What Is A Deck Prism On Ships?

A Deck Prism was fitted into a ship’s deck to illuminate the areas where sunlight could not penetrate. For centuries, sailing vessels used the reflection and refraction from deck prisms to provide light on board wooden ships. Though candles and kerosene lamps were also used, these posed a hazard.

Meanwhile, the glass prism dispersed the natural sunlight into the space below from a small deck opening without compromising the vessel’s seaworthiness.

Usually, it hung below the overhead and dispersed light in a sideward fashion. The top was flat and had a flush with the deck, ultimately becoming a part of it. The lens shapes were naturally derived from handmaking the glass on an iron.

To get the areas lit brightly, the glass was made colourless with manganese oxide while the purple hue of some specimens found was caused by years of exposure to UV rays.

Onboard colliers or coal ships, these prisms were used to keep an eye on the cargo holds. A fire was lit which was then reflected by the prism and could be seen on the ship’s deck even during the day.

What were the important features of Deck Prism?

• The deck prism was generally built of glass, like any other conventional prism.
• The prism deck was mounted on a roof, where sunlight could be best reflected or on a holding device.
• Sailing vessels had more than one deck prism for better lighting.
• The deck prism was flat-bottomed, making it mountable.
• It was tapering at the top and had many faces for the light to be reflected.

While most prisms were orbital and flat-bottomed, some were rectangular and flat-bottomed.

One of the most famous vessels with prisms shedding ship light was the American whaling vessel, Charles W. Morgan. The vessel which operated during the early 1840s, has been re-modelled and placed in the Mystic Seaport repository. While being restored from 1977 to 1984, it was found that many original deck prisms of the ship had cracked or were damaged. Molds were taken from the original 19th century prisms and hand-cast replicas were produced.

Replica deck prisms from this ship are now sold in the U.S and come in decorative packaging, perfect for awards, anniversaries or even graduation gifts for maritime enthusiasts.

Many maritime repositories have old deck prisms as artefacts. In the present age, electronic nautical equipment is available and ships are well-lit, however, this piece of equipment is an intrinsic part of the maritime heritage and has acquired a significant place in nautical history.

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