Measurement of Wave Force on Lighthouse Structures

To obtain numerical results of wave force, Thomas Stevenson designed the marine dynamometer in 1842– an instrument capable of self-registering the force of waves^[1]. As Stevenson noted, 'there is no contest to which the old proverb "fas est ab hoste doceri" is more applicable than in resisting the surge of the ocean'^[1], indicating the importance of understanding the ocean's power through direct measurement^[1].

The dynamometer consists of a cast-iron cylinder (D, E, F, Fig. 2) bolted to the rock, featuring a circular flange (D) with an access door^[1]. A circular disc (A) on which the waves impinge is connected to four guide rods (B) that pass through a circular plate (C) screwed to flange D and through holes in the cylinder's bottom^[1]. Inside the cylinder, a steel spring is attached to plate C and connected to a small circular plate K secured to the guide rods B^[1]. Leather rings (T) serve as indices, registering how far the rods have been pushed through the holes or, in other words, how far the spring has been drawn out by the waves' action^[1].

The following formula is used for graduating the instrument^[1]:

d = (D * a) / W

Where:

• d = the length in inches on the proposed scale, corresponding to a force of one ton per square foot acting on the disc^[1]
• W = weight stated in tons, which is found by experiment to produce a given amount of yielding of the spring^[1]
• D = number of inches yielded by the spring with weight W^[1]
• a = area of the disc in square feet^[1]

The dynamometers were typically placed to be immersed around the ¾ tide mark in locations providing considerable depth of water^[1]. Discs varied in diameter from 3 to 9 inches, though 6 inches was common, and spring strengths ranged from 5 to 10 lbs per inch of elongation^[1]. Instrument lengths varied from 14 inches to 3 feet, and their readings were subsequently standardized to a value per square foot^[1].

Observations using the marine dynamometer commenced in 1843 at Skerryvore rocks in Argyllshire and at the Bell Rock in the German Ocean in 1844^[1]. These observations helped determine the relative forces of summer and winter gales, as well as the greatest recorded forces in the Atlantic and German Oceans^[1]. However, it is important to note that the values obtained are applicable to areas of limited extent and should not be extrapolated to large surfaces of masonry^[1].

Experiments at Dunbar harbor revealed that the greatest wave force was exerted at high water level, with forces decreasing quickly above and below this level^[1]. Furthermore, instruments placed to measure the recoil force of waves indicated that the force of recoil could be three times greater than the direct force of the waves before they reached the wall, due to the concentration of energy produced by the resistance of the masonry^[1].