Fishing Craft Technology

Unit 4 - Boat building materials and construction of boat

4.5. Hull Protection method
There are different methods of hull protection is use in India.
1) Oils:
Oils like sardine oil, cod liver oil, cashew nut shell liquid oil etc., are used either singly or in combination with others to protect the hulls. This is a common practice for indigenous boats on the east and west coast of India. These oils are smeared periodically.

2)Dammar batu:
Dammar batu is a glossy organic substance extracted from certain selected trees. It differs from gum in that it is not soluble in water. Solid pieces of this resin are powdered and mixed with ground nut oil with continuous stirring. Damma batu hardens the surface of wood and retards moisture changes. The application of this resin is common in all types of boats in Sothern Gujarat and Northern Maharashtra. The fishermen in South Gujarat apply lime plaster and Maharashtra fishermen use antifouling paints over the resin to make it effective against marine borers.

3) Coal tar:
It is used for protection of indigenous boats on the eastern coast of India.

4) Copper Sheathing:
This is commonly used on mechanized boats in India.

5) Aluminium alloy sheathing:
It is becoming popular in India for protection of mechanized boats.

6) FRP sheathing:
It has been introduced in India for the protection of mechanized boats. All these methods have their own merits and demerits. The oils and Dammar batu are cheaper and are easy to apply. However, they are not effective against borers. Coal tar though easily available, cheap and effective against borers and foulers makes that boat heavy and its surface becomes sticky, creating problems in painting. Aluminum alloy is light and cheap but prone to corrosion in seawater. Hence anticorrosive paint has to be applied. FRP sheathing does not corrode, is impervious to rot and does not leak at the seams. But constant abrasion is likely to cause thinning out of sheathing. Moreover, the method of sheathing involves special techniques and the hull needs preparation in a specific way. Copper sheathing is the most ideal for protection of underwater part of the hull as anticorrosive and antifouling paints are not required. However, because of the exorbitant cost, fishermen are looking for alternative material.

Steel:
Steel as a boat building material came to general use during the period between 1845 and 1880. Steel was the first material considered to replace wood. The first steel ship was built in the year 1878. The adaptability of steel as construction material for large vessels is well known all over the world. It is one of the most remarkable materials with many advantages if used intelligently.
Basically steel is an alloy of Iron and carbon. The categories of steel vary from soft and malleable, which can be easily bend and twisted, to hard and brittle as glass. The plain low – carbon steels containing from about 0.10 to 0.20% carbon are more than adequate for most of the boat construction needs. Stainless steel is not good as boat building material but it has excellent uses for rigging fittings, fastenings, propeller shafts and other applications. They are not always at their best in salt water though they are strong, tough and most corrosion resistant.

(i) General properties of steel:
The strength is the first and greatest attribute of steel. On strength weight basis steel is excellent. By alloying with metals like manganese, its strength could be increased but then they are difficult to handle due to stiffness. High tensile steel made by alloying with metals like manganese has strength of about 2.6 – 3.1 kg/cm2.

However because of its stiffness, it needs special techniques of welding. These are not used extensively because of the cost factor except occasionally for the sake of speed, manoeurability etc., in naval vessels. A majority of steel in ship is medium steel. Although some parts of the structures may be of high tensile steel. The specific gravity of steel is 7.84. On the basis of weight alone, steel would be at a considerable disadvantage. It is heavier per cubic meter than wood, aluminum or FRP.
The mild steel is ductile, will elongate to 30-40%. Its ductibility and strength are the opposite sides of the same coin. The mild steel which is commonly used for boat construction has the advantages of cheapness, high mechanical properties and ease of cold working. However it is more prone to corrosion against which it needs efficient protections. It weighs 64-1445 Kg/m3 and has a tensile strength of 4735 kg/cm2. The effect of high temperature is negligible as its melting point is around 1150 – 1200oC.
Steel is fire resistant and it can survive a fire that would be catastrophic to hulls made with other materials. Because of the relatively small frames and other structural steel members and the wider spacing possible, coupled with the feasibility of building water and fuel tanks integral within a steel hull, a considerable gain in living or working space is available in steel hulls.

