Fishing Craft Technology (1+1)

Unit 4 - Boat building materials and construction of boat

4.2. Wood
The timber is being used as a boat building material since time immemorial. Nearly 5000 species of Indian tropical trees are known to yield timber though only about 400 species are actually suitable as boat–building material. About 200 of them can be put to commercial use of which a few have been found ideally suited for the construction of modern mechanized boats. Hence selection of the timber is very essential.

General consideration in selection of timber

• Select Seasoned stock: Seasoned timber in which moisture content is within prescribed limits is preferred except in the case of steam bent frames where green stock is better as it can be bent easily.
• Prefer air dried wood compare to kiln dried wood: In kiln drying, oils of wood having resistance to fungi and borers leak out or evaporate. Hence they are susceptible to dry rot. As such air-dried wood is preferred.
• Avoid sapwood: Sapwood is softer and less durable than heart wood. It is recognized by its light colours as compared to heart wood. Hard heart wood is free from natural defects and blemishes and with straight grain is good for boat construction.
• Prefer radial cut timber than tangential cut
• Select timbers having high rot resistance: Rotting is severe problem in tropical areas. High resistance to rot can also be achieved pressure impregnation of timbers.
• Choose timbers which have high dimensional stability: A timber that is not stable will shrink and swell excessively resulting in opening of seams with resulting leaks causing great strain in the fastenings and making planks buckle. Stability is measured by tangential shrinkage which should be less than 4%.
• Timber must have good strength: The weight of the timber gives an indication of its strength. Moderately heavy timber with a weight of 562-722 Kg/m3 at about 12% moisture continent is good enough.
• Timbers should be able to take nails and screws without splitting: The problem of splitting, when nailing the planks can be solved by pre boring the holes for nails and screws.
• Workability of the timber: Cutting, planking and working with wood should not be cumbersome
• Ability to take finish
• Select those timbers which can take preservatives
• Cost and availability

Specific requirements of wood for various components of boat
For hull planking, the wood used is subject to severe conditions of biological deterioration and electro–chemical degradation. It should be especially durable against marine bore’s attack, stable in shape and size, good nails screw holding power, easy to work and shape as the planks are butt joined and it should have low swelling, shrinking and warping.
Decking is subject to alternate wetting and drying. Therefore, timbers should have high dimensional stability, resist delay and insect attack, should not wear out easily, take polish satisfactorily, easy to shape and work as decking is butt jointed, should not crack under alternative wetting and drying and good resistance to impact and abrasion . For both hull and deck planking light weight combined with moderate strength is preferred.
For keel, keelson and Hog keel, heavy timbers with high strength are used. Single piece wood is preferred to avoid joint. The timber used should have resistance to decay and insect attack. It should not develop severe cracking and splitting as the timber used in this area is subject to stagnant and humid air conditions.
The wood used for ribs must be very tough stock resistance. Possess good bending qualities, durable as that in keel, it is subject to stagnant humid air conditions and it should be free from, knots, cross grains etc., which impair the bending and strength properties. For stringers, transom, beam and floor timbers, the timbers with considerable length should be available and those must posses resistance to delay and insect attack under humid air conditions prevailing inside the boat. Light weight with moderate strength is required. Knees require very strong and hard timber having resistance to decay and insect attacks. Oars, paddles and masts require long length elastic and tough but not heavy timber and it should be durable against decay and insect infestation.

Wooden fishing vessel
No single material is ideally suited for all types, sizes and areas of operation. Each material has its own merits and demerits. There are vast resources of wood available in India. In spite of this factor, good quality timber suitable for boat construction is becoming scarce and dearer with proper selection, seasoning and treatment by preservatives many secondary timbers like van teak, mango etc., can be brought to extensive use in boat building.


Properties of wood
Wood consists of hollow fibres, the walls of which are composed of cellulose and lignin. The strength of wood depends on the direction, size arrangement and appearance of the fibres and also its moisture content. Timber with straight and close grain with 10 to 15% moisture is the strongest. It is 16 times stronger in direction of grain than across. The specific gravity of wood is about 1.56 regardless of species. In spite of this, dry wood of most species float because a large part of it is occupied by all cavities and pores. Since wood decays in normal working condition due to attack of fungi, marine borers and insects it has to be used in boat building after proper treatment and preservation.
The strength of any wood is closely related to the amount of moisture it contains. Wood in very wet condition and absolutely dry condition will have poor strength. Usual permissible range of moisture is 12-18% with maximum strength found at 15%.

Merits and demerits of wood
Merits
Wood has been used in boat building exclusively from time immemorial. Wood presents certain inbuilt and Inherent advantages like positive buoyancy, easy workability, treatability and good strength. Inherent elastic properties of wood help in bending and shaping when heated and steamed. It can shrink and swells without rupture. Wood can also stand sudden applied shock. It also has clean and smooth finish apart from being highly durable with proper treatment. Care and maintenance the life of the wooden boat can be increased. It is good heat insulation. Easy to fit and drill holes.

Demerits
Wood being a hygroscopic material is subject to rapid changes in physical conditions resulting in dimensional instability. It absorbs nearly 30% of its weight. Being an organic material it is subject to several types of deterioration. Though strong, its strength is not uniform as it depends on the orientation of its grains. It is unhygienic as wooden decks and fish holds harbor millions of micro–organisms. It deteriorates easily due to mechanical wear and tear and is easily destroyed by fire. Constant protection against fungi, bacteria, insects, borers and foulers is needed. It also requires careful selection, seasoning and storage sharp curves are not possible and wastage is also considerable.