Ferrocement, a composite of metal and concrete, is one of our favored building materials. Because the strength to weight ratio is not as good as steel, ferrocement structures end up being heavier and slower, but that’s fine for our needs – in fact, that’s just the direction we want to compromise. So I was interested to stumble across a free online book: Fishing Boat Construction: 3. Building a Ferrocement Fishing Boat, from the UN FAO Fisheries and Aquaculture Department.
For example, it contains a section on various materials for building boats in the 3rd world:
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.
Note that while ferrocement boats are currently made using a laborious hand-application process, the techniques of the Monolithic Dome Institute demonstrate that ferrocement construction can be automated to some degree with inflatable forms and shotcrete (spray concrete), suggesting future economies of scale.