29.09.2015, 01:32:52 (GMT+1)
Hydrofoil device has major fuel-saving potential
CFD study confirms value of game-changing, energy-reduction technology
A fixed foil fuel-reducing device installed below the ship stern offers significant potential as an exciting, game-changing energy and cost-saving innovation.
Hull Vane hydrofoil technology, developed originally for an America's Cup sailing yacht, influences the stern wave pattern and creates hydrodynamic lift which is partially oriented forward. This helps to reduce vessel resistance. The device reduces fuel consumption by optimising the water flow around the rear of the vessel.
According to the Netherlands company responsible for the development and fine-tuning of this energy and emissions-saving technology, overall performance depends on ship length, speed of travel and hull shape in the aft sections. However, tests have already confirmed that fuel savings of between five and 15 per cent are achievable for many commercial and naval vessels - and in some cases the fuel consumption benefits could be as high as a 20 per cent reduction.
A computational fluid dynamics (CFD) study was carried out by the Defence Materiel Organisation (MDO) of the Royal Netherlands Navy on an offshore patrol vessel (OPV). Based on the operational speed profile and power curve of the ship, it was determined that most fuel is used at speeds between 15 and 20 knots despite the fact that the OPV sails at less than 15 knots for 86 per cent of the time.
Therefore the hull vane was optimised for a speed of 17.5 knots. Because the vessel has a pronounced trim wedge - which is incompatible with a hull vane device - the trim wedge was partially modified to agreed DMO limits. CFD tests were carried out to determine resistance at five, 12.5, 17.5 and 22.5 knots which produced impressive resistance reductions of 1.3 per cent, 13.7 per cent, 15.3 per cent and 11.1 per cent respectively.
When multiplied by the operational speed profile this worked out at an annual saving on fuel and emissions of 12.5 per cent. This would also reduce CO2 emissions by 1,000 tons annually for each ship which, say the device's developers, makes this underwater ship technology an extremely cost-effective carbon dioxide emissions abatement measure.
To test out the device's impact on the vessel's operating performance, a seakeeping analysis was carried out in typical wave conditions. This revealed a four per cent cut in heave motion, a seven per cent reduction in pitching movements and a 13 per cent reduction of vertical accelerations on the helicopter deck. As vertical accelerations are the limiting factor for heli-operations, the fixed foil technology can apparently also increase the operational envelope of each vessel.
A further benefit, demonstrated during sea trials, is that the turbulent wake zone behind the stern was reduced by 50 per cent which makes launching and recovering rigid hulled inflatable boats (RHIBs) through the stern slipway both easier and safer. In addition, the range of the vessel can be increased by up to 17 per cent, from 5,000 to 5,850 nautical miles.
"The results of the full-scale trials matched exactly what we had calculated - savings in fuel of between 12 and 14 per cent," says hull vane creator Dr Pieter van Oossanen, founder of Van Oossanen Naval Architects. "The hull vane is most effective for ships with moderate speed. Ships in this category include supply vessels, ferries, container vessels, naval vessels and faster merchant ships."
The fuel-saving capabilities of the hull vane technology are due to four distinct factors - thrust, wave reduction, trim correction and reduced pitching. This pioneering marine innovation works with an upward flow at the stern which enables the foil-shaped appendage to generate a forward thrust force which reduces the vessel's resistance. Furthermore, the fuel-saving device also reduces the running trim and the ship-generated stern wave. It also dampens both pitch and heave motions in waves, further reducing resistance created by the vessel's motions.
The hull vane is a potential game-changing cost and performance efficiency innovation for suitable new-build vessels, especially as the installation costs are likely to be quickly offset by propulsion power savings. However, developers say it is also easily fitted to existing vessels, even at sea so companies can avoid unnecessary drydock expenses.
"There are no moving parts on the hull vane and therefore no maintenance costs," says Bruno Bouckaert, global sales director at Hull Vane B.V. "The hull vane is a capital investment which will provide predictable returns during the lifetime of the vessel."
A paper written about the CFD study - 'A Life-Cycle Cost Analysis of the Application of a Hull Vane to an Offshore Patrol Vessel' - was presented by Bruno Bouckaert at the FAST international conference on Fast Sea Transportation in Washington DC earlier this month.