# Impact Force Mechanism of a Trimaran Model and Its Experimental Validation
## 1 Introduction  
Trimarans have excellent stability, seakeeping, and rapidity and are widely used in high-speed transport and in warships. Compared to the mono-hull ship, it has the advantages of better overall arrangement for the wider flight deck, stronger survivability as a kind of warship, and smaller resistance as a type of transport. In adverse sea conditions, trimarans' sheaves will move due to strong wind and a high wave load. Although the duration of the impact pressure is relatively short, its peak value is very large. The mild impact pressure will cause the deformation of or fatigue damage to the hull structure, and the serious impact pressure will even be a threat to the safety of the hull and may result in serious marine accidents. For military ships, the bang will affect the ship's electronic equipment, and for civilian ships, the bang will affect the passengers' experience and cause unnecessary panic. To date, many trimaran accidents have been caused by water slamming, and thus, the prediction of the slamming load due to water has received much attention. For example, the Onomichi maru, weighing 66,874 tons, sank due to water slamming in the early 1980s. It is therefore necessary to find a way to reduce the water-slamming pressure.  
Research on water slamming dates back to [PERSON] in 1929. It involves potential flow theory [1], the conservation of momentum [2], and elastomer angles, as well as the changes in elastic effect when a wedge-shaped body enters the water [3]. The model structure for water-slamming experiments can be a sphere or cone [4], structures with different sections [5], a flat bottom [6], a V shape with an areal ship profile [7], a large wedge plate frame with different stiffnesses [8], or a wedge [9]. In the last decade, the research on the slamming force has focused on simulation [10, 11]. For example, using CFD-FEA is a possible way to simulate the slamming and the green water loads on a sailing ship, and it is a method that combines computational fluid dynamics (CFD) and finite element analysis (FEA) solvers [12]. Furthermore, the simulation of green water and the slamming loads of ships shows that high-frequency components of the freakwaves have a significant impact on the frequencies of peak pressure in the bow and deck area [13]. After finding ways to perform the simulation, people were able to predict water-slamming loads in different situations. Arbitrary Lagrangian-Eulerian (ALE) finite element method is one of the ways to perform the prediction, and it was used to study the impact between the flat panel and calm water [14]. However, by comparing the results from the experiment and the simulation, it was found that the numerical results might be larger than the experimental results by as high as 21% [15]. As the accuracy of the prediction of the slamming pressure was unacceptable, the fully coupled ALE algorithm was used to improve the accuracy [16]. However, the discretization of ALE will cause uncertainty in the simulation [17]. Another approach to the prediction of the slamming loads was presented by combining the seakeeping theory and CFD technology [18].  
Some recent studies on trimarans are available. The peak pressure value was found to increase when the heel angle enlarges on the lower side of the trimaran [19].  
There are many articles on simulations of the slamming pressure and the factors that influence the slamming pressure in the literature. However, little progress has been made in the field of reducing water slamming [20]. One of the possible ways to reduce the slamming pressure is to use the air-cushion effect [21]. In this paper, flexible cladding is placed in certain places where the peak water-slamming load is huge in order to decrease its influence on the ship.