Ferrocementbåde by Hans C. Bredahl

 

A little known, but definitely not dull, area of application for Ferro cement is boat building. This is despite the fact that cement boats have been built for the past 100 years in sizes ranging from the smallest canoes and kayaks to large motorised torpedo boats. If you study the subject of Ferro cement, sooner or later you will come across the names of Joseph Luis Lambot and Peir Luigi Nervi. These two; Lambot is French and Nervi Italian, created the foundation for modern Ferro cement boat building techniques. Lambot built a rowing boat in Ferro cement, or as he called it, ”ferciment”, in 1848. We know that that boat was used extensively for at least 50 years. After disappearing for many years, it was found by chance at the bottom of a lake in the vicinity of Paris in 1955, still intact and seaworthy after more than 100 years. Nervi developed Lambot’s ideas in the 1940’s. He called the new material Ferro cement. Through attempts and experiments with thin plates reinforced with different types of nets, he discovered that the result was a material with remarkable elasticity and strength, well suited to boat building. It is Nervi’s boats and their successors, especially in New Zealand, Australia and Canada, who we can thank for the development of the modern Ferro cement technique.

 

Regardless of which building method or combination of methods one uses to build a Ferro cement boat, the end result should always be a hull that fulfills the following requirements:

1. Smooth, neat lines in accordance with the model and drawing.
2. Regular thickness of the hull with all the reinforcement covered by a 1-2mm thick layer of cement mortar.
3. Armature weight between 400 and 600 kg/m hull material.
4. Equal weight distribution in the armature with a high level of form stability, completely filled with high quality cement mortar.

 

Despite many directly contradictory perceptions amongst Ferro cement constructors and builders about how the reinforcement and molding of a hull should be carried out, the uniformity of the material and the composition of the finished hull are fundamental. If the finished result is a hull, which fulfills the requirements of the above definition, then all is well.
A Ferro cement boat can be built in two months, either upright without a wooden frame or in a wooden frame, where the boat lies upside down. The armature, which consists of reinforced steel with several layers of chicken wire, mesh wire or similar, is formed so that it follows exactly the shape the boat needs to have. Painstaking accuracy is required for the building up of the armature, because a skew or bowed armature cannot be straightened after molding. The wire armature is bound or welded together and thereby attains sufficient stiffness and accurate dimensions.

 

The mortar for the molding must be of good quality. We recommend Low alkaline cement that is mixed in the a proportion 1:2 with pure sand with the largest grain size between 2-3mm dependant on the dimensions of the armature and with at least 8-10 weight percent sand less than 0,125 mm. The water/cement ratio must be around 0,35 and not over 0,40 to ensure that it is strong and watertight. If a super-plastifying additive is used then the water content can be reduced by 20-25%. The mortar can be applied either by hand or with a spray. Vibration can assist in ensuring that the mortar is distributed evenly so that the wire armature is filled completely. After 28 days of hardening the internal buttressing can be removed and the adaptation can commence. The boat owner can now begin to see that he is the owner of a boat. A carefully executed cement boat will, after the final surface treatment with e.g. epoxy lacquer, look just as beautiful as a ship built out of steel, aluminium or fiberglass in a shipyard. The stability on the water is excellent because of the low point of gravity and weight distribution, which is similar to that of a boat built out of wood in an old fashioned way. The molding of the hull is often done communally. It is therefore not unusual, when a molding party is called, that 30-40 men and women meet up to participate in the molding, which should preferably take place in one go, “wet in wet”.

In Denmark, Ferro cement boats are built mostly by people without any prior knowledge of the subject, and the construction time for a boat can vary between about 6 months to 4-5 years.
In the Danish waters and also on the World’s seas, there are several Danish-built Ferro cement boats sailing today, and many more are under construction all around the country.