Launching of any structure is quite an anticipated occasion depicting the hard work paying off and victory after years of scrupulous construction and numerous man hours deployed. And when it comes to a near self-sustained vessel sailing on the notorious ocean or river waters, the celebration is unimaginable. It is one of the most important procedures in the entire chain of ship construction processes.
But even a subtle miscalculation or error may compromise the ship’s launching and mourn the spectators’ faces as they witness something going horribly wrong. To obviate any such risk ship launching is meticulously planned and everyone directly involved is quite punctilious.
TYPES OF SHIP LAUNCHING
Various types of Ship Launching Methods are used depending upon the feasibility of ship and geographical parameters.-
Stern First
This method dates back to ancient times and is one of the most familiar methods for ship launch. The prerequisites involve arranging the slipways to be nearly perpendicular to the shoreline. Nowadays reinforced concrete mats are used as slipways to provide sufficient strength.Construction is done on temporary cribbing so as to give access to the hull’s outer bottom.As preparation for launching, a pair of standing ways with greased surfaces is erected under the hull and out into the barricades.A pair of sliding ways is placed on top, under the hull, and a launch cradle with the bow and stern poppets is erected on these sliding ways. Common mechanisms for releasing the vessel in launching ceremony include weak links designed to be cut at a signal.On launching, the vessel slides back, down the slipway until it floats by itself.
Image courtesy: Google Images.
Sideways
This type of launching is used where the water channel is not feasible for lengthwise launching. The slipways are built so that the vessel is side-on to the water and is launched sideways. However more sets of launching assisting construction are required to support the ship and this act as a downside. Rigorous calculations have to be made to check the stability of the vessel as it touches the water surface to avoid capsizing of the vessel due to lateral moment created during launching process.
Image courtesy: Google Images.
Air Bags
This method has the upper hand of requiring less permanent infrastructure, risk and cost and is quite safe and innovative. A series of inflated tubes placed under the hull deflate to cause a downward slope into the water. The airbags made of reinforced rubber layers have high load capacity and are usually cylindrical in shape with hemispherical heads at both ends. The Xiao Qinghe shipyard was the first to manifest this type of launching in 1981.
Image courtesy: Google Images.
Float Out
Though not technically recognised as a ship launching method, this method is most widely used one among the shipyards. Ships built in drydocks are launched simply by filling the dock with water and the vessel is 'Floated out'. Thus it is a simple, effective and safe procedure. though the initial investment is high.
Image courtesy: Google Images.
LAUNCHING CURVES
Predictions of the movement are vital to ship’s safe control. A set of six curves is prepared to predict the behaviour of the ship during launch. They are curves plotted against the distance of travel down the slipway for end launching process
1.Weight
2.Buoyancy
3.Moment of Weight about fore poppet
4.Moment of Buoyancy about fore poppet
5.Moment of Weight about after end of groundways
6.Moment of Buoyancy about aft end of groundways
Image courtesy: Basic Ship Theory (K.J. Rawson & E.C. Tupper).
The Important features of these curves are as follows-
·At the point at which the moment of buoyancy about the fore poppet equals the moment of weight about the fore poppet, the stern lifts.
·The difference between the weight and buoyancy curves at the position of stern lift is the maximum force on the fore poppet.
·The curve of the moment of buoyancy about the aft end of the ways must lie wholly above the curve of the moment of weight; the least distance between the two curves of the moment about the aft end of ways; gives the least moment against tipping about the end of ways.
·Crossing of the weight and buoyancy curves before the after end of ways, indicates that the fore poppet will not drop off the end of the ways
Thus launching, though a celebratory and ceremonial event requires a lot of background calculations with minimal errors so as to be able to predict the vessel’s motion in advance. Each parameter needs to be assessed with utmost accuracy to prevent stability loss or any undesirable accidents on the launching day.
Here is a video of various ship launches recorded. They are fascinating to watch, but at the same time, they involve a long calculation and thinking process and a huge effort of engineers to accomplish the task.
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 LHV which is by definition perpendicular to the direction of flow of water, and a drag force vector DHV in the direction of the flow. The sum of these vectors FHV can be decomposed into an x-component and a z-component:
LHV + DHV = FHV = Fx,HV + Fz,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:
LHV = CL * ½𝜌V2A
DHV = CD * 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) ∗ LHV – cos (𝛼+𝛽+𝜃) ∗ DHV
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) ∗ LHV + sin (𝛼+𝛽+𝜃) ∗ DHV
With this, the influence of the hull vane on the running trim can be derived using:
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)
As it is known to us that a surface ship has 6 degrees of freedom, viz. surge, sway, heave, roll, pitch and yaw. Surge, sway and heave represent the translational motion of a ship along the x-axis, y-axis and z-axis respectively. Roll, pitch and yaw are associated with the rotational motion of a ship about the x-axis, y-axis and z-axis respectively. The picture shown below depicts the 6 degrees of freedom of a surface ship.
