How to Make a Concrete Boat Float
Archimedes' Principle says that, for a boat to float, the water displaced by the boat has to weight more than the boat. This means that a concrete boat that weighs three tons has to displace more than three tons of a water. To figure out the amount of water your boat needs to displace, apply some simple multiplication and division, as well as some facts about building a concrete boat; then start building.
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Things You'll Need
- Calculator
- Ferro-cement boat plans
- Rebar
- Chop saw
- Wire ties
- Wire mesh
- Tin snips
- Cement
- Hand grinder
- Marine epoxy primer
- Marine epoxy paint
Instructions
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Buy a plan book for ferro-cement boats. While your local library may have books on the subject, you'll need to have a plan book of your own that you can write or draw in. Your local bookstore may have---or be able to order---a ferro-cement boat plan book.
Another source for plan books is Hartley and Brooks (hartley-boats.com/). Members of the Society of Naval Architects and Marine Engineers, the firm is known for its ferro-cement boat designs.
Concrete boats are called "ferro-cement" boats due to their construction from cement---concrete without rock or other aggregates---that's laid up over wire mesh and rebar made from a ferrous (iron-based) metal, such as steel.
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Add up the weight of all the material called out in the boat plan you select, plus the weight of paint and primer, steering and power systems and any creature comforts you propose to add. Divide that total by the number of cubic feet (length times width times molded depth) in your boat to determine the boat's weight per cubic foot.
Since you may eventually use your boat in water of any density, build it for the water least likely to support it. Fresh water weighs 62.4 lbs. per cubic foot. This means that each cubic foot of your boat must weigh less than 62.4 lbs..
To determine how much of your boat will be above the water (called its "freeboard"), divide this result by 62.4. The result will represent the percentage of your boat's hull that will sit below the water line. If your boat's weighs 30 lbs. per cubic foot, divide 30 by 62.4. The result, 48.07, means that about 48 percent of your hull will sit below the waterline.
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Cut the steel rebar with the chop saw to the lengths called for in the boat building plan. Assemble the steel rebar matrix with wire ties according to the plan.
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Cut the wire mesh with tin snips to fit over the rebar. Attach the wire mesh to the rebar with wire ties.
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Mix the cement according to the manufacturer's directions, and "lay up" the cement on the rebar/wire mesh matrix by hand to the thickness specified by the boat plan. (Applying the cement by hand helps prevent voids when it hardens.)
Level the surface with a hand grinder once the cement has set completely, taking care not to "thin out" the thickness of the hull. Seal the concrete, inside and out, with marine epoxy primer and paint.
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Add the steering and power systems, seats and other "comfort features."
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Tips & Warnings
Plans for any boat will include such information as the length, beam (width) and molded depth---the depth of the finished boat, allowing for the curvature of the hull between the sides and keel (the backbone of the boat).
Ferro-cement boats, like any boat where the hull acts as its own supporting frame, face a danger of cracking if driven aground or onto rocks or a reef.
This project involves the use of both hand and power tools in cutting, chopping and grinding a variety of materials. It also involves the use of epoxy paints and a variety of construction materials. Wear protective gear such as goggles, gloves, long sleeves and long pants, and observe appropriate safety principles.
