Concrete slab bottom

Ballast is what allows sailboats to carry sail, by keeping their masts more or less vertical in spite of the forces generated by the sails. Keelboats carry ballast in their keels, and the usual high-end material for keel ballast is lead, which is 17 times denser than water. Lead is a grey, weak, toxic metal. Some sailboat builders cast an entire keel out of lead with some bolts sticking out the top, and bolt it onto the underside of the boat with a backing plate and some nuts. Treating lead as a structural material always seemed like an odd choice to me. On such boats, when the keel snaps off and sinks the rest of the boat floats upside-down, trapping the crew, who then spend however long it takes for rescue to arrive, soaking in salt water polluted with diesel fuel and battery acid.

Such prospects never appealed to me, so when circumstances forced me to buy a production boat (a yacht, if you will) I opted for one that had an integral fiberglass shell and a steel slug in the keel for ballast. At least the keel isn't made of extra-heavy putty and won't snap off. An additional feature of lead-keeled boats is that the keel has a tendency to gradually disappear. When immersed in salt water, lead gradually turns into lead oxide, which is a light, fluffy material that easily washes away. Although some lead keels are covered with a fiberglass skin, some are simply painted, and many a marine surveyor, tapping around the keel, has seen his hammer go right through the paint and the fluffy white powder underneath. This is all very profitable for the recreational boating industry, I am sure.

On my previous boat, HOGFISH, the ballast consisted of lead bricks which the builder cast into precise shapes that fit between the frames under the cabin sole. They were certainly very effective as ballast, and since the bilge was usually dry, they did not deteriorate at all over a 30-year period. But with QUIDNON the choice of ballast is completely open (as is the quantity of it, which will be determined experimentally using a model). But supposing I used lead ballast, and used, say, 10,000 lbs of it, at about $2/lb for reclaimed led shot (which then has to be cast, generating toxic fumes in the process) that would be $20,000. So if the construction budget is $50,000, 40% would be spent on ballast? No, thank you!

Lead makes good ballast because it is compact, but QUIDNON, which will have around 3000 cubic feet of interior space, is not exactly cramped, and so using lead ballast because it is more compact than other options would be a false efficiency. Steel scrap embedded in concrete would work just as well, and many a backyard boatbuilder and cost-conscious ocean navigator has used it with great success.

But there is another consideration: one way of saving money is to make sure that each element in the design does more than one job, and as many jobs as possible. So, for instance, on QUIDNON the boom gallows will not only hold up the booms, but will also provide ventilation (they will be hollow and have openings for air), hold