Cruise ship operators are turning to transverse thrusters to maintain position when the ships stop offshore but cannot drop anchor and to improve manoeuvring and mooring in busy harbours
Cruise companies are devising ever more interesting experiences for their passengers and when they need to do things like stop dead, far offshore, they rely on thrusters to hold these giants of the sea stationary in what is known as the ‘operational area’.
Wärtsilä will supply no less than seven of its latest thrusters, along with the rest of the propulsion systems, to each of the new vessels for Geneva-based MSC Cruises. Under construction at Chantiers de L’Atlantique, the LNG-fuelled ships will be launched in 2022 and 2024.
Wärtsilä could be heading for world domination in transverse thrusters for cruise ships. By early 2019 the company had booked orders for more than 60 of these thrusters for 12 cruise ships under construction in four shipyards. All these orders landed in a 12-month period.
“This represents the dominant share of newbuild vessels within the cruise sector during this period,” notes Lauri Tianen, director for thrusters at Wärtsilä Marine.
But why the relatively recent attraction of transverse thrusters for cruise ships? There are two main reasons: first, they are vital for maintaining position when the ships stop offshore but cannot drop anchor, which is increasingly the case in environmentally-controlled waters; and second, they make for easier manoeuvring and mooring in often busy harbours.
Cruise ship captains routinely conduct a risk analysis before entering a port. This concerns planning for anything that may go wrong and a corresponding work-through of every anticipated manoeuvre. As captain Jose Vilarinho of the 593 m-long Azamara Quest, a relatively small cruise ship of 30,277gt, told Cruiseline.com: “We can handle the waves and the swells and the rain, but the worst thing for me is the wind. In port it is a very strong enemy because the ship is stopped and any wind can impact it.”
This is why Wärtsilä’s latest transverse thrusters – the WTT series – cover a power range up to 5,500 kw, big enough for a cruise ship.
Much smaller vessels of course may need heavyweight thrusters, like the steerable versions used on tug boats that require high levels of bollard pull. Powered by a six-cylinder Wärtsilä engine and equipped with WST24 steerable thrusters, two 32 m-long new tugs operated by the Turkish Directorate General of Coastal Safety on the Bosphorus Strait of Istanbul achieve a bollard pull of 75. Normally, an eight-cylinder engine would be required to achieve a 75tbp, Wärtsilä points out.
And at the other end of the scale, it is impossible for the biggest containerships to work in restricted waters without the aid of bow and stern thrusters, collectively known as manoeuvring thrusters. Because they are essentially propellers with a transverse axis of rotation instead of a longitudinal one, these hydraulically powered thrusters allow these vessels to make remarkably precise and accurate movements.
And with environmental obligations in the forefront, Wärtsilä’s latest transverse thrusters comply with tough international standards relating to discharges into the sea. The secret here is the new generation of bio-lubricants, the chemistry of which is tightly defined and regulated.
Importantly, the thrusters’ hydraulic system is integrated for lubrication purposes. According to Wärtsilä, integrated hydraulics are more reliable and easier to maintain. Also, they save space. In a clever trick, the hydraulics are integrated into the e-motor foundation and the thruster’s gearbox does double service as the oil tank: “This eliminates the conventional standalone hydraulic power unit”, Wärtsilä says.
Wärtsilä also has ease of construction in mind with its latest brand of transverse thrusters. As well as providing all the essential documentation for installation and commissioning, these thrusters are designed to be married to the hull with a minimum number of welds and fewer pipes. For easy access all components requiring periodic maintenance are installed in a single location. And, compared with conventional thrusters, the panels and cabinets take up to 50% less space.
The power-to-weight ratio has also been improved, the latest thrusters being smaller and lighter which makes installation faster and less costly.
With a broad range of power from 500k w to 5,500 wk in its portfolio, it is little surprise that Wärtsilä has delivered well over 3,200 of its transverse thrusters around the world. The latest versions come in 14 power sizes that can be installed in the bow or stern in vessels of all types, including merchant, cruise and ferry as well as offshore support vessels.
As the number of thrusters on board ships grows, control systems have had to keep pace. Wärtsilä’s latest system, ProTouch, bristles with features that help the bridge manage the thrusters via a full range of levers, touch-screen interfaces, displays, indicators and modules that can be mixed and matched to suit any propulsion requirements.
