a porthole for my boat?

If you own a boat and are interested in purchasing a porthole, you have a variety of alternatives; many of which are dependent on the type of boat you own, the purpose of the porthole, and the location of the porthole on your boat.

Modern, functional portholes are commonly available in three distinct styles: traditional circle portholes, decorative oval portholes, and rectangular portholes with rounded edges. 

The type of porthole you select is mainly determined by the style you like, although different brands and designs are better suited for installation on specific portions of the ship, such as the hull, deck, or coach roof.