HULL VANE- THE CONCEPT

INTRODUCTION

The history of Hull Vane can be traced back to 1992 when it was first used in full-scale trials of a catamaran. Surprisingly, the results of the test showed that the vessel had reduced bow-up trim and resistance and, this had driven interest among the engineers to further carry out research on this device.

At first, the questions that strike our mind are what actually is a Hull Vane?

How does it look like?

What is the purpose of using it in ships?

And where actually is it used on ships?

 All these questions are answered in this article to give a general idea about Hull vane, its geometry and purpose.

Model used for hull vane model test 
Image courtesy- Google Images

Hull Vane is a fixed, resistance reducing foil attached to the hull below the water line near the stern of the ship.

In order to increase the fuel efficiency of the ships, the hull resistance must decrease. The concept of hull vane first struck the mind of Dr IR. Pieter Van Oossanen of the Netherlands and the first patent was filed by him in the year 2002. Since then a number of tests, for the optimization of this device, have been carried out using model tests, CFD and full-scale trials. The results were remarkable and showed fuel reduction in excess of 20% for yachts and 5% to 10% in other vessels mostly naval vessels, merchant ships, cruise ships, etc.

Hull vane animated concept

Image courtesy- Google Images

During the trial of the catamaran in 1992, it was found that the vessel wasn’t acquiring its required speed due to excessive trim and wave generation. By placing a foil in the steepest part of the interacting wave system aft of the midship, reduced the bow-up trim and the resistance significantly. Since then a number of tests, trials were carried out for a range of vessels like container ships, Ro-Ro vessels, Supply vessels, cruise ships, etc. and the results show a decrease in resistance to 26.5% to an increase of 9.5% which clearly indicates that the device is not suitable for all kinds of vessels. 

The second application of the Hull Vane, on the 2003 IACC yacht Le Defi Areva.
Image courtesy- Google Images

In 2014, two vessels equipped with Hull Vane were launched. A 55 meter supply vessel Karina manufactured by Shipyard De Hoop in the Netherlands and 42m yacht built by Heesen Yachts. The required engine power was reduced by

15% in the former and in the latter vessel a resistance reduction of 23% was significantly observed.

HULL VANE - WORKING PRINCIPLE

In this section, we will discuss how the Hull Vane actually does what it has been designed to do. We can observe four prominent effects of hull vane on the vessel dynamics.

THE THRUST FORCE

It is based on the basic foil theory. A schematic overview of the forces on the Hull Vane is given in the below figure.

Schematic overview of the forces on the Hull Vane in a section view of the aft ship.
Image courtesy- Google Images

α is defined as the hull vane inflow angle ( the angle between the inflow and the chord line), β is defined as the hull vane angle ( the angle between the chord and the body fixed x-axis). The vessel displayed in the figure is at zero trim.

The foil creates a lift force vector L_HV which is by definition perpendicular to the direction of flow of water, and a drag force vector D_HV in the direction of the flow. The sum of these vectors F_HV can be decomposed into an x-component and a z-component:

L_HV + D_HV = F_HV = F_x,HV + F_z,HV

If the x-component of the lift vector is larger than the x-component of drag vector, the resulting force in x-direction provides thrust. The lift and Drag Forces can be estimated using:

L_HV = C_L * ½𝜌V²A

D_HV = C_D * ½𝜌V²A

If θ is defined as the trim angle (the angle between the body fixed x-axis and the inertial x-axis) the thrust force that is generated by the Hull Vane can be derived by the equation:

F _{x, HV} = sin (𝛼+𝛽+𝜃) ∗ L_HV – cos (𝛼+𝛽+𝜃) ∗ D_HV

TRIM CORRECTION

The force in the z-direction affects the trim, and especially at higher speeds, this trim reduction proves to have a large influence on the total resistance of the vessel. This effect can also be achieved with interceptors, trim tabs, trim wedges or ballasting. Similarly, to the force in the x-direction, the force in the z-direction can be estimated using:

 F _{z, HV} = cos (𝛼+𝛽+𝜃) ∗ L_HV + sin (𝛼+𝛽+𝜃) ∗ D_HV

With this, the influence of the hull vane on the running trim can be derived using:

𝛿𝜃 = t𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑚𝑜𝑚𝑒𝑛𝑡 / 𝑟𝑖𝑔ℎ𝑡𝑖𝑛𝑔 𝑚𝑜𝑚𝑒𝑛𝑡 𝑝𝑒𝑟 𝑑𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑡𝑟𝑖𝑚

≈ F_Z ∗ 𝑎𝑟𝑚 / 𝐺𝑀_L ∗ 𝛥 ∗ 𝑔 ∗ sin (1°)

Not only the trim reduction itself has a positive influence on the hull’s performance, but the trim also affects the angle of attack of the water flow on the hull vane.

REDUCTION IN WAVES FORMATION

The flow along the hull vane creates a low-pressure region on the top surface of the hull vane. This low-pressure region interferes favourably with the transom wave, resulting in a significantly lower wave profile. The wave reduction is so significant that it can be observed by eye. The reduction of waves not only leads to a more beneficial resistance, it also leads to less noise on the aft deck, and to a lower wake. The former is mainly beneficial for yachts and the latter for inland shipping, where wake restrictions limit ship speeds in ports or other enclosed areas.

