OutSail Shipping Launches Wingsails to reduce cargo ship fuel consumption
Sails powered ships for millennia; but then the convenience of energy-dense fuels displaced sails. As ship speeds eventually exceeded wind speeds, the consensus became that sails had no place in shipping and were relegated to hobbyists and sport. Fast forward a century and a half, and maritime shipping, like all other industries, is facing a reckoning to mitigate the greenhouse gasses produced by their activities.
The International Maritime Organization (IMO) has introduced new regulations which use a vessel’s Carbon Intensity Indicator (CII) to grade ships. This grading scale becomes more aggressive over time, and any ship with a poor grade must take corrective action. The corrective actions can be as non-invasive as reducing speed (aka: slow steaming) or as extreme as a retrofit to use a different, cleaner fuel source. This costs millions and takes a ship out of commission for months, and it’s difficult to ensure your (now more expensive) fuel is available at every port of call. Ship owners are hedging their bets that slow steaming will dominate their future, with ship order books full to reflect the increased capacity needed when containers take 20% longer to cross the ocean.
Aerodynamically, sails are simply vertical wings. Wind blowing across the vessel causes the sail to generate lift and drag, and the resultant vector has some forward component to pull the ship through the water. However, if the wind comes from an angle too close to the direction of travel, there is no thrust. As an added complication, the sail only sees the relative wind. If the ship travels faster, the wind will appear to come from closer and closer to the direction of travel, even if the true wind is coming from perpendicular to your course! Despite this, standard sails can still produce forward thrust as long as the wind is at least 20 degrees off from directly in front of the vessel. This is how our sails can still save power, even on a fast moving vessel.
There are many sail technologies out there. A common question we get asked is “Are you going to use flettner rotors/suction airfoils?”. Both of these technologies use power supplied by the ship to increase the lift produced by a surface; rotor-sails spin, and suction airfoils…suck? Each of these have a place, especially at low vessel speeds. But our customers ask us for a solution that works for container ships cruising at the relatively high speed of 22kt. At these speeds, the relative wind is almost always ahead of you, so lift/drag becomes more important. Powered sails suffer from poor lift/drag, both from the high induced drag from very high lift coefficients, and system losses from drawing on ship’s power. So no we are not going with flettner rotors/suction airfoils. While they are the new exciting technology on the block, if you factor in their power usage and high drag ratio, they are just not as practical as a simple sail.