Lighthouse Construction and Illumination: A Comprehensive Overview

Maritime engineers must consider the unpredictable forces of the ocean when designing these towers^[1]. Towers built on rocks in the sea are not equally assailed, necessitating an understanding of wave heights in relation to the length of 'fetch,' which is the distance from the windward shore^[1]. Observations begun in 1850 indicated that the height of waves increased nearly in the ratio of the square roots of their distances from the windward shore^[1]. However, waves do not continue to increase indefinitely; the maximum recorded height in the Atlantic was 43 feet, measured by Dr. Scoresby, who also noted their width, time interval, and velocity^[1].

Waves can exert tremendous force at significant heights^[1]. For example, at the Bishop Rock Lighthouse, a bell was broken off at 100 feet above high water, and at Unst Lighthouse, a wall was toppled at 195 feet above the sea^[1]. Further illustrating this, at the Fastnet Light, stones were dislodged from a cliff at 82 feet above sea level, and sheets of water and spray could shut out daylight at 118 to 122 feet above the sea^[1]. The most extreme example of wave force was at Wick breakwater, where concrete masses weighing 1,350 tons and 2,600 tons were displaced^[1].

To quantify these forces, marine dynamometers were designed in 1842 to measure wave impact^[1]. These self-registering instruments consist of a cast-iron cylinder with a circular disc that waves impinge upon, guide rods, and a steel spring to measure the force^[1]. Observations at Skerryvore Rocks in the Atlantic Ocean showed that winter gales exerted forces more than three times greater than those in summer, averaging 2086 lbs. per square foot compared to 611 lbs^[1]. The greatest force recorded at Skerryvore was nearly three tons per square foot during a heavy gale in 1845^[1].

Early lighthouse designs faced numerous challenges, as evidenced by the repeated failures at Eddystone Rocks^[1]. Mr. Henry Winstanley began work on a tower in 1696, completing it in 1700, only to see it disappear in the great storm of 1703^[1]. John Rudyerd, a silk mercer, constructed a timber tower in 1706 that stood for 46 years^[1]. John Smeaton commenced construction of his stone tower in 1756, finishing in 1759^[1]. Later towers, like the Bell Rock, Skerryvore, and Fastnet Rock lighthouses, incorporated lessons from these earlier works, using innovative designs and durable materials like granite^[1].

Alan Stevenson noted that stability should depend more on weight than strength, following nature’s analogy^[1]. He highlighted the importance of a low center of gravity, suggesting a triangular form to ensure stability^[1]. Important design considerations include: sufficient mass to prevent overturning, circular horizontal planes, straight or curved vertical planes to allow free wave movement, and determining height based on visibility requirements^[1]. For soft rock foundations, towers should have curved profiles, while hard rock foundations permit more varied designs^[1]. The best tower position isn't necessarily the highest point but should offer maximum protection from the greatest fetch and deepest water^[1].

The Eddystone rocks provide excellent examples of both successful and unsuccessful lighthouse designs^[1]. Winstanley’s tower was critically flawed in design. Rudyerd's tower was notable for its simplicity and careful adaptation of different parts^[1]. The carpentry was well-constructed, and the tower was well-bolted to the rock, and was constructed on a level base^[1]. Smeaton's tower used dovetailed joints for the stones, averaging one ton in weight, but its sharply curved profile threw away diameter, and the arched floors were a source of weakness^[1].

The shape of the rock can significantly modify the force of waves acting on a tower^[1]. The experience at Dhu Heartach Lighthouse, where blocks were torn out by a summer gale at the same level as the glass panes in Winstanley’s first Eddystone Lighthouse^[1], demonstrates this principle. This further suggests that the height of the waves' impact depends on the relation between wave height, rock configuration, and the seabed's configuration near the lighthouse^[1]. Therefore, a rock can protect a tower or increase wave force and cause them to strike higher than expected^[1].