Any depression or bulge in the developing hull must be leveled.
Building a concrete boat may sound pretty freaky . . . but it works and the job is easier than you might think. The finished craft has some real advantages over conventionally constructed boats too: it won't rust or rot, sharp rocks don't punch holes in it and the vessel just keeps on getting stronger for the next 30 years or so!
What you do is build a wooden frame in the shape of the boat you want, tack on several layers of chicken wire and metal rods . . . and then cement over all your mistakes (you'll have a lot more leeway with this process than if you were working with wood or fiberglass). Once the reinforced or "ferro" cement sets up, you'll have a seaworthy hull that's both dirt-cheap and virtually maintenance-free. And, if you keep the thickness of the troweled-on pour of cement down to less than an inch (which provides plenty of strength), the shell will weigh about the same as a similar hull constructed of wood.
The ferrocement process seems ideally suited for that large boat you never thought you could afford. Pool your spare change and weekends with a few friends and you can build the hull of a 36-foot fishing boat in 700 man-hours for a materials cost of less than a grand! Or bring in a 50-foot work boat hull—with deck and bulkheads—for less than 2,000 man-hours and about $4,000.
This all sounds pretty implausible and it was . . . until the idea of constructing watercraft from concrete was rediscovered in the 1940's by an Italian engineer named Nervi. The concept was later picked up by some New Zealand experimenters and brought to North America by John Samson when he established a ferrocement design and supply business in Canada. Several hundred—if not thousand—such vessels have now been launched or are currently abuilding on this continent.
Almost any watercraft—sloops, ketches, cutters, power cruisers, tugs, trawlers, houseboats, you name it—can be built from ferrocement. If you're boat-wise and already know something about this construction technique, you can probably adapt regular boat plans to the process. Otherwise you may want to buy drawings and instructions tailored specifically for ferrocement.
Yes, it seems certain that ferrocement boats are here to stay . . . still, a few words of caution are in order before you dash off to "pour your own".
Many experts in the field advise against using the process on very small designs—say an 18-foot boat—because the thin-hulled craft that result are not (in the experts' opinion) seaworthy. We know of several people who've constructed exactly such designs, however, and they have no complaint. Then again, they weren't amateurs either.
It should also be noted—even when constructing a bigger ferroconcrete craft—that the final cementing: plays a very critical part in determining the ultimate strength of the vessel. If you can find the cash to hire a professional plasterer to do the job, by all means do so.
The first step is building a temporary platform to use as a foundation for building. This holds the ribs, or "station frames" which will later support and give shape to the concrete hull as it's poured.
The station frames must be cut, plumbed and leveled carefully because the finished hull can only be as true as the skeleton underneath. Two people working full time can bring a ferrocement boat to this point in about one week.
Once the station frames are correctly positioned, wood strips are nailed over the ribs.Slowly, the skeleton begins to resemble a boat's hull. Once the planking is in place, any depressions or bulges must be leveled. It takes around two weeks to reach this point if two people work on the boat full time.
Next, a coating of strong polyethelene is applied to the hull mold. This will keep the cured ferrocement from sticking to the form. Wood "knock outs" will eventually be replaced with portholes.
After the polyethelene, 20-gauge, one-inch mesh (common chicken wire) wire is applied. Four layers of wire mesh is stapled right to the hull planking.
Horizontal metal rods are then stapled in place over the chicken wire. The horizontal rods are spaced two inches apart, and will be covered with vertical rods spaced on six inch centers. A final two layers of wire mesh is then applied.
Now it's time to cement! When the great day arrives, you'll be wise to hire some pros to help you handle that final - and all important - step.
With enough wiling hands, it only takes a few hours to plaster the average ferrocement hull... just make sure the best people are on the trowels!
As the cementing progresses, you must turn your attention to thought of curing the job. Some builders prefer to spread damp blankets over the new shell and keep the covering continuously moistened for 28 days. Others erect a polyethelene "dome" over the fresh hull, insert a steam generator, build the temperature inside to 160 degrees Fahrenheit and maintain the steam-temperature level for 24 hours.
The cured hull is turned upright, and the form removed. The shell is now ready to be finished inside. A good schedule is to pour a hull one summer, fit out its interior the next winter (when shelter is appreciated) and sail away the following spring.