Actually, Stena had some ship proposals a few years back that proposed dual engines and
propellers for reliability and increased maneuverability when transiting canals. These same
ships also had proposals for an air cavity underneath the hulls, like a catamaran.
But the cavity held a layer of air underneath the hull that would reduce hull friction and
therefore fuel consumption. So it produces less pollution.
Some additional concepts call for an air lubrication system, high pressure air is bubbled beneath the
hull near the bow, which would also create a reduction of hull friction, and gain a 6% reduction in fuel consumption. Mitsubishi, and NYK have been testing the systems over the last two years.