Similarly, ZF has developed a new condition-monitoring system that keeps an eye on the status of a vessel’s thrusters and, says ZF, helps extend their service life. Based on sensors that are embedded at strategic places in the thrusters, the system measures tell-tale vibrations. If the vibrations are big, they could indicate wear and tear in the bearings and gears in the thruster’s upper and lower gearbox.
“The results of the measurements help operators identify early on which components need to be replaced or maintained,” ZF explains in a release. “This means repairs can be planned in advance and potential damage to components can be avoided, all of which prevents downtime and shortens maintenance cycles.”
And like Wärtsilä’s latest thrusters, ZF’s oil-cleaning systems are keeping pace with increasingly stringent anti-pollution regulations in the era of environmentally acceptable lubricants (EALs). According to the manufacturer, the system “ensures oil quality and prolongs oil life”.
It is also a matter of keeping the sea out. “Particularly with biodegradable oil, it is important to keep contaminants such as water out of the system so as to prevent downtime caused by mechanical failure, ” explains Drew Orvieto, ZF’s senior manager for commercial fast craft in North America.
After 40 years of concentrating on rugged thrusters, Kongsberg Maritime’s latest tunnel thrusters, unveiled in mid-2019, are purpose-designed for an easier life than the hard-worked versions that the company originally developed for offshore support vessels. “These thrusters are high-end, high-value products that do not correspond well with the requirements of the merchant market,” explains Bård Bjørløw, vice-president global sales and marketing.
In other words, it would be like using a sledgehammer to crack a nut.
Kongsberg’s latest range is aimed at cruise ships, ferries, roro vessels and container ships, rather than drill ships. Highly standardised and using the minimum number of components, they are available in two basic sizes that cover four propeller diameters and power outputs ranging from 1000 kw to 5000 kw, ideal for intricate manoeuvring in ports and holding station in the middle of an ocean.
“With the new tunnel thrusters, we have achieved a more cost-effective solution for newbuild and retrofit application to existing commercial ships [which] rarely require dynamic-positioning functions and only operate their thrusters for a limited period,” adds Mr Bjørløw.
Kongsberg must have got the designs right because in late February 2020, the maritime division picked up a significant contract to provide the propulsion for three of Mystic Cruises’ pocket-sized adventure vessels, with a fourth to follow.
Destined for sailing in polar regions, the vessels will be almost entirely Kongsberg-propelled. The manufacturer will supply the main and auxiliary engines, motion control, electric systems and other equipment. Importantly, the thrusters will allow the vessel to hold position without the benefit of anchors, which could rip up fragile ecosystems lying on the seabed. Kongsberg will deliver the equipment through the next two years.
As the global fleet responds to the exigencies of regulators, passengers and its own sense of responsibility for the management of the world’s oceans, thrusters will become as indispensable for cruise ships as they have been for offshore supply vessels.
A system for all ships
Keep it simple and sea-kindly. That is the principle behind Kongsberg Maritime’s new Promas thrusters. Taking a unified approach, the manufacturer has integrated the propeller and rudder into a single system so that the hydrodynamic efficiency is improved. “Virtually any type of vessel will benefit from this integrated propulsion and manoeuvring system,” the manufacturer claims. “In general, the efficiency gain is between 3 and 8% for single-screw vessels and 2 and 6% for twin-screw vessels.”
Mystic Cruise’s new polar vessels will be equipped with Promas propulsion. “This is a shafting solution with a ‘hub cap’ and twisted rudder for maximum efficiency,” a spokeswoman said. “Together with our hybrid electric solution, this form of propulsion has proven to be very flexible and reliable.” It is also claimed to be fuel-efficient, particularly for Mystic Cruises’ economy-sized cruise ships. For maximum manoeuvrability the vessels will also feature two of Kongsberg’s tunnel thrusters in the bow.
The so-called ‘hub cap’ is an important element of the Promas system. A carefully shaped appendage, it is fitted to the propeller in a way that helps streamline the flow onto a bulb that is fixed to the rudder, also for streamlining purposes. The principle, explains Kongsberg, is to reduce the flow separation – effectively turbulence – that occurs directly behind the propeller. “The result is an increase in propeller thrust because previously wasted energy is recovered from the flow,” engineers explain.
But why the twisted rudder? As many trials have shown, the shape improves efficiency and, above all in a thruster, manoeuvrability.
According to Kongsberg’s sums, the payback period for Promas is short and installation is simple. In the endless quest for bottom-line efficiency, it is certainly one of the answers.