Marine Propulsion Methods
It’s Fun Fact Friday and today we’re looking at marine propulsion, or more simply, how vessels move in the water. Paddles and sails were the first marine propulsion systems, powered by humans rowing or capturing wind to move the boat. Rowed galleys with sails even combined those two methods. In the 1800s, steam engines were developed and came into wide use, followed by diesel engines and gas turbine power in the 1900s. Nuclear reactors started showing up in the 1950s in large vessels like icebreakers and military, failing to reach widespread commercial use. Electric motors and LNG power have become more popular as of late, and combining methods into dual-fuel and hybrid vessels is now common. Let’s take a look at some of these technologies and how they’ve led to the ships we see today.
Tilting rotor sails on SC Connector. Image Credit: Riviera.
Sails and Oars
For many years ships were powered by sails or oars. Most ships used for travel or merchant services used sails to travel on the wind. When wars were fought by ramming ships or fighting hand-to-hand, galleys became popular as they provided better maneuvering. Naval guns in the 1500s brought ships back to sail, where they stayed for a few hundred years. Sails function as a sail on a tall mast, controlled with ropes, and are making a comeback in different forms as people seek more efficient ways of travel.
The first successful commercial steamboat was built in 1807, the North River Steamboat. They rose in popularity, mostly being used for river and short distances because of high weight, low power, and poor reliability. There are many types of steam engine such as the side-lever, grasshopper, crosshead, waking-beam, steeple, and others - enough for an entire article just dedicated to steam. For this overview, steam engine technology developed along with marine technology. Paddles gave way to screw propulsion and wooden hulls were replaced with iron, then steel, thereby allowing ships to grow and require ever more powerful engines. At their core, steam engines use cylinder technology that gives power to a crankshaft to power a ship.
Triple expansion steam engine. Image Credit: Wikipedia
Steam engines evolved into steam turbines, which are still used today. Steam turbines can be powered by coal, fuel oil, or even nuclear power. In 1897, Sir Charles Algernon Parsons’ marine steam turbine increased the power-to-weight ratio and produced a whole new line of high speed liners in the early 20th century. Around that time, coal was replaced by fuel oil as the primary fuel for these engines - a much more convenient and less manpower intensive solution.
LNG carriers are built with steam turbines. They store liquefied natural gas at extremely low temperatures and must draw some “boil-off” gas to maintain pressure and temperature. This gas can be used to fuel the ship, making more steam for the ship’s turbines. There is also a process for re-liquifying LNG in the works, but it’s mostly being directed at diesel carriers since they burn diesel while hauling LNG.
Nuclear powered steam turbines use water heated by a nuclear reactor to create steam. The biggest detriment to nuclear steam power is the cost - upfront and maintenance costs are much higher than other fuels, even with no emissions and fuel price security. Nuclear is now limited to military vessels and some icebreakers like the Arkitka class, with over 75,000 horsepower. No worries about arctic conditions negatively impacting their fuel because nuclear-powered vessels, such as aircraft carriers and submarines, don’t have to refuel for months at a time. They also produce no emissions and can travel quickly and quietly. The additional room that would be taken up by fuel can be used for other storage, such as aircraft fuel on a carrier. Due to the long list of benefits, including no carbon emissions and faster cruise speeds, interest is growing in using nuclear for commercial vessels.
Nuclear aircraft carrier. Image Credit: The Atlantic
Since about 1960, most ships have been built to run on diesel fuel. Reciprocating diesel engines have found uses connected directly to the prop shaft, used with reduction gearboxes, or producing electricity to power electric motors to move vessels. The first diesel engine went into use in 1903, but their popularity really grew with turbocharging, which increased their power to weight ratio. Large merchant vessels are usually powered by a slow two-stroke diesel engine or a medium speed four-stroke engine. Two-stroke engines have a smaller footprint, but are taller than their four-stroke counterparts, so four-stroke engines see more use in passenger applications like cruise ships.
Dual-fuel engines can be powered by LNG, marine diesel, or heavy fuel oil. LNG is becoming more popular as we seek to reduce emissions, but multiple fuel options make vessels more flexible as they travel. LNG is also the most efficient fuel, but finding a fueling station can be more difficult than other types of fuel.
Battery electric power was first pioneered in the late 19th century in small lake boats. Lead-acid batteries powered propellers that moved the boats. Battery electric systems provided limited range, and diesel-electric hybrid systems became much more popular. As a result of the growing popularity of hybrid systems, a recently launched ship operates on battery power alone, with a 40-mile range.
World’s largest hybrid vessel. Image Credit: Marine Insight.
The most popular propulsion types are sail, paddle wheel, propeller, and jet. Sails use the wind, one of the oldest methods for traveling via boat, and are coming back with new technology. Paddle wheel systems use a large wheel with multiple blades with the bottom of the wheel underwater. Screws are a more efficient way of navigating, but paddle wheels can easily traverse shallow waters and by spinning the wheels in opposite directions, rotate a vessel in place. Screws, known as propellers, are widely used today in ships of all sizes from a small bass boat with one small propeller to huge ships with anywhere from 1-4 huge props. Pump jets are the last form of propulsion, and frequently used in personal watercraft like jet skis and speedboats, and can even use thrust vectoring to steer a vehicle.
We have come full circle with propulsion methods from early sails and paddles to modern nuclear vessels and back to sails to augment other propulsion types. Companies like AirSeas produce an automated kite that deploys to use wind power to tow large ships with wind to reduce fuel usage. Norsepower even has rotor sails that use the Magnus effect to spin large cylinders, producing power for ships that offsets fuel usage. The early focus of marine power systems was the most space efficient use of power, which is now being replaced by environmental efficiency. As ships continue to evolve, we’re excited to see what new propulsion systems will be in store in the future.
Air sail on a container ship. Image Credit: AirSeas.