Formula to Predict Transmission for P‐Type Floating Breakwaters

The aim of this paper is to define a simple and useful formula to predict wave transmission for a common type of floating breakwater (FB), supplied with two lateral vertical plates protruding downwards, named π‐type FB. Eight different models, with mass varying from 16 to 76 kg, anchored with chains, have been tested in the wave flume of the Maritime Laboratory of Padova University, under irregular wave conditions. Water elevation in front and behind the structure has been measured with two arrays of four wave gauges. Our starting point for the prediction of wave transmission was the classical relation established by Macagno in 1954. His relation has been derived for a box‐type fixed breakwater assuming irrotational flow. Consequently he significantly underestimated transmission for short waves and for large drafts. This paper proposes an empirical modification of his relation to properly fit the experimental results and a standardised plotting system of the transmission coefficient, based on a simple non‐dimensional variable. This variable is the ratio between the peak period of the incident wave and an approximation of the natural period of the heave oscillation. A fairly good accuracy of the prediction is found analysing the data in the literature relative to variously moored π‐type FBs, tested in small scale wave tanks under regular and irregular wave conditions.