(ii) Merits:
Steel is a highly versatile material. It offers significant savings in the construction of standard designs in large numbers. By adopting latest techniques in welding, larger sections of steel can be fabricated with maximum efficiency and minimum wastage of material particularly for the construction of bigger boats beyond 20 m length. Steel offers high uniform strength. It is possible to make all shapes by cutting, bending, welding and riveting. It can even be bent to double curves but this is expensive and requires special skill and time. A special advantage of steel craft is that they can be easily lengthened.
Steel is an ideal material to cope with ice afloat where other materials usually fail. Another attractive feature of steel is the ease with which they can be altered. It works out cheaper for size above 18 m. Construction of steel vessel is much faster than other materials. In comparison with wood, repair, cost is less in steel vessels. Moreover, steel is readily available in a wide variety of shapes and sizes. It is easy to fit out and easy to make strong connections.

(iii) Demerits:
The main disadvantage of steel is its corrosion because of marine environment. Hence constant maintenance is needed. As the specific gravity of metal is high, vessels constructed with steel is heavy. It is brittle adjacent to the weld. Another disadvantage with steel hulls is the difficult to getting the compass to work well. A skilled worker is necessary.


Aluminium:
Use of aluminum in construction of boats and ships is not new and in fact dates back as far as 1890. Earlier attempts did not meet with success because right type of material and method of construction were not well known. The first sea–going hull of aluminum was built in France in 1892. The advancement in metallurgical science together with latest techniques in welding has made the extensive use of aluminum possible in fishing vessels since 1930.

(i) Properties:

• Aluminum becomes harder and stronger with time
• It’s melting point is as low as 600oC
• Extremely ductile and malleable. It will bend or deflect more easily than steel.
• Alloys used in boat construction are called marine alloys like Al–magnesium alloy. Al- silicon alloys etc. Both these alloys are resistant to sea water corrosion.
• Pure aluminum is rather soft and weak and has yield strength of approximately 2,273 kg, but alloys have yield strength of about 40,000 Kg.
• It has ultimate tensile strength of 17,273 kg/cm2 and weight of 2,713 kg/m3
• Special quality alloy used is very light with specific gravity of 2.7 as against 7.8 for mild steel.
• It is tough and resilient.
• Pure aluminum is a white metal with a bluish tinge and has good resistance to corrosion on account of formation of a protective film of oxide. However, pure metal is too soft for construction value.
• Modulus of elasticity is more or less constant for all aluminum alloys i.e. 6960
• Kg/mm2
• Properties deteriorate at high temperature as it has low melting point (600oC)
• Easy to work with, can be formed, welded, riveted or bolted into any form.

ii) Merits:
The greatest advantage of aluminum is its light weight. The savings in weight when aluminum is used can result in an increased carrying capacity in a cargo vessel. Its use in super structures lowers the centre of gravity of a vessel. Its use in hull construction often results in the saving of fuel due to reduced displacement. It resists sea–water corrosion without water absorption. As regards its corrosion resisting properties, it is better than copper, zinc, steel etc., It is impervious to worms, rotting and rusting. Due to its good elastic modules, aluminum has a greater impact resistance than steel. It requires very little maintenance. Welding of aluminum is extraordinarily fast. The welding speed is three times that of steel though aluminum requires pre-welding preparation for cleaner surfaces.
Aluminum is non- porous and non-peeling and provides most desirable hygienic conditions for fish handling. Aluminum has a much better scrap value than steel. This is very little wastage material. Aluminum vessels have better manoeurability due to less weight. Greater speed is also possible. They have greater load carrying capacity if the same design is used as that for wood or steel.

