98 Ship Design for Efficiency and Economy

Figure 3.5 Effect of a change in number of engine cylinders on the cost of the ship

Figure 3.6 Annual fuel and lubricant costs (kf C kl) as a function of number of engine cylinders and ship's length

3.4 Discussion of some important parameters

Width

A lower limit for B comes from requiring a minimum metacentric height GM and, indirectly, a maximum possible draught. The GM requirement is formulated in an inequality requiring a minimum value, but allowing larger values which are frequently obtained for tankers and bulkers.

Length

Suppose the length of a ship is varied while cargo weight, deadweight and hold size, but also AM L, B=T, B=D and CB are kept constant (Fig. 3.7). (Constant displacement and underdeck volume approximate constant cargo weight and hold capacity.) Then a 10% increase in length will reduce AM by 10%. D, B and T are each reduced by around 5%. L=B and L=D are each increased by around 16%.

For this kind of variation, increasing length has these consequences:

1.Increase in required regulation freeboard with decrease in existing freeboard.

2.Decrease in initial stability.

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Figure 3.7 Variation of midship section area AM with proportions unchanged

3.Better course-keeping ability and poorer course-changing ability.

4.Increase in steel weight.

5.Decrease in engine output and weightÐirrespective of the range of Froude number.

6.Decrease in fuel consumption over the same operational distance.

Increase in the regulation freeboard

The existing freeboard is decreased, while the required freeboard is increased (Fig. 3.8). These opposing tendencies can easily lead to conflicts. The freeboard regulations never conflict with a shortening of the ship, if CB is kept constant.

Figure 3.8 Effect of length variation on the freeboard. Fa D freeboard of basis form, Fb D freeboard of distorted ship, Fc D desired freeboard after lengthening

Reduction in initial stability

The optimization often requires constant initial stability to meet the prescribed requirements and maintain comparability. A decrease in GM is then, if necessary, compensated by a slight increase of B=T, reducing T and D somewhat. This increases steel weight and decreases power savings.

Course-keeping and course-changing abilities

These characteristics are in inverse ratio to each other. A large rudder area improves both.

Increase in steel weight, decrease in engine output and weight, decrease in fuel consumption

These changes strongly affect the economics of the ship, see Section 3.3.

100 Ship Design for Efficiency and Economy

Block coefficient

Changes in characteristics resulting from reducing CB:

1.Decrease in regulation freeboard for CB < 0:68 (referred to 85% D).

2.Decrease in area below the righting arm curve if the same initial stability is used.

3.Slight increase in hull steel weight.

4.Decrease in required propulsion power, weight of the engine plant and fuel consumption.

5.Better seakeeping, less added resistance in seaway, less slamming.

6.Less conducive to port operation as parallel middlebody is shorter and flare of ship ends greater.

7.Larger hatches, if the hatch width increases with ship width. Hatch covers therefore are heavier and more expensive. The upper deck area increases.

8.Less favourable hold geometry profiles. Greater flare of sides, fewer rectangular floor spaces.

9.The dimensional limits imposed by slipways, docks and locks are reached earlier.

10.Long derrick and crane booms, if the length of these is determined by the ship's width and not the hatch length.

Initial stability

GM remains approximately constant if B=T is kept constant. However, the prescribed GM is most effectively maintained by varying the width using Muhlbradt's¨ formula:

B0

B D

C .CB=CB0/2 1 C 1

C D 0.12 for passenger and containerships

C D 0.16 for dry cargo vessels and tankers.

Seakeeping

A small CB usually improves seakeeping. Since the power requirement is calculated for trial conditions, no correction for the influence of seastate is included. Accordingly, the optimum CB for service speed should be somewhat smaller than that for trial speed. There is no sufficiently simple and accurate way to determine the power requirement in a seastate as a function of the main dimensions. Constraints or the inclusion of some kind of consideration of the seakeeping are in the interest of the ship owner. If not specified, the shipyard designer will base his optimization on trial conditions.

Size of hold

For general cargo ships, the required hold size is roughly constant in proportion to underdeck volume. For container and ro-ro ships, reducing CB increases the `noxious spaces' and more hold volume is required.

Usually the underdeck volume rD D L B D CBD is kept constant. Any differences due to camber and sheer are either disregarded or taken as constant over the range of variation. CBD can be determined with reasonable accuracy

Optimization in design 101

by empirical equations:

D

CBD D CB C c 1 .1 CB/

T

with c D 0:3 for U-shaped sections and c D 0:4 for V-shaped sections.

