Believe it or not, boats exist where the hull consists of cement--and they don't sink! Ferro-cement describes a modern process first started in 1855 by French boat makers. The old process involved creating a steel wire frame, covering it with cement plaster, and letting the plaster harden into a hull. Today's cement boatbuilding process works in similar fashion and provides for the cheapest method of building a large boat. As a result, it is frequently used by amateur boat builders.
- Skill level:
Things you need
- Chicken wire
- Grid wire
- Metal wire
- Wire snippers
- Chop saw
- Cement mixer
- Cement temporary carrier
- Cement boat designs or design book
Utilise plans provided by a boat designer detailing all the framing steps necessary for a boat design. Buy a related book on cement boat design so that you can use the documents while working on the boat. Purchase sufficient wood and supplies to build the frame. Purchase rebar, chicken wire and grid wire to provide a metal frame if desired.
Establish a clear working area big enough to fit the size of boat to be built. Build an initial bracket with wood, nails and hammer large enough to hold the boat frame as it comes into creation. Begin building the boat frame consistent with the boat design plans obtained earlier.
Build a boat hull frame of either wood or metal wire to provide the skeleton of the boat. Cut the rebar to match the steel bar lengths of the design with a chop saw or grinder. Tie the rebar together with metal wire or use nails and brackets to connect the wood frame. Spread the chicken wire across beams and main wires to fill in the space gaps of the hull skeleton. Connect it to the frame with metal wire and snippers.
Purchase sufficient stock of ferro-cement to address the size of boat being built. Mix the ferro-cement as provided on the cement bag instructions with water. Use a man-size cement mixer to produce large amounts of ready wet cement. Begin to trowel the cement onto a temporary board and then to the chicken wire on the frame. Spread the cement from section to section so that the cement forms a layer about an inch thick at least.
Continue trowelling the ferro-cement onto a cement temporary carrier and then onto the hull frame so that all the wiring and frame is covered up by a thick, complete layer. Disregard the need for holes and vents while cementing (these can be drilled into the dry cement later). Move from section to section until the entire hull is cemented per the boat designs. Check various areas to confirm they are an inch thick. Let the cement hull dry.
Contract for a professional cementer or plasterer to apply the final cement layer and smooth out the final surface. Allow the professional to look for and repair any weak areas, cracks, or insufficient layer spots that may exist. Pay the contractor once the work is completed to satisfaction.
Use a grinder to smooth out the cement areas inside the hull after it is completely dry. Use marine paint primer and surface paint to place a protective layer on the cement, especially where it will be exposed to water. Drill all the necessary holes for plumbing, steering, the rudder and ports. Install the necessary equipment into such holes so that all are plugged and sealed once done. Prepare the boat hull for transportation to a boat yard for further internal and above waterline construction.
Tips and warnings
- For a cement boat to float, it needs the water it displaces to equal more in mass weight than the weight of the boat itself. While boat design books will provide this data for you, depending on the size of boat design, you can also calculate it by measuring the weight of everything used to make the boat hull versus the weight of cubic foot water displaced. Remember that salt water weighs different than fresh water.
- Ferro-cement boatbuilding works best for boats larger than 20 feet in length. Small boat designs tend to have insufficient hull strength to manage at sea and can fail at the wrong time, particularly if constructed by an amateur builder for the first time.
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