Stavangerfjord/Bergensfjord ferry profile
and you would not be able to do this unless it was a research project, but CFD allows you to make a computerised model of the ship to set up physical boundaries.” The bulb created by CFD was then tested in a towing
plank to verify it. It was found that the CFD results for bulb were 99 per cent the same as those from the physical tank and “this is regarded as correct as you can get,” Ms Stangeland commented. Furthermore, the bulb optimised by CFD reduced
resistance by five per cent compared to the original bulb that the team started off with, which would have been used as a basis if the CFD software had not been deployed. “This is very good. By reducing resistance by five per cent, fuel costs are reduced considerably,” Ms Stangeland said. The bulb selected stands out from the normal model as it
is more elongated, as this was found to be the most optimised design for the ship and the route it is being deployed on. In addition Fjord Line’s Mr Larsen explained that a Rolls-Royce Promas system was installed between the propellers and the front of the rudders, which features a hub cap, rudder bulb and twisted rudder. In the Promas system a tapered hubcap is fitted to the hub of the propeller and leads the water flow onto a bulb, which forms part of the spade rudder. The rudder has a twisted leading edge, optimised for the flow from the propeller, which converts into additional forward thrust some of the swirl energy in the slipstream that is normally lost. “This creates a better slip as the water leaves the propeller and rudder with less turbulence and resistance,” Fjord Line’s Mr Larsen commented. Indeed, the result of using the Promas system is an increase in propulsive efficiency of about six to eight per cent, depending on application, Rolls-Royce pointed out, leading to reduced fuel consumption and emissions. An added bonus of creating the least resistance possible is also the fact that passenger comfort is boosted. “To have the waves to hit the hull in the perfect manner to avoid too much motion was beneficial as of course we want to reduce the amount of seasickness among the people using these ferries, especially considering this is a very rough sea passage,” Mr Kalkvic said. Another benefit of Rolls-Royce's system is that the radial distribution of hydrodynamic loads
Stavangerfjord’s LNG tanks being installed
on the propeller blades can be modified, reducing tip loading and helping to cut the intensity of blade pressure pulses and associated noise and vibration. At the time of writing, Stavangerfjord was due to start its service at the end of May and Bergensfjord to be delivered in the fourth quarter of this year.
Asked what one of the biggest challenges was in deciding
to switch to LNG, Fjord Line’s Mr Larsen explained that bunkering was a major issue, due to strict regulations, the safety aspects and also simply because LNG is “new for everyone”. Fjord Line has chosen Skangass to be its LNG supplier, which will use Risavika Harbour in Stavanger, Norway, as the bunkering facility for the vessels. This has smoothed the path for the ferry operator as the LNG facilities are just 300m away from the terminal. “It’s always difficult being a pioneer,” Mr Larsen summed up. PST
The two ferries were initially designed for diesel retrofit/ dual fuel engines but Fjord Line switched to LNG
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