Understanding Cruise Ship Hull Design

With the advent of aviation as the most viable and fastest means of commercial mass transportation more than half a century ago, passenger shipping has whittled considerably except for a few routes and is mostly prevalent for tourism and recreation.

Passenger vessels are either cruise ships or ferries. Ferries are smaller and accommodate fewer passengers over short distances. Cruise vessels are bigger, more luxurious, complex and carry thousands of people across exotic locations for vacations.

While different in many aspects, both have some similarities.

In this article, we will explore the basics of hull design of cruise ships.

1. Hull Design Characteristics

There are some critically specific denominators when it comes to all passenger cruise vessels:

Hull Form

The fine hull form is the foremost characteristic of all cruise vessels, irrespective of operations and size. This is owing to the speed considerations of cruisers that need to cater to a stringent timetable not only expected by the passengers but also to the liner itself.

Finer hull forms exhibit remarkable resistance characteristics, both in terms of wave and frictional resistance and thus endow cruise vessels with higher speeds. While finer hull forms remain an obvious choice for vessels when it comes to dealing with better speeds, it is a challenge for common commercial vessels like tankers or bulk carriers as improved resistance characteristics come with a trade-off in terms of usable hull volume spaces for cargo stowage, measured as Net Tonnage or NT.

Bluffer hull forms, as in almost all tankers or bulkers, cater for more space in the hull ahead of midships, and these incur more significant profit margins in terms of higher freight volume carried in one charter voyage from point A to B. However, they come at a cost of speed and, thus, consequently, more voyage times.

Advantages of Large Superstructures

The volume space reduction in the main hull of finer-form vessels like passenger ships or cruise ships is essentially compensated by the fact that most of the amenities, facilities, provisions, and even accommodation spaces in the majority are in the superstructure.

Modern cruise ship designs have even larger superstructures and the usable tonnage spaces within the hull are arranged in a way such that a fine hull-form neither affects the pax. number carrying capacity of the vessel nor the disposition of tankage and essential spaces designated for machinery, systems, stores, etc.

2. Stability and Seakeeping

Understanding The Location of the Vertical Centre of Gravity

The most crucial aspect in terms of a cruise vessel from a basic design point of view is understanding the location of the transverse vertical centre of gravity or the VCG.

From the first principle, the greater the vertical extent of a body, the higher the tendency for the vertical centre of gravity or VCG to be at an elevated height above the baseline.

Consider a block of 10 meters and another block of 5 meters in height (assuming the same width and length). Who has a higher G? Of course, the first one being taller, has a higher position of G as compared to the latter. Now, consider another case. Consider two blocks of both 10 meters but of dissimilar widths.

Who has a greater VCG value? The narrower block has an elevated G value compared to the broader one. The interplay of the dimensions and form of a body brings about the difference in the vertical position of the CG.

VCG In Cruise Ships

A modern cruise vessel design has a problem in terms of VCG from both angles. First, owing to larger and taller superstructures, and second, the finer hull form, as discussed above.

The subsistence of both these factors tends to elevate the VCG location of cruisers at dangerously high levels from a geometric point of view, increasing the chances of the intact transverse stability of the vessel being compromised.

External Loading

Due to the form of the vessel, the effects of hydrostatic and hydrodynamic loading are more pronounced. Moreover, owing to the large superstructure of the vessel, and greater freeboard (as compared to bluffer vessels like tankers or bulker), cruise vessels also essentially have a greater windage area.

What is the windage area? Windage area is the sum of all surface areas of a vessel that are exposed to the weather, that is the part of the vessel above the waterline when viewed from a particular direction. Thus, the action of wind forces on a passenger cruiser, with greater superstructures and increased freeboard, is far more than a tanker or a bulker.

At very high sea states, the hydrodynamic and aerodynamic loading on the vessel is very high. This induces large degrees of force on the vessel that compromise not only the intact static stability index of the vessel but also the dynamic.

Dynamic stability is nothing but seakeeping. Owing to persistent time-varying induced large forces and moments over time, the vessel tends to freely move in all its degrees of freedom, the lateral motions being the most critical as it deals with the transverse behaviour of the vessel, passenger comfort and most importantly, the uprightness of the vessel against capsizing.

Lowering The G Position

Hence, the biggest challenge of a cruise ship design is to lower down the G as much as possible notwithstanding the problems due to its inherent form that cannot be changed. How is that done?

The answer lies in two directions.

1) The basic design point of view

Ballasting is the easiest and most reliable way to bring down the G value. Modern cruisers adopt meticulous ballasting measures such that not only is the displacement enough to cater to the buoyancy requirements of the vessel to stay afloat but also the freeboard is decreased significantly owing to the greater distribution of mass elements at the bottom region.

The large depths of the hulls of cruisers provide a greater margin to increase weight such that the freeboard as well as the reserve buoyancy of the vessel is enough to safely stay afloat at a higher deep loading displacement value.

At the design stage, the tankage of the cruise vessels is disposed within the hull in the most optimized way such that the maximum ballasting can be carried out to drag down the VCG value safely and at the same time, not compromise on the flotation of the vessel.

While tankage is a part of deadweight, the lightweight distribution of the vessel is also equally important. The general arrangement of a cruiser is critically predetermined and must be designed meticulously such that the maximum possible lightweight can be dumped below the waterline, that is within the limits of the Net Tonnage volume spaces of the hull.

Apart from engines and machinery, a passenger vessel is characteristic of numerous systems and utilities that cater to the lives carried on board. From cooking to freshwater to refrigeration to air-conditioning, the essential systems required for such vessels are very extensive and need to be properly disposed of in dedicated spaces.

Thus, all such systems are placed in the compartments of the hull, and distributed properly such that the G value remains desirable.

2) Construction point of view

From the construction point of view, the structural weight of the superstructure is minimized as much as possible such that the G does not shoot up much but at the same time, the strength and integrity are catered to. Light and tough materials such as superior grades of Aluminum or lightweight steel breeds are mostly used for superstructures in passenger cruise ships.

3) Other Means

However, apart from standard basic design practices, other means of bringing down G must be also employed. These mostly include technologies ranging from fin stabilizers to bilge keels to anti-roll tanks that generate forced time-varying counter-moments to mitigate the effects of external moment loadings such that any form of transverse motions, either static or dynamic, is corrected at the right time.

Damage Stability

Damage stability is the other pillar of stability that deals with the integrity of the vessel after structural damage. Without delving into the intricacies of the damage stability, in a nutshell, it is sufficient to say that a higher degree of subdivision is desired in cruise ships, as in the event of damage and accidental flooding, the vessel does not lose its buoyancy much.

Remember Titanic? The century-old tragedy is still considered more to be an aftermath of the design rather than what caused it. Experts all over the years have postulated that if the subdivision of the vessel had been better, the iceberg impact would still not have been catastrophic as the flooding of the compartments could have been contained within proper limits of the force-buoyancy static equilibrium.

While this is the case for all vessels, in other commercial ships like tankers or bulkers, compartmentalization can only be extended to feasible limits as the proper disposition of the cargo spaces is another challenge. Too much subdivision tends to further increase the structural weight, something not acceptable for high-displacement vessels.

The damage stability considerations of a cruise vessel are done under stringent class regulations and adhering to guidelines approved by IMO that are more particular than those of other commercial ships since, in this case, it is a matter of dealing with numerous lives on board.

You might also like to read-

• Understanding Vessel’s Hull Speed And Its Determination
• What are triple-hull vessels?
• Different Parts Of A Ship’s Hull
• Types of Hulls Used For Vessels

About Author

Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.

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