Dutch naval architect Dr. Ir. Pieter van Oossanen and his team of naval architects and hydrodynamicists in Wageningen have come up with something that could have pretty significant implications, or at the very least least give other naval architects something to talk about when they meet up at the pub after work.
As the above diagram shows, the flow of water off the stern of a ship travels upward as well as aft as a ship moves through the water. At the same time, energy is lost by the wave created by the stern.
Pretty straightforward stuff, but when you add a foil to the picture things start getting interesting.
With the foil in the flow of water coming off the stern, the angle of attack of the water hitting the foil creates lift perpendicular to the surface of the foil with a net forward thrust vector after you account for drag. At the same time, the lift from the foil helps raise the stern out of the trough keeping the bow trimmed down, and at an optimum level.
In a conversation today with Bruno Bouckaert, Global Sales Director at the Hull Vane, he notes that tests have shown the stern wave is reduced and the pitching and heave motions are reduced as well, further increasing the efficiency of the hull.
Even more interesting is that when the ship heaves, the foil acts similar to the way a windsurfer would gain forward momentum by pumping the sail to windward. This heave motion actually helps to propel the ship forward ever so slightly.
The ideal candidates for this technology are ferries, container ships, cruise ships, patrol boats, supply vessels, navy vessels, large motoryachts, reefer ships, car carriers and RoRo vessels, according to the company. In these cases, fuel savings between 5% and 15% are common, and in some cases, fuel savings of 20% have been achieved.
Comparative sea trials on MS Karina, a 55 m Fast Supply Intervention Vessel, showed a reduction in shaft power of 10% at 12 knots up to 15% at 21 knots with the application of a Hull Vane.
With today’s news about the X-Stern from Ulstein, it’s important for naval architects and ship owners to realize that ship design, and in particular, hull design, is a field that continues to evolve.
Perhaps a new idea for ocean racing yachts?
To be honest, that was the first thing I thought of, but I would say it’s unlikely because the stern sections of modern race boats are very flat and the water coming off them would have minimal angle of attack. The added frictional drag would probably outweigh any possible gains.
In addition, the fin would have to be installed fairly far below the waterline to ensure it doesn’t breach the surface when the vessel pitches forward, yet the farther below the waterline, the less angle of attack to the water flow and less efficient it will be.
Then again, Oracle Racing showed everyone that 70 foot catamarans could match race on San Francisco Bay on foils doing 45 knots. So maybe the Hull Vane is something to be considered?
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February 17, 2021
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