Shipbuilding & Ship Repair Articles

Let us talk a bit more about the application of non-ferrous materials in ship construction. The previous article covered the use of the GRP while in this one we will deal with concrete. Concrete consists of a mixture of stone aggregate bonded by a hardened cement, and Portland cement is normally used for marine applications. The aggregate consists of sand, gravel, and crushed stone. Specific gravity of concrete normally varies between 2.2 and 2.5, primarily depending upon the sizes and density in the stone mixture. Lighter weight concrete with specific gravities in the 1.6 to 2.0 range are made by using clay and shale aggregate. The ratio of water to cement is one of the most significant factors in determining concrete quality and properties.

Ordinary structural concrete with a water/cement/ratio approximating 0.40 by weight is usual for marine work. The long term durability of concrete in sea water has been well established on the basis of service experience and testing of samples from structures that have been submerged. However, in certain situations when concrete is ex¬posed to sulfate in soils or fresh water, it may react with the sulfate, and degrade. Sea water, however, minimizes or prevents such deterioration. Where sulfate deterioration is of concern, special sulfate resistant concretes are used.

Ferrocement

Ferrocement is a form of reinforced concrete wherein layers of steel mesh are used as the reinforcing medium. The material has been used for making small boats up to 50m with skin thickness of 10 mm to 40 mm. The low cost and availability of the mesh and concrete ingredients make ferrocement particularly attractive where sophisticated industrial facilities are not available. Its most extensive commercial use has been for fishing vessels up to 50 m in length.

In the present article we will have a short talk about one of the non-metallic materials used for construction ship hulls. Though not used in ship building as widely as the steel, for instance, the these plastics are becoming more are more popular and therefore deserve due attention of the ship designers and shipbuilders.

Glass reinforced plastics, GRP, are a form of fiber reinforced plastics, FRP, which were introduced for marine structural applications in the 1940's in the form of Navy personnel boats. Since that time GRP have found widespread acceptance for yachts and small boats such as fishing trawlers up to 34 m in length. Although the future of glass reinforced plastics for larger ship structures is very promising, economic factors, and to some degree, questions of durability, limit their applicability.

Reinforced plastics used for ship structures are composed of glass fibers embedded in unsaturated polyester resins. Properties of GRP that are particularly useful for marine service, and have led to their extensive use for small boats, are high strength-to-weight ratio combined with good resistance to deterioration upon prolonged exposure to sea water. Lower maintenance costs for GRP hulls compensate for their relatively high initial cost as compared with steel or wood.

In this article we will talk about the engineering and design in the shipbuilding industry of today. First of all, let's see what exactly the engineering department of the shipyard normally deals with. The engineering department is called upon to aid in preliminary planning due principally to the numerous applicable rules and regulations which must be satisfied and to the complexity of both yard and ship-board equipment, it is important to note that new regulations.

These rules and regulations would normally include internationally recognized regulatory documents published by such entities as, for example, the Intergovernmental Maritime Consultative Organization (IMCO) and the U. S. Coast Guard (USCG) and many others, have greatly increased the amount of engineering work necessary for both the preliminary and final designs of the ship. Engineering works are normally started at the earliest possible time, even before schedules are prepared. That is very important for the smooth engineering design and subsequent construction of the vessel.

Scheduling methods used in shipbuilding are unique to each yard and generally reflect practices developed from experience. An overall schedule which is most useful to both management and production departments is one which highlights major tasks and events, and which shows the sequence of work and the relation of the various tasks to each other and to the whole project.

The basic principle in network flow is the task-to-task relationship. That is, task С cannot start until its two prerequisite tasks A and В are completed. There is, of course, the usual task-to-time relationship for each task. These principles have always been employed in one form or another throughout industry, but the computer has now made it possible to utilize to the fullest these principles in network form.

Network Flow Scheduling technique is often used for controlling large, complex, and possibly non-repetitive projects. Examples of the technique are PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method), both of which provide a means of representing graphically the different operations that make up a project. These networks can be revised to show the effects of adjustments to a schedule necessitated by changes in design, delays, etc. It is also possible to treat the network statistically in order to obtain an idea of the probable longest and shortest times for completion of a project.

Preliminary planning is done at the time of bidding and before contract signing. The first step is to determine dates for such key events as keel laying, launching, and delivery. Due to uncertainties of the final design, the material market, and the general labor situation, it may be desirable to modify these dates in order to provide a margin of time in meeting definite commitments. This is a management prerogative.

Key dates are usually shown on a shipway schedule chart; these dates become increasingly critical when the number of shipways is decreased. Estimating the shipway schedule for new designs is difficult, especially if development work is necessary of if new facilities are required. Experience is extremely valuable in this connection because there is seldom time for any in-depth analysis.

For any efficient operation, whether the plans are prepared in the shipyard or by a subcontractor, the engineers and draftsmen must work closely with the building yard. The only way to minimize potential problem areas and to ensure a maximum degree of success is to consider each step in the construction and scheduling process, no matter how trivial the step may appear. This applies to machinery installation and outfitting as well. It is imperative that a design engineer be familiar with all of the pertinent factors affecting production, such as maximum size and weight of plates, sub-assemblies. and erection units. But most important is the time and effort needed to plan construction at the very beginning of the design stage.

In order to provide some flexibility in design and construction, specifications are often written to give the con¬tractor reasonable options, such as to use a casting, a forging, or a weldment for a stern frame or to use either radiography or ultrasonic testing for weld inspection. In other cases, the specification will describe the construction method or system preferred by the contractor.