iii) Demerits:
Aluminum is not compatible with other conventional metals, more so in sea – water due to electrolysis. This can be prevented by coating the surface with insulating materials as neoprene, p.v.c, rubber etc. These non-absorbent materials prevent the flow of galvanic current necessary to sustain attacks. Zinc chromate coat also prevents galvanic corrosion in salt water. Fire near an aluminum vessel is dangerous. It will seriously reduce the strength of the boat, because of its low melting point. Aluminum requires surface preparation prior to painting. Antifouling paints have to be carefully selected for aluminium vessels because those containing mercury should not be used, as mercury destroys aluminium by forming an amalgam. The surface needs thick barrier paints like zinc chromate prior to painting. The initial cost of aluminium vessel is much higher than steel. However, about 50-60% can be saved in weight which reduces the cost difference. It lacks the stiffness of steel. The thickness of hull must be 1½ to 2 times than that of steel for the same stiffness or the same strength.
Aluminium welding is a tricky business. As the metal oxidizes rapidly, oxygen must be excluded from the weld during fusion process. This calls for skill in welding. Construction of aluminum boats requires a greater technical knowledge and skill than is required for any other vessel.

Ferro cement:
Ferro cement was first introduced as a boat–building material in 1847 in France by Joseph Louis Lambot. However it took professor Pier Luigi Nervi of Italy in 1940 to successfully develop a Ferro cement boat. Since then it is finding application as a boat building material in many countries. Ferro cement is a highly versatile form of rein forced concrete made of wire mesh, sand, water and cement which possess unique qualities of strength and serviceability. It consists of a reinforcement of a number of layers of galvanized iron chicken wire mesh over an arrangement of mild steel rods and fully plastered with a mortar mix of sand and cement combination. The American Bureau of shipping defines Ferro cement as “A thin, highly reinforced shell of concrete in which the steel reinforcement is distributed widely throughout the concrete so that the material under stress acts approximately as a homogeneous material.

(i) Properties:

• The specific gravity of Ferro cement is 2.4 to 2.6
• The steel content works out to be 22-25% (562 Kg/m3) while the mortar mix is about 70-75% (2409 Kg/m3), water is added (by weight) in the water–cement ratio 0.22 – 0.4. Water is added to give it the required plastic–like consistency. Construction involves curing after mixing one part cement in two parts of sand and then water.
• Ultimate thickness of shell after careful use of mortar mix will range between 18 mm and 38mm.
• Like concrete, its properties are those of (i) Mortar (ii) Reinforcement (iii) Special arrangement of the reinforcement in the member (iv) Absolute dimensions of the member.
• The reinforcement averages about 385 Kg/m3 i.e., good tensile strength in all directions.
• High steel quality and concrete gives better strength/weight ratio.
• The boat consists of layer of wire from 1.25 to 2.5cm plus 5 gauge steel rods laid fore and aft and athwart the job. These rods are the main strength. Pins made of 16 gauge U shapes are used with the arm of the U about 8 cm long. A pin has to be placed wherever rods intersect.
• FC is essentially RC but it exhibits different behavior from conventional RC in performance strength and potential application that it must be classed as completely separate material. It differs from conventional reinforced concrete in that its reinforcement consists of closely spaced multiple layers of steel mesh completely impregnated with cement mortar.
• It differs very much from the reinforced cement concrete in making greater proportional use of steel over cement.
• Ferro cement can be formed into section less than 2.5 cm thick with only fraction of an inch of cover over the outer most mesh layer. The conventional concrete is cast into sections several cm thick with about 2.5 cm of concrete cover over the outer most steel rods.

(ii) Merits:

• Easy to fabricate into complex shapes without the use of moulds.
• It has good strength.
• It is water proof and rust proof.
• Has corrosion resistance.
• A unique feature of FC is that it becomes stronger with increasing age especially under constant contact with water.
• FC boats, if properly built will have the longest life in water.
• Cost of FC boats is much less than the conventional wooden hull.
• Construction procedures are simple require simple tools and ordinary manual labour.
• Basic materials like cement, steel, sand etc., are readily available throughout the world.
• Easy to repair, repaired by cleaning the damaged part and then plastering with an epoxy binding agent.
• Resistance to fire and marine borers.
• Hull does not need any protection; paint is given only for aesthetic reasons.
• Highly resilient.
• High energy–absorbing property compared to other materials.
• Only short training is required for the labour, if a skilled supervisor is present. Construction needs only unskilled labour.

