# Measuring Maritime Paint Thickness under Water Using THz Cross-Correlation Spectroscopy
## 1 Introduction  
The shipping industry accounts for 2.7-2.9% [1; 2; 3] of all anthropogenic greenhouse gas emissions (GHG), equivalent to 1076 million tonnes of CO2e each year [1; 3]. As a result, the industry is subject to increasing regulations and pressure to reduce GHG emissions in the interest of reducing its global carbon footprint [4]. Major organizations, such as the International Maritime Organisation, are committing to a reduction of GHG of 50% by 2050 compared to 2008 [5], fueling the demand for technical solutions that can help increase the fuel efficiency of large vessels such as container ships. A vital component to ensure the efficiency increase is the ability of vessel operators to constantly monitor the quality and condition of the vessel paint during docking sessions and active operation. The underlying reason is that the paint highly influences the hydrodynamic properties of the vessel, most prominently by minimizing the drag through the prevention of fouling [6]. Fouling is the accumulation of organic material such as biofilm, algae, and barnacles [7]. Fouling on vessel surfaces greatly affects the overall drag, increasing fuel consumption by up to 30% in extreme cases [8; 9]. The practical way to prevent fouling is to apply antifouling paint on vessels as the outermost paint layer [7]. This paint is applied on top of anticorrosive and binding layers. The most common antifouling paint is by design \"self-polishing\" [10], which means that the paint slowly deteriorates to expose new clean antifouling paint, thus removing any fouling accumulation. In cases where the antifouling paint wear is higher than anticipated, or the paint layer is thinner than estimated, the self-polishing process leaves the vessel partly unprotected [8]. Consequently, ships are sandblasted and repainted upon dry-dock sessions, but this is a costlyprocess due to labor, paint, rerouting the vessel, and the lost operating time [11], which encourages the shipping companies to extend the periods between docking sessions as long as possible. In turn, it has been estimated that 10% of the fuel consumption of ships stems from inefficient surface performance of the hull [12]. Since most antifouling paint is self-polishing, its condition can at any point during vessel operation be well-described by its remaining paint thickness. Shipping companies can consequently use continuous paint thickness monitoring to optimize the fuel efficiency of their vessels, leading to a reduction in GHG emissions. On top of the GHG emission increase, biofouling on ship hulls leads to invasions of nonindigenous marine species in foreign waters. This can lead to the introduction of new infectious diseases and threaten the biodiversity of freshwater, estuarine, and marine ecosystems [13]. By knowing the exact remaining thickness of antifouling paint, it is possible to establish procedures that can effectively mitigate the introduction of invasive species by reducing the amount of biomaterial attached to container ship hulls [14]. Currently, there are several methods used to measure the full-stack thickness of ship paint including all layers given as one number. A fast, reliable, and accurate way of doing this is a device utilizing the Hall effect and eddy currents to measure a wide variety of dry paint types [14,15]. Ultrasound has also proved to be able to accurately and reliably measure the thickness of ship paint [16]. In the case of wet paint, which is relevant upon paint application during a dry-dock session, another used method is simple pins of different heights pushed through the paint, called a \"Wet Film Gauge\". The number of pins in contact with the paint corresponds to the absolute thickness of the paint [17]. One drawback of this method is that the thickness of the wet paint shrinks, as the paint dries. Therefore, a wet paint thickness measurement can be unreliable [18].