 Wave pattern on the 55 meter supply vessel without Hull Vane (top) and with Hull Vane (bottom) at 20 knots
Image courtesy- Google Images

 Comparison of the wave profile of the 55 meter supply vessel without Hull Vane (left) and with Hull Vane (right) at 13 knots.

Image courtesy- Google Images

Image courtesy- Google Images

REDUCTION OF VESSEL MOTIONS IN WAVES

Another significant effect of the hull vane is that it dampens the heave and pitch motions of the vessel. When the vessel is pitching bow-down the stern of the vessel is lifted and the vertical lift on the hull vane is reduced by the reduced angle of attack of the flow. This counteracts the pitching motion. Similarly, during the part of pitching motion in which the stern is depressed into the water, the vertical lift on the hull vane is increased. This again counteracts the pitching motions and similarly, it also dampens the heave motions.

 Image courtesy- Google Images

The advantage of reduction in motions is that the added resistance due to waves is reduced, which makes the hull vane even more effective in rough waters. As the motions are reduced, it increases the comfort levels onboard the vessel, safety and range of operability in various sea states.

 INFLUENCE OF POSITION OF THE HULL VANE

According to Moerke, if the Hull vane is fitted too close to the hull, it might lie in the boundary layer thus reducing the lift it generates. In addition to it, the low-pressure region on the upper side of the hull vane is reflected on to the hull and additional pressure resistance is created on the hull. Hence, the resistance of the combination of the hull and hull vane increases. After carrying a number of CFD analyses, it was found out that if the hull vane is placed behind the transom of the vessel, the pressure reflection can be reduced along with a slight reduction in the thrust generated by the hull vane.

Another consideration in the positioning of the hull vane is the angle of water flow near the stern of the vessel. The largest angle of attack can be achieved by placing it in the steepest part of the transom wave. But at high speeds, this location is found to be too far aft of the hull. An Additional complication is that this optimal location is very dependent on the wavelength and thus on the ship speed. In the vertical direction, a higher angle of attack can be achieved by placing the hull vane closer to the hull which is restricted by the free surface effect on the lift generated by hull vane, slamming by waves and pitching motions if it is placed too close to the water surface.

Hull Vane fitted behind the transom
Image courtesy- Google Images

INFLUENCE OF SHIP SPEED AND SHAPE OF THE HULL

According to Moerke, the hull vane is more effective at higher speeds. This statement was also supported by MARIN as they observed a power reduction of 3.3% at 17 knots (Fn 0.21) and up to 10.2% at 21 knots (Fn 0.27) for a 169m container vessel during its model tests. For high Froude numbers, the results in saving are much better. Also from tests and trials, it has been found out that hull vane is most favourable for Froude numbers in the non-planing region, between 0.2 to 0.7. 

Comparison of Resistance for a 42m, 47m and 55m motor yacht and 300m container vessel fitted with and without hull vane.
Image courtesy- Google Images

The addition of hull vane adds to the wetted surface area, the friction resistance thus increases in comparison to a vessel without hull vane. Above Fn 0.2, pressure resistance becomes more dominant. Therefore, best results are obtained for a range of 0.2 to 0.7 Fn. At higher Fn, the force generates by the hull vane creates an unbeneficial bow-down trim.

According to Moerke and Zaaijer, if the buttock angle is increased, the angle of attack of the flow to the hull vane increases and the lift vector is directed more forward increasing the resulting decomposed force in the x-direction. Also, the effect of pressure is minimized if the water column near the transom is maintained as much as possible. The leading edge of the hull vane experiences a lower hydrostatic pressure than the trailing edge when it is positioned below the front of the transom wave. The shape of the stern of the ship also a major role. Flat buttocks are considered ideal as they ensure a uniform flow.                                             

HULL VANE AND SHIP TYPES

The effectiveness of the hull vane is also dependent on the ship type as stated earlier. It is not very effective for bulk and crude oil carriers. For vessels less than 30m LOA, the investment costs are high as compared to savings using a hull vane.

Ideally, hull vane is best suited for medium and large-sized vessels operating at moderate or high non-planing speeds like the ferries, supply vessels, cruise ships, patrol and naval vessels, motor yachts, reefer ships, Ro-Ro vessels, car carriers and container vessels.

Hull Vane

Image courtesy- Google Images

The hull vane is a fuel saving device aimed to lower the pressure resistance which is the dominant component at higher speeds. CFD computations, model tests and sea trials have shown potential resistance reductions of more than 20% depending on the ship speed and hull shape, especially on merchant ships with resistance reduction between 5% and 10%.

This is a hull vane documentary video. It will give better insight about the concept of hull vane.

                    Video courtesy- Hull Vane Bv (YouTube channel)

Acknowledgements:

Hull Vane (Van Oossanen Naval Architects, The Netherlands)

Article by: Kushagra Gupta