1.1 Ferrocement: a material for fishing boats
1.2 What is ferrocement?
The increasing cost and scarcity of durable boatbuilding timbers have affected the construction of fishing craft around the world. The developed world has by and large witnessed the transfer from traditional wooden boatbuilding methods to either less conventional wood construction techniques (e.g., plywood or wood laminates) or non-wood materials such as fibre reinforced plastic (FRP), steel, aluminium and ferrocement. These techniques generally favour less labour intensive methods of construction. In the developing world where timber is still the predominant boatbuilding material, the scarcity and high cost of good quality timber have not meant that less wooden boats are being built, but rather that building quality has deteriorated through the use of poor quality timber. At the same time, however, attempts have been made to diversify construction methods with varying degrees of success. This publication is intended to benefit those who are considering ferrocement construction; it is assumed that those who use the book are already conversant in small fishing vessel construction.
The choice of boatbuilding material will be made after considering all alternatives and their characteristics: other criteria will include the availability and cost of raw materials, the degree of skill required from the labour force, the range of designs that can be built with the process, the ease with which those designs can be adapted, the replication of the production process, and the relative ease of repairs. This can be summarised by the following categories:
a) availability2. Labour:
d) testing facilities
a) availability3. Site:
a) suitability of location4. Design:
a) in-house5. Cost:
c) suitability for operational area
d) acceptable to prospective purchasers
a) competitively priced6. Marketing and Sales:
b) value for money
7. After Sales Service:
a) maintenanceLater sections of this document cover many of the above categories more fully as regards the construction of ferrocement craft.
In recent years the destruction of forests has now excluded various species of good quality boatbuilding timber from general use, reducing availability and spiralling costs upwards. For wooden fishing boats, the added burden of sheathing protection against marine borers particularly in warmer waters, and ice damage in cold waters, also need to be taken into account in their final costs.
Plywood offers the chance to achieve strength with lightness and is easily maintained and repaired. However, its flat sheets impose design limitations in small craft and marine grades are not widely available.
GRP (glass reinforced plastic) or FRP (fibre reinforced plastic) construction, whilst having a wide market in developed countries particularly for leisure craft, has been held back in some underdeveloped countries for various reasons including high setting up costs, requiring factory air conditioning, dust, humidity, and quality control. The shelf life of materials, availability of materials for carrying out repairs, and high taxes on imported materials, present their own local difficulties Whilst fibreglass boat hulls can be mass produced and the mould costs amortized over a number of craft, for limited numbers and changes in design, it is much less advantageous compared with other boatbuilding materials.
Despite good weight and strength qualities, aluminium alloy requires greater technical knowledge in its construction to prevent exposure to dissimilar metals and great care whilst welding or bending, as well as taking account of alloy fatigue so as not to affect the materials strength. Availability of the correct alloy in many countries will also be a problem. Fishing boats made from aluminium have tended to be built in industrialised countries with the technical back-up to hand.
Steel presents the builder with an easier material to work with: there is plenty of information to consult; the materials are usually readily available worldwide; setting up costs can be fairly nominal; and good construction provides robust boats. However, in all phases of construction, steel needs to be nurtured against corrosion attack from initial storage to final painting stage, particularly in underdeveloped countries where maintenance, if carried out at all, is very often poorly done, resulting in a reduced life span. Good maintenance results in a longer life and a considerable increase in maintenance costs to keep the material in reasonable condition. Furthermore, high initial capital outlay and relatively high technology are required for plant, machines and tools. In the design, the shape of the craft will to an extent be governed by the equipment available to bend and shape the steel to acceptable limits, and the acceptance that generally speaking steel fishing boats would be constructed from 14 m L.O.A. upwards.
Ferrocement is a flexible and durable form of construction. It is easy to repair and possesses many features that help produce a well founded fishing boat. It is particularly suitable for moderate to heavy displacement designs with well rounded sections. However, good supervision in all aspects of construction is desirable, whilst in general the labour can be semi-skilled. Because ferrocement materials are analogous to the building construction industry, material availability worldwide is generally very good and cheap. Despite some poor construction in various places over the past twenty five years, which has hindered the acceptance of the material somewhat, there are enough fishing boats constructed in ferrocement around the world to give substantial claim for ferrocement to be positively accepted in fishing boat building.
Ferrocement is a technical term, not to be confused with ordinary reinforced concrete. It might be defined as a composite material consisting of a matrix made from hydraulic cement mortar and a number of layers of continuous steel mesh reinforcement distributed throughout the matrix. The basic parameters which characterise ferrocement are the specific surface area of reinforcement, the volume fraction of the reinforcement, the surface cover of the mortar over the reinforcement and the relatively high quality of the mortar.
Ferrocement behaves like reinforced concrete in its load bearing characteristics, with the essential difference being that crack development is retarded by the dispersion of the reinforcement in fine form through the mortar. This makes the material of interest in boat construction and it has been established that when cracking takes place it results in a wide distribution of fine cracks which, in combination with the high alcalinity of the cement rich mortar, inhibits corrosion in the reinforcing steel.
The main advantages of ferrocement are low cost, the low level of skills required for hull construction, and reduced maintenance with increased resistance to rot and corrosion when compared to wood and steel.
Claims of low cost can only be substantiated where the second advantage of low level of skills required can be exploited, for example, in developing countries with a large unskilled, underemployed and low paid labour force. In industrial nations, where there is little difference between the cost of skilled and unskilled labour, these advantages are less apparent or even disappear, explaining the relatively slow growth in the use of ferrocement. The advantages in developing countries are more obvious, and this explains FAO's interest in the material.
The main disadvantages of ferrocement are its weight and poor impact resistance. However, these disadvantages only restrict the application, of the material, but need not detract from its potential. In the case of heavy displacement workboats over 11 m LOA, the increased weight is of reduced importance; for such craft over 15 ms, the weight will be no more than that of a similar size steel vessel. Poor impact resistance can be largely overcome at the design stage by provision of appropriate hull protection members.
It may well therefore be asked why ferrocement has not been used more widely? This is due mainly to three factors: bad publicity due to poor amateur and professional construction; publication in the early years of outlandish claims for strength and low cost of construction, which in some cases could not be substantiated; the heavy rise in labour costs in industrialised countries which affected what is generally speaking a labour oriented material. Although today with the range of building techniques for ferrocement expanding, the labour cost factor need not play such an important part.