(iii) Demerits:

• Not well suited for light displacement vessels less than 10 m long.
• Low impact resistance.
• Slightly heavy.
• Needs more HP to propel the vessel.
• The weakest feature of FC is low resistance to penetration by a sharp object which is called punching. This is different from impact.
• It cannot be readily adapted to mass production.
• Difficulty in making the holes after construction. Holes in the hull for water inlets are best moulded in, when the hull is constructed.
• Diesel cannot be used in FC hulls as it attacks concrete and cement mortar. To prevent this interior of tanks must be coated with epoxy or diesel–resistant tar, some time steel tanks are to be used.
• Corrosion occurs inside, if adequate mortar is not applied inside wire mesh over the reinforcement. When mortar is forced through many layers of mesh, it is difficult to ensure complete and uniform penetration. It voids or holes are left behind, it can result in corrosion of mesh after entry of water. Special care is needed to see that steel reinforcement is not exposed. If so, quick repairs are necessary.
• As with GRP hulls it is very difficult to assess the quality of a FC hull once it has been built. Hence careful quality control and inspection is needed, if the finished hulls are to be of an acceptable standard.
• Heat transfer into fish hold is more than wood and hence an insulation of 1½ times, the thickness used in wooden boats is needed.
• Noise transfer is more. Problem is overcome by using sound– proof insulating material in bulk heads.

Fibre glass rein forced plastics (FRP Fishing Vessels):
GRP was used for the first time in USA in 1946 in boat construction. GRP is a combination of two basic materials consisting of reinforcing agent like glass fibre in the form of thin fibre and a plastic resin capable of impregnating fibres. The resulting material is known as GRP. The term reinforcement plastic materials refers to any plastic material (Polyester resin) whose physical properties have been upgraded by the addition of some auxiliary material, fibres of glass etc., In fact reinforced plastics are analogous in several respects to reinforced concrete where the low tensile strength of concrete is upgraded with steel rods. This material is ideally suited for mass production.
The raw materials used in GRP boat building are (a) Fibre glass (b) Plastics (c) Catalyst (d) Accelerator (e) Other substances which includes (i) Thixotrophic paste (ii) Fillers (iii) Pigments
There are different resins – Polyester, epoxy vinyl etc., but polyester is in common use. It is a colour less liquid which can be stored for a considerable period in a cool place. It can be activated by addition of small quantities of two other chemicals known as accelerator and catalyst. Although a number of thermosetting resins such as phenolics, melamine, silicones and epoxies are used, polyester resins have found the widest use in reinforced plastics today.
Unmixed polymer resins will have shelf life of at least three months when kept closed at 300C but once activated it should be used within 15-60 minutes based on mixing ratio. Polyester resin when activated goes through five stages before it is regarded as fully cured. They are (i) Liquid stage (ii) Gel stage (iii) Set stage (iv) Cured stage and (v) Final cured stage.
The whole process is mainly influenced by the activators, their properties of mix and working temperature. Under tropical Indian conditions this period may be extended for 10-15 days. Preparation of polyester with glass reinforcement is in the ratio of 3:1 i.e., 67% resin by weight and 33% of glass fibre by weight. Polyester is a thermo hardener, a thermo setting plastic material which, after the initial action of required heat and pressure cannot be changed by the application of more heat or high pressure. It is an ester formed by the reaction of an acid and an alcohol. Complex esters are known as polyesters which are cheaper, serviceable and more popular. Epoxy though stronger is tricky to handle and is more expensive.
Glass fibres are made from a basic product of “continuous filament” produced by special technique of mechanical drawing of molten glass. They are available in different forms like unidirectional, bidirectional and random directional. The most widely used fibre glass is chopped strand mat consisting of strands about 50 mm long held together in random mat form, using adhesive binders. This binding agent dissolves on contact with resin allowing the resin to penetrate the fibres to the full depth of the mat. Glass fibre serves purely for reinforcement, while the thermosetting resins help in proper binding of the material. Greater strength of laminate can be achieved by using woven glass mat called woven rovings.
The process of converting polyester resins from liquid to solid state involves chemical reaction with the addition of catalyst (methyl ethyl ketone peroxide) and an accelerator (cobalt nephenate) to polymerization at room temperature. Polymerization generates its own heat so that the lamination work with fibre glass reinforcement is cured at room temperature. Quantities of catalyst and accelerator added vary in relation to the weight of the resin, thickness of laminate and working temperature. Normally 1% accelerator and 4% catalyst are added by volume.
Other substances which can be added to the resin are thixotrophic paste, fillers, pigment and release agent. Thixotrophic paste is added to prevent run of resins on a vertical surface. Fillers are the powders such as china clay, chalk or alumina which make the resin opaque, reducing its cost, improving abrasion resistance surface hardness and also reducing exotherm which is the liberation of heat due to chemical reaction during manufacture. Pigment is added to the first layer on the whole layup to give the desired colour. A suitable release agent must be applied over the working surface of the mould to prevent sticking of the resin.