With the initial assumption of constant underdeck volume, the change in the required engine room size, and any consequent variations in the unusable spaces at the ship's ends and the volume of the double bottom are all initially disregarded. A change in engine room size can result from changes in propulsion power and in the structure of the inner bottom accommodating the engine seatings.

The effect on cost

A CB variation changes the hull steel and propulsion system costs. Not only the steel weight, but also the price of the processed ton of steel is variationdependent. A ton of processed steel of a ship with full CB is relatively cheaper than that of a vessel with fine CB.

The specific costs of hull steel differ widely over the extent of the hull. We distinguish roughly the following categories of difficulty:

1.Flat areas with straight sections in the parallel middlebody.

2.Flat areas with straight sections not situated in the parallel middlebody, e.g. a piece of deck without sheer or camber at the ship's ends. More work results from providing an outline contour adapted to the outer shell and because the shortening causes the sections to change cross-section also.

3.Slightly curved areas with straight or curved sections. The plates are shaped locally using forming devices, not pre-bent. The curved sections are pre-formed.

4.Areas with a more pronounced curvature curved only in one direction, e.g. bilge strake in middlebody. The plates are rolled cold.

5.Medium-curved plates curved multidimensionally, e.g. some of those in the vicinity of the propeller aperture. These plates are pressed and rolled in various directions when cold.

6.Highly curved plates curved multidimensionally, e.g. the forward pieces of bulbous bows. These plates are pressed or formed when hot.

Decreasing CB complicates design and construction, thus increasing costs:

1.More curved plates and sections, fewer flat plates with rectangular boundaries.

2.Greater expenditure on construction details.

3.Greater expenditure on wooden templates, fairing aids, gauges, etc.

4.More scrap.

5.More variety in plates and section with associated costs for storekeeping and management.

An increase in CB by 1CB D 0:1 will usually increase the share of the weight attributable to the flat areas of the hull (group (1) of the above groups) by 3%. About 3% of the overall hull steel will move from groups (3)±(5) to

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groups (1) and (2). The number of highly curved plates formed multidimensionally (group (6)) is hardly affected by a change in CB. The change in weight of all curved plates and sections of the hull depends on many factors. It is approximately 0:331CB hull steel weight.

Speed

The speed can be decisive for the economic efficiency of a ship and influences the main dimensions in turn. Since speed specifications are normally part of the shipping company requirements, the shipyard need not give the subject much consideration. Since only the agreement on trial speed, related to smooth water and full draught, provides both shipyard and shipping company with a clear contractual basis, the trial speed will be the normal basis for optimization. However, the service speed could be included in the optimization as an additional condition. If the service speed is to be attained on reduced propulsion power, the trial speed on reduced power will normally also be stated in the contract. Ships with two clearly defined load conditions can have both conditions considered separately, i.e. fully loaded and ballast.

Economic efficiency calculations for the purpose of optimizing speed are difficult to formulate due to many complex boundary conditions. Schedules in a transport chain or food preservation times introduce constraints for speed. (For both fish and bananas, for example, a preservation period of around 17 days is assumed.)

Speed variation may proceed on two possible assumptions:

1.Each ship in the variation series has constant transportation capacity, i.e. the faster variant has smaller carrying capacity.

2.Each ship in the variation series has a constant carrying capacity, i.e. the faster variant has a greater transportation capacity than the slower one and fewer ships are needed.

Since speed increase with constant carrying capacity increases the transportation capacity, and a constant transportation capacity leads to a change of ship size, it is better to compare the transport costs of 1 t of cargo for various ships on one route than to compare costs of several ships directly.

Essentially there are two situations from which an optimization calculation can proceed:

1.Uncompetitive situation. Here, speed does not affect income, e.g. when producer, shipping company and selling organizations are under the same ownership as in some areas of the banana and oil business.

2.Competitive situation. Higher speed may attract more cargo or justify higher freight rates. This is the prime reason for shipowners wanting faster ships. Both available cargo quantity and freight rate as a functions of speed are difficult to estimate.

In any case, all variants should be burdened with the interest on the tied-up capital of the cargo. For the uncompetitive situation where the shipowner transports his own goods, this case represents the real situation. In the competitive case, it should be a lower limit for attractiveness of the service. If the interest on cargo costs are not included, optimizations for dry cargo vessels usually produce speeds some 2 knots or more below normal.