Two aims of advanced boat development are fuel efficiency through reduction of drag and protection of the environment against pollution through use of ‘cleaner’ fuel sources. Materials may also include carbon-fiber composite multihul. 20–30% GHG-emissions reduction for shipping is possible and may occasion economical and health benefits. Reduction in drag may offer significant fuel efficiency. Advancements in technology such as aluminum alloys or carbon-fiber composites may reduce hull weight (and hence displacement and drag) for a given payload. A bulbous bow may be positioned just below the waterline to modify the way water flows around the hull. Large ships with bulbous bows have a twelve to fifteen percent better fuel efficiency than similar vessels without them. Drag may be reduced by decreasing the effective wetted area of the hull. In Skjold class patrol boats this is achieved by augmenting the buoyancy by incorporating a fan-blown skirted compartment between the two rigid catamaran-type hulls. This design permits lower displacement and therefore reduced drag at high speeds. A related concept is the Air Cavity System. Creating an air cavity on the underside of a hull may reduce drag by up to 15%. Fuel cell powered boats may benefit from the higher thermodynamic efficiency of fuel cells compared to internal combustion engines (40-60% vs 20-25%).