SANDWICH VERSUS SINGLE SKIN : Material Concept of a Patrol Boat in a Life Cycle Cost Perspective

Abstract: This report describes the Master Thesis Project "Single Skin Versus Sandwich: Material Concept of a Patrol Boat in a Life Cycle Cost Perspective" performed at Kockums AB, Karlskrona, and reviewed and graded at the Royal Institute of Technology, Stockholm. There are both economic and environmental gains of developing fuel-efficient (light) vessels. Kockums have successfully produced ships in sandwich composite material, which is a light and stiff but expensive material concept. Building a vessel in single skin composite might result in a lower total life cycle cost due to several factors. Kockums wish to acquire more information of the affecting factors thus they have initiated this project. The project includes analyzing the accumulated cost of a concept patrol vessel while changing five variables: class notation ("Patrol" or "Passenger"), operational profile (10 or 35 knots), material concept (sandwich or single skin), material (carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP)) and choice of propulsion system (controllable pitch propeller (CPP), Inboard Performance System (IPS) or water jet) resulting in 48 versions of the vessel. First, the structural arrangement was adapted to the design loads of each version by iteratively seeking the maximal structural utilization of the elements. This was done by using a computational tool (RSTRUCT) that allows for effective scantling calculation. Then, the material, production and operational cost were determined for each version and the break-even points in terms of years of operation were found. The results gave insights concerning the characteristics of the different material concepts. The single skin versions were found to be both heavier (70 %) and more costly in terms of material and production cost (17 %) in relation to their sandwich equivalents. The break-even points between CFRP versions and GFRP versions were ranging between 4 and 14 years, depending on operational profile. For example, for a very low speed profile passenger vessel the break-even point was 40 years, implying that the GFRP version was the most beneficial choice. Regarding propulsion choices, the IPS system was the best choice for every version due to its high overall propulsive coefficient in a broad speed range.