Hydraulics continue to be used heavily in marine applications, as they offer the power density, safety and shock resistance needed in these environments.
By Josh Cosford, Contributing Editor
If you’re fortunate enough to own any variation of recreational watercraft, or if you’ve been lucky enough to sail an ocean in a colossal cruise ship, you’re likely to think little about how they operate. Other than small boats with outboard motors, there really isn’t much mechanical technology to see for passengers on watercraft. You’re either enjoying a beverage while showered with spray while sitting atop the finest vinyl upholstery of a speed boat or gazing at the chandelier hanging five stories above you in an ocean liner’s atrium. If you ask me, both options sound like great ways to spend the day.
With the mechanical systems of boats so thoughtfully tucked away, you’d be understandably surprised by the variety of systems and devices that use hydraulics. In some large ships, you’d be more hard-pressed to find something that moves without hydraulic motivation. So perfectly suited to marine conditions is hydraulics that it will likely retain its position atop the hierarchy indefinitely.
Hydraulics offer more than power density
Let’s start by covering the benefits of hydraulics for marine applications, which, believe it or not, goes far beyond the power density advantage. Yes, hydraulics still offers the most power in the smallest package, at least from the actuator’s perspective. Hydraulic power units often occupy significant real estate, especially aboard large ships, but that doesn’t take away from how they provide the highest force or torque to any remotely mounted actuator.
Mechanical systems simply don’t have the power or reliability when operated in wet and often cold oceans. Electrical actuators do not have the power to handle the tasks called upon them at a massive scale, such as powering and rotating azimuth thrusters (Figure 1), which themselves may be many stories tall. Although electric propulsion is becoming more common, the steering and drive systems on large thrusters are primarily hydraulic, as well as the retraction systems, where required.
Electronic control continues to saturate the marine environment, just as with industrial and mobile hydraulic machinery. The level of sophistication and control is extraordinary when using computer-controlled hydraulics, so expect to see modern cruise ships, tankers and aircraft carriers all employing electro-proportional control over their hydraulic systems. However, you truly won’t see 8000 kW propulsion systems that are electric-only.
Reliable and safe
Even as electric functions permeate the dry side of a ship’s hull, much of what goes on above deck must be reliable in even the most punishing ambient conditions, including intense saline spray, bone-chilling cold, and the baking sun. Anchor handling, deck machinery, winches (Figure 2), and ramps/gangways are all perfect for hydraulic actuation, to name a few.
Part of what goes into a robust, reliable marine hydraulic system is the oil used. Although some boats may never see any given extreme, such as a tropical fishing boat, others travel through every extreme on nearly every trip, such as transpacific cargo ships. To withstand heat of the Arabian Sea then arrive weeks later at the port in Montréal, a marine hydraulic oil must exhibit an excellent viscosity index to withstand both. Viscosity index describes oil’s ability to maintain its rated viscosity through severe temperature fluctuations.
Additionally, all marine oil formulae are recommended to adhere to the EPA’s and the U.S. Fish and Wildlife Department’s toxicity tests for marine life. In such tests, the EPA will measure the lethal concentration of a substance that kills 50% of the test organisms within a specified period, usually 96 hours. The U.S. Fish and Wildlife Department’s test is similar but also covers chronic toxicity related to survival, growth and reproduction. So long as the oil meets these criteria, synthetic hydraulic fluids may also be suitable rather than only bio-based oils.
Anchors aweigh
The primary marine hydraulic applications are some of the most unique in our realm, while others are clever takes on old fluid power concepts. Many other machines employ hydraulics for steering and winch systems, for example, but you’ll find only anchor handling and lifeboat davits within the marine environment (obviously).
Large vessels have large anchors, as you would assume. Some ships’ anchors weigh many tons, so the handling systems are not only robust but also engineered to withstand huge tensile forces and shock loads. So large are some ship’s anchors that they require entirely specific vessels to transport them. Anchor handling and towing winches are complex hydraulic machines capable of hundreds of tons of pulling force and many times higher braking force.
An anchor handling and towing winch is more than a powerful hydraulic motor and a drum. It must also control the drum when letting go the anchor, as you can imagine a 15-ton chunk of iron has the potential to accelerate under gravity even through viscous water. Hydraulic braking allows such a feat to limit velocity and arrest the paying out of the anchor should an unsafe situation arise. Also, hydraulic motors using brake valves are valuable tools for limiting payout speed.
Similar to a cylinder’s counterbalance valve, any load-induced pull on the hydraulic motor creates higher pressure at the outlet port and reduced pressure at the inlet work port. Because the brake valve requires a pilot pressure signal from the inlet work port, the load-induced pressure differential causes the brake valve to close until pressure at the inlet port increases again. The pressure rise once again opens the brake valve, allowing the motor to turn and the anchor to once again pay out. The concept sounds digital in nature, but in reality, a delicate balance is achieved where the brake valve is metering flow perfectly to balance pilot pressure with load pressure.
Anchor windlass equipment is specific to the raising and lowering of anchors, but anchor handling and towing winches are more versatile since they may be used to operate many other winching functions. The windlass portion of such a device can be disengaged with a clutch system, allowing work drums to perform other tasks, such as rope or cable winching for mooring, towing or crane functions. As you’d expect, the clutch cylinders may also be hydraulic.
You’d be hard-pressed to find a better application for radial piston motors (Figure 3) than anchor handling systems. They’re powerful, efficient, reliable and available in the giant sizes required to pull enormous anchors at 30-40 feet per minute. If that doesn’t sound terribly fast, remember that even the anchor chain can weigh as much as the anchor itself. Plus, excessive force on the chain links and gypsy (the winch teeth that pull on the anchor) could lead to stress failure while increasing the danger to the deck crew.
One small caveat about anchors, especially those on large ships: they’re simply not used on many new, advanced ships. With the precise electronic control of azimuth thrusters positioned throughout the hull, the network of GPS, gyroscopes, motion reference units (mrus), and wind sensors feed real-time data to the Dynamic Positioning System. The DP System simply holds the ship in place without any need for an anchor.
Safe deployment of lifeboats
Should something go wrong, you’ll be glad that hydraulics are a primary and essential part of lifeboat davits. The davit (Figure 4) is a lifting and lowering system that safely takes passengers in lifeboats from the shipside to the ocean surface. Looking much like a twin crane, hydraulic cylinders lift the lifeboats and extend them outward where they can clear the ship.
As expected, hydraulic motors may also lift and lower the boat. You can bet your dingy that brake valve-controlled motors will prevent the cables from lowering too rapidly, which risks slamming passengers hard onto the water.
You’ll notice in Figure 4 that the mast has an accumulator mounted to the side. Herein lies a great advantage to hydraulics — high-power energy storage. Should the state of a ship in an emergency be so dire as to offer no electric power, the accumulator provides all the energy the davit needs to lift, extend and lower the lifeboat.
One of many options
I must mention, of course, to prevent a flurry of criticism and opposing viewpoints, that not every application discussed is the exclusive domain of hydraulics. There are electric and mechanical versions of thrusters, winches, and davits, but in my humble opinion, hydraulic options are the most desirable.
No matter your preference, hydraulics will always be a critical source of power and motivation for marine applications, especially for huge ships such as tankers, cruise ships, and container ships. The power, control and reliability of this mature technology is too hard to ignore and will permeate marine technology for decades to come.
MARY – as you can see, I had some trouble with the subheads on this one… would it be possible to remove 2 lines of copy from the first column (after “figure 2” as im trying to keep that on this spread, but before “anchors aweight”), that would be super. It should fix both of the wonky subheads and copyfit for us. please let me know if thats not possible!!
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