Marine Application

From ships to submarines to mining the sea floor worldwide, certain plastics and RPs can survive the sea environment. This environment can be considered more hostile than that on earth or in space. For water surface vehicles, many different plastics have been used in designs in successful products in both fresh and the more hostile seawater. Boats have been designed and fabricated since at least the 1940s. Anyone can now observe that practically all boats, at least up to 9 m (30 ft) are made from RPs that are usually hand lay-up moldings from glass rovings, chopper glass pray-ups, and/or glass fiber mats with TS polyester resin matrices. Because of the excellent performance of many plastics in fresh and sea water, they have been used in practically all structural and nonstructural applications from ropes to tanks to all kinds of instrument containers.

Boat

In addition to their use in boat hull construction, plastics and RPs have been used in a variety of shipboard structures (internal and external). They are used generally to save weight and to eliminate corrosion problems inherent in the use of aluminum and steel or other metallic constructions.

Plastic use in boat construction is in both civilian and military boats [28 to 188 ft. (8.5 to 55 m)]. Hulls with non-traditional structural shapes do not have longitudinal or transverse framing inside the hull. Growth continues where it has been dominating in the small boats and continues with the longer boat boats. The present big boats that are at least up to 188 ft long have been designed and built in different countries (USA, UK, Russia, Italy, etc.). In practically all of these boats low pressure RP molding fabrication techniques were used.

Examples of a large boat are the U.S. Navy's upgraded minehunter fleet, the "Osprey" class minehunter that withstands underwater explosions. Design used longitudinal or transverse framing inside the piece hull. It has a one piece RP super structure. Material of construction used was glass fiber-TS polyester plastic. The designer and fabricator was Interimarine S.P.A., Sarzana, Italy. The unconventional,

Figure 4 : Examples of materials for deep submergence vehicles

Figure 4 : Examples of materials for deep submergence vehicles

unstiffened hull with its strength and resiliency was engineered to deform elasdcally as it absorbs the shock waves of a detonated mine. Its design requirements included to simplify inspection and maintenance from within the structure.

Underwater Hull

On going R&D programs condnue to be conducted for deep submergence hulls. Materials of construcdon are usually limited to certain steels, aluminum, dtanium, glass, fiber RPs, and other composites (Fig. 4.53). There is a factor reladng material's strength-to-weight characteristics to a geometric configuration for a specified design depth. Ratio showing the weight of the pressure hull to the weight of the seawater displaced by the submerged hull is the factor referred to as the weight displacement (W/D) ratio. Submergence materials show the variation of the collapse depth of spherical hulls with the weight displacement of these materials. All these materials, initially, would permit building the hull of a rescue vehicle operating at 1800 m (6000 ft) with a collapse depth of 2700 m (9000 ft).

When analyzing materials for an underwater search vehicle operating at 6000 m (20,000 ft) with collapse depth of 9000 m (30,000 ft), metals are not applicable. Materials considered are glass and RP. The strength-to-weight values for metals potentially are not satisfactory. One of the advantages of glass is its high compressive strength; however, one of its major drawbacks is its lack of toughness and destructive effect if any twist, etc. occurs other than the compression load. It also has difficulty if the design requires penetrations and hatches in the glass hull. A solution could be filament winding RP around the glass or using a tough plastic skin.

These glass problems show that the RP hull is very attractive on weight-displacement ratio, strength-weight ratio, and for its fabrication capability. By using the higher modulus and lower weight advanced designed fibers (high strength glass, aramid, carbon, graphite, etc.) additional gains will occur.

Depth limitations of various hull materials in near-perfect spheres superimposed the familiar distribution curve of ocean depths. To place materials in their proper perspective, as reviewed, the common factor relating their strength-to-weight characteristics to a geometric configuration for a specified design depth is the ratio showing the weight of the pressure hull to the weight of the seawater displaced by the submerged hull. This factor is referred to as the weight displacement (W/D) ratio. The portions the vehicles above the depth distribution curve correspond to hulls having a 0.5 W/D ratio; portion beneath showing the depth attainable by heavier hulls with a 0.7 W/D.

Based on test programs the ratio of 0.5 and 0.7 is not arbitrary. For small vehicles they can be designed with W/D ratios of 0.5 or less, and vehicle displacements can become large as their W/D approach 0.7. By using this approach these values permits making meaningful comparisons of the depth potential for various hull materials. With the best examination data reveals that for the metallic pressure-hull materials, best results would permit operation to a depth of about 18,288 m (20,000 ft) only at the expense of increased displacement. RPs (those with just glass fiber-TS polyester plastic) and glass would permit operation to 20,000 ft or more with minimum displacement vehicles.

The design of a hull is a very complex problem. Under varying submergence depths there can be significant working of the hull structure, resulting in movement of the attached piping and foundation. These deflections, however slight, set up high stresses in the attached members. Hence, the extent of such strain loads must be considered in designing attached components.

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