6 degrees of freedom of a ship (Courtesy- Google images)
Ship rotational motions just seem normal. Almost each and every vessel experiences motion in the seaway and the magnitude of the motion depends on the efficiency of the designer behind it.
Guess what would have caused such a macabre to the containers?
Well, obviously, one would say it's due to extreme rolling motion the ship has encountered, probably due to the resonance of the ship’s natural frequency with the encountered wave frequency in the seaway.
Generally, beam waves (waves perpendicular to the ship’s centerline axis or the x-axis) are the reason behind roll motion of the ship.
But what if this is rolling is not caused due to beam waves? It seems illogical, but here is what we will be discussing in this article, rolling due to head waves (along the ship’s centerline axis or the x-axis). This phenomenon is also called as Parametric Rolling resulting due to resonance and certain special processes as discussed below.
Parametric Rolling – The phenomenon
Now, assume a ship is moving in a straight line with zero drift angle and a wave is encountered opposite to the direction of its motion. Now if the wavelength of the waves is same as the length of the ship then major transient changes will occur in the water-plane of the ship, which would consequently change the transverse stability of the ship. Generally for a typical ship design, looking at the plan view, the midship is made fuller and the aft and forward ends are fine-form to balance with the cargo carrying capacity (or space requirement) and streamlined shape of the vessel. Also, the local breadth of the ship at any station decreases as one goes down the waterline to the keel.
When the wave crest is at amidship, due to the shape of the ship, the aft and forward ends of the ship contribute less to waterplane area. So stability decreases as we know the transverse stability of a ship is directly proportional to the waterplane area. And when the trough is amidship, stability increases as crests prevail at the forward and aft end of the ship. This transition in transverse stability takes place with a certain frequency which gives rise to parametric rolling. The picture below illustrates the change in waterplane area with respect to wave position.
Image courtesy – Team LSD
For a particular location and loading condition, natural roll period of the ship is a function of waterplane area, mass and added moment of inertia and hence remains constant. Assuming the frequency of the head wave remains constant; hence we know that there are particular bands of frequencies which cause parametric rolling. But the fact that we have ignored here is that there is a relative motion between ship and waves, which consequently changes the frequency with which a wave hits the ship. The actual frequency with which the waves hit the ship in a seaway is known as Encounter frequency.
ωe=ω- (ω2*U*cos μ)/g
ωe is Encounter frequency
μ is wave heading angle of the ship
U is velocity of ship ω is angular frequency of the wave
One of the fundamental conditions for the parametric roll to set up is that the encounter frequency of waves should be twice that of natural roll frequency of the ship. So, there will be a certain set of ship speeds, heading angle and wave frequencies which will cause parametric rolling. Under this frequency condition, when the crest is amidships, the stability decreases, so the ship would roll, but after half the natural frequency of roll of the ship, stability increases abruptly due to increase in waterplane area, so the restoring force contributes to further rolling. This small roll angle slowly turns to a large parametric roll.
The image below shows parametric rolling of a ship (damping neglected). The changing GM values pertain to changing waterplane which we have already discussed. More water plane area means more GM. As the transverse stability and natural roll come near in phase, the roll angle increases. If the two are in-phase then resonance occurs. The image depicts the above-mentioned concept.
Image courtesy- of Thesis paper; Parametric roll instability of ships by Irfan Ahmad Sheikh, University of Oslo
Hull Form Impact on Parametric Roll
Also, there are certain types of hull forms, which are susceptible to parametric rolling and same can be inferred by analyzing previous voyage data. Due to improvement in hull designs for better cargo capacity and flow efficiency, the bow flare, stern overhangs and fuller amidship are introduced in ship design. Due to this when waves travel along the length of the ship, gradients in waterplane area are very significant. This results in parametric rolling.
Parametric Roll- Havoc and Prevention
The picture of the container ship we saw at the beginning is of container vessel APL China. Due to parametric roll, 60% of her cargo was lost to waters.
To tackle this serious issue, most of the ships are now being engaged with sensor systems which alarm the crew about the possibility of parametric roll and paves a way to take immediate actions. Although, this phenomenon can never be eliminated, so other measures are adopted to reduce rolling amplitude of the ship such as bilge keel, stabilizing fins, anti-roll systems, etc. Since the phenomenon depends upon the encounter frequency, a ship can simply change its speed or heading angle to counter parametric roll especially to avoid resonance.
Although this phenomenon takes place very rarely, it’s consequences force the naval architects to incorporate preventive measures. With the help of simulation software, the response characteristics of the ship in different wave conditions can be easily predetermined, which helps in techno-economical motion based designing of ships.
The following video shows an experimental demonstration of parametric roll for a ship model in a wave flume. Hope it helps you to visualize the havoc it creates in large vessels.