(i) Properties:

• Minimum tensile strength of FRP is 1050 Kg/cm2
• Specific gravity is 1.6
• Weight per cubic meter 1717 Kg/m3
• Modulus elasticity is 1.2 x105 Kg/m2
• Quality and performance of GRP depend to a large extent on the skill and care employed during its manufacture. In this respect it differs from other construction materials like steel or aluminium, where the quality of the finished product depends on the nature of raw materials themselves.
• Humidity and temperature control are essential while working with FRP. Low working temperature increases the curing time and high temperature reduces the pot life of resin. Pot life of resin is its storage period after the catalyst and accelerator are added. Normal optimum ambient temperature of FRP processing is about 28OC. The relative humidity should be as low as possible, not above 68% because the moisture carried by the glass fibre on the resin can affect the quality of the end product.

ii) Merits:

• Light weight.
• Highly durable.
• High strength to weight ratio.
• Can be fabricated to any desired shape and size.
• Material is free from corrosion both in air and water.
• No deterioration due to fungi and borers.
• Speedy construction.
• Due to light weight, it is possible to achieved greater speed; increased fish hold capacity and lesser H.P.
• Very little surface finishing required after moulding.
• Excellent quality control possible.
• Impervious to moisture.
• Being homogeneous structure, there is no leakage. Hull is one piece without seams and joints
• High chemical resistance.
• High weathering resistance.
• In case of any accident, the damage caused will be mainly localized resulting in easy repairs.
• Smooth glossy surface finish results in minimizing frictional losses and there by resulting in fuel economy.
• FRP lends itself ideally to mass production.
• The low modulus of elasticity of FRP is beneficial in absorbing energy from impact loads such as slamming.
• Due to the incorporation of the pigment in the gel, painting of the hull can be avoided. The plastics used to form FRP laminate can be provided with a variety of colours which eliminates the need of paint for many seasons.
• Though initial investment is high, because of minimum expenditure on maintenance and prolonged service life, FRP ultimately proves to be the most economical material for fishing boats.

(iii) Demerits:

• Higher initial cost. Moulds needed in the manufacture work out to be quite expensive, because of the utmost perfection that is required
• Unlike other yards, the concepts of FRP boat yard is elaborate and complicated besides being expensive.
• Fabrication needs special technical knowledge.
• Environmental controls are necessary to achieve maximum results.
• Some of the essential raw materials have low storage life and hence require storage under strict environmental control
• The resistance of FRP to abrasion is poor, considering its use in conjunction with wire ropes, otter boards etc., but this can be solved by incorporating additional weather plates wherever necessary.
• FRP boats have significant hull deflection adversely affecting the engine and shafting.
• Greater fire danger.
• If the defects are not seen while laminate is laid up, this will reflect at later stage of use when it is difficult to rectify. Water or oil finds their way into laminate if the edges of the laminate are not sealed.
• The material lacks stiffness as the modulus elasticity of conventional FRP is less than 1.4 x 105 Kg/cm2 compared to 2.1 x 106 Kg/cm2 for steel and 7x105Kg/cm2for aluminium
• Needs protection against foulers.
• Low modulus elasticity may lead to vibration.