An answer for the squeeze

Wärtsilä’s Joost van Eijnatten explains that market pull has been scaling up cruise ship sizes; however he points out this inevitably has an impact on manoeuvrability especially in smaller, space-limited harbours. It’s meant that the bow and stern thrusters have a heavier job on their hands: “Past a certain size cruise ship, about 140,000gt, four bow thrusters have almost become the norm; these usually are sized to 3MW each, so that’s 12MW in total.” He points out “four large units, each roughly 3m in diameter, takes up a lot of space in the narrowest part of the hull”. However this is where the new Wärtsilä WTT-40 transverse thruster comes in. “If you can change those four units over to three larger thrusters of 3.4m diameter giving 4,000kW each, you can install them more efficiently,” he says, “For example one tunnel less means you can place them a little further toward the bow... and since the ‘virtual combined thrust’ is in the middle, there’s an extra manoeuvring efficiency that comes from the more forward position.”   It seems like a simple enough solution with a lot of benefits: “The higher the propeller diameter, the more effective these thrusters become. It’s partly a case of lower power density across the controllable pitch propellers, this helps to keep the noise and vibration within limits,” he says. However it takes finesse, he adds: “The hydrodynamics become really important when you move up in power - it’s actually much easier to meet the noise and vibration requirements for a smaller thruster than a large one.” Because of this, Wärtsilä’s experience with the huge Oasis of the Seas was very useful. As well as the engines, Wärtsilä developed a set of 5.5MW bow thrusters for the outsized 225,282 GT cruise vessel: these are, says Mr van Eijnatten, among the largest in the world - at the time it was pointed out that the combined, 22MW bow thrusters totted up more power than onboard most cargo ships. However, it obviously gave Wärtsilä a taste for scale and at the same time allowed resources to be pointed at extensive model testing and CFD analysis, “something that we are using more and more,” he explains. This resulted in optimising the flow around the thruster housing. However, he explains “the key element” is the flow into the entrance: “The better you can smooth the transition from the hull to the circular tunnel, the more effective the thrusters will be. But this isn’t just a question of making it look smooth, you have to calculate flow speed and pressures to get the shape right.” He adds, because of this it’s best to get together with the ship designers at a very early stage – the installation simply won’t yield the greatest possible efficiency should it be an afterthought. Traditionally, these transverse thrusters have CPP propellers with the angles set through a hydraulic actuating cylinder, but this entails a complete tank and pump set, placed in close proximity to the thruster itself. Although it had already been seen that the hydraulics could be much more compact, allowing for them to be fitted onto the motor foundation, he explains there was a certain ‘Eureka’ moment when it was realised there are already large oil baths used to lubricate the gear set: “It’s a small step to say, omit the separate tank and filter and instead use the existing lubrication oil.”   So, this new ‘all-in-one’ development gave the thrusters fully integrated hydraulics, providing circulation, filtering and cooling of the lube oil alongside the pitch setting function. “The whole trick is that you have better oil quality, better lubrication, higher reliability, and a limited number of components as each of them take on three or four functions.” This streamlining results in a reduced number of pumps and filters, cutting down the space needed in the thruster room “one benefit for both the designer and yard is that the installation is a lot easier and simpler”, he concludes. More, the Wärtsilä WTT-40 is able to comply with the US EPA's (Environmental Protection Agency) VGP2013 regulations. These state that when a vessel is sailing in US waters, environmentally acceptable lubricants (EALs) must be used in all oil-to-sea interfaces, such as the propeller shaft seal of transverse thrusters. He admits this does require some modifications: “The EALs are different in make up to the mineral oils traditionally used. So we need to make sure everything inside is compatible with EAL fluids and this means selecting slightly different components.” “It’s one more thing you need to take it into account from the beginning,” says Mr van Eijnatten. The first WTT-40's have been ordered and are scheduled for delivery during 2016.