Fishing Craft Technology

Unit 4 - Boat building materials and construction of boat

4.5. Fouling and boring organisms
Factors causing deterioration of wood
(1) Rot:
Rotting is natural phenomenon caused by micro-organisms like fungi, which cause decay in wood. Organic material of wood is absorbed by these fungi, which causes disintegration of the cell walls followed by changes in colour, texture and strength properties. Wood becomes soft and spongy, due to rotting, besides other changes in the physical and chemical characteristics. Use of untreated wood together with inadequate ventilation and drainage lead to decay. Most of the decay in boats in salt water occurs above the waterline where water can enter but not evaporate and at joints where grain is exposed. In fresh water, interior members below the water line are also often subject to decay.
Some of the preventive measures to avoid rot are usage of heartwood, seasoned and treated wood, allowing proper ventilation of the wood surfaces possibly at all times and eliminating entry of water into the boat.

(2) Marine borers:
Certain group of animals like teredo, martesia etc., bore into the under part of the wooden hull and destroy the wood in the long run. They consume cellulose, a constituent of wood. The damage caused by marine borers is not so easily detectable till it has progressed to a dangerous level. Use of antifouling paints or use of under water sheathing made of copper, FRP or aluminum act as preventive measurers. Besides, if vessel is moored in fresh water for a few weeks or taken out of water altogether for about there weeks, they will perish. If the damage caused is not very severe, the holes could be filled with wooden plugs or with any sealing compound. However it is advisable to replace a member in case of extensive damage.

(3) Marine foulers:
Certain organisms settle on the submerged part of the hull there by affecting the propulsions and efficiency of under water acoustic devices. These organisms otherwise called foulers include microscopic ones like bacteria, fungi, diatoms rotifers and macroscopic ones like coelenterates flatworms, tubeworms, branacles and molluscs. As the settlement of foulers affect the propulsion the speed of the boats is reduced and fuel consumption gets increased. This forms a negative aspect in the economical operation of boats. If the affected boat is kept in fresh water for a considerable time, many of the foulers are cleared. However, adherent shells of organisms like branacles and bivalves remain attached. For complete removal of foulers, they have to be scrapped during dry docking. Metallic sheathing like copper or application of antifouling paints on the submerged portions also prevents settlement of foulers.

(4) Corrosion:
Corrosion is the gradual disintegration of metal due to chemical or electro – chemical attack by atmosphere moisture or other agents. This phenomenon is prevalent in steel vessels and also wooden vessels having metallic sheath to the underwater part of the hull. The corrosion results in thinning of plates and reduction of speed of vessels due to roughness caused on the surface of boat.

Control of corrosion:
The corrosion can be controlled by the following means.

(a) Material selection and design:
Metals that show potential differences should be positioned in such a way that the anodic area is large as compared to cathodic area. For example, iron screws in a brass plate will be quickly attacked but brass screws in an iron plate would be acceptable.
(b) Use of alloys which resist attack:
Alloys immune to all corroding media are preferred. However, no single alloy satisfies this condition. Copper base alloys and stainless steel alloys are resistant to corrosion.
(c) Galvanic or sacrificial system:
Sacrificial plates are used to protect the structure. Normally Zn is used to protect steel, cast iron, brass and bronze.
(d) Efficient paint system:
Adequate coating of anticorrosive paints affords protection against corrosion. The affected parts are chipped, brushed and cleaned to remove rust and mill scale. Subsequently the surface is painted with anticorrosive paints. In case of severe corrosion, the affected plates are replaced by new ones.
(5) Mechanical damage:
The hull of a vessel gets damaged after collision with other boats, structures and jetties. This kind of damage shortens the life of the boat besides posing a threat to the lives of the crew and others on the vessel. Careful handling of the vessel by competent crew would help to avoid collisions. Mechanical damage should be attended to immediately.