Cruising ships

An approximation to their dimensions

In my post of April 8, 2010 in this blog, entitled "Preliminary dimensioning of merchant ships," I included a procedure to calculate a first approximation of most significant particulars of 9 different merchant ship types, but I omitted a very important one, which is the passengers cruising vessel (hereinafter Cruising Vessel (CV)). One friend suggested me to extend the procedure to include the CV, because of the interest presently have these vessels, which are among the few that can successfully compete in Europe with eastern shipyards. Following this suggestion I have prepared a preliminary CV's dimensioning procedure, but I warn from just now that the data I base on have been obtained from Internet sites and some of them are not quite reliable, as I shall comment further on.

This estimate is based on statistical analysis and regressions of ships in service, and relate each vessel's particular with a single parameter, that in this ship type is the number of passengers accommodated in two-low-beds cabins . The resulting particulars should, therefore, be considered as a first approximation, to get an idea of a new design dimensions or to serve as an initial basis for a further definition by the usual design procedures.


I am now commenting the most important aspects of this dimensioning and at the end is there the form to get the dimensions.

Number of passengers (Pax)

This is the single input data the user must enter and from the same the program estimates the most important particulars, deduced from a statistical analysis of a number of CVs whose data I have taken from Ref.1 and Ref.2. It is important to emphasize that we are refering to the passengers accommodated in two-low-beds cabins, as many CVs also offer some cabins with additional overlaid beds, so increasing the number of passengers and sometimes (but not always) both figures are displayed (eg 2.000 / 2.400). Naturally there are CVs with different accommodation standards, with larger cabins and more public rooms, leading to larger vessels. This explains, in part, the significant scatter seen in the adjacent figure, which relates gross tonnage GT with Pax.

Gross tonnage GT

This is the most significant parameter of a CV's size, and is used to relate it with Pax. The sample of available CVs ranges ships built from year 2000+, with 400-5.400 Pax, whose GTs are about 12.000 to 225.000. But I've also included a project, the 8.000 Pax "Princess Kaguya" (Ref.3), but with very few published dimensions, just to give a rough idea of this "monster size."

Total Length

Total Length (Lt) is the most significant dimension of CVs and that which is most often shown in brand publications. On the one hand length Lt is fundamental to define the necessary size of the mooring berth and on the other is more attractive to the public than the length between perpendiculars Lpp, much smaller, about 17% in this type of vessel. I have related Lt with tonnage GT, as shown in the enclosed graph.

Breadth

These vessels have a relatively small breadth, with a ratio Lt/B from 7 to 9, that is much higher than in cargo ships ranging from 5 to 7. The breadth considered in this program is the highest in the underwater hull, but in many CVs accommodation deckhouses have a larger breadth in order to increase the living spaces area.

Ships between 60.000 and 100.000 GT (carrying 1.500 to 2.600 pax) typically have a breadth of 32.30 m to allow passage through the present Panama Canal.

Draft

Draft is very important because of the harbours limitations in passenger embarking piers. It varies only a little with increasing vessel size. Most of the sample CVs have a loaded draft between 7 and 9 meters, as shown in the enclosed graph.

Depth

Depth is not a significant dimension in CVs, because they have a lot of decks and none of them gets clearly defined. Consequently, I have not included depth in the displayed particulars.

Speed

There is a great uniformity in the speed of CVs, from 20 to 22 knots, with notable exceptions as the Queen Mary 2, with 30 knots published. Always it is doubtful whether the published speed is in cruise conditions, i.e. a service speed, or if, conversely, it is a speed in ideal trial conditions with engines at maximum power. Moreover, published propulsive power figures often are not clearly defined. It is therefore difficult to reliably compare the published speed with what can be calculated by standard design methods.

The speeds shown in this program are cruising speeds in service conditions, having been estimated by regression between published speeds and ship sizes, represented by GT.

Propulsion

All CVs are powered by at least two propellers. Older ships used to have two propellers rotated by shaft lines driven by medium speed diesel engines through reduction gears. Today ships are generally moved by azimuthal propellers(pod), which incorporate an electric motor, with the great added advantage of increasing the ship maneuverability. Very large CVs are driven by 3 or 4 pods.

These ships require a large power generation for hotel services such as lighting, air conditioning, galleys and so on. Therefore it is normal to install several diesel-electric sets that distribute production between propulsion motors and hotel services.

Block coefficient CB

CVs are vessels of fine hull forms with a CB ranging from 0,55, the smallest, to 0,78 the largest, although most are about 0,70. As the CB or displacement are seldom published, I have considered CBs deduced from the Towsin formula (see Ref.4. pag. 584), that is suitable for this type of vessel.

Price

I have included an estimated ship price (in millions of U.S. dollars) based just on her size (GT) and derived by regression of published prices and tonnage GTs, as reflected in the accompanying graph. Obviously these prices are only an idea of what can be a cruise ship value, as prices are those published in Internet, in different years, regardless of such important factors as inflation, situation of shipbuilding market, possible aids and so on. In particular, prices of vessels larger than 225.000 GT, result from extrapolations and therefore have a very questionable reliability.

References

1- List of cruise ships. Wikipedia. http://en.wikipedia.org/wiki/List_of_cruise_ships
2- Cruise-vessels.com http://cruise-vessels.com/drupal/en/node/2
3- Absolut cruceros. http://www.absolutcruceros.com/el-crucero-mas-grande-del-mundo-surcara-las-aguas-en-2012-el-princess-kaguya/
4- Basic design of merchant ships (El Proyecto Básico del Buque Mercante), by R. Alvariño, J.J. Azpíroz, M. Meizoso. Fondo Editorial de Ingeniería Naval. Colegio de Ingenieros Navales y Oceánicos. 2ª edición Madrid 2007

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Estimating dimensions of a Cruising Vessel

Number of passengers (400 to 8.000):

Total Length(m)
Breadth (m)
Draft(m)
Block Coef.
Gross Tonnage GT
Sevice Speed (knots)
Propulsion Power (Kw)
Price aprox.(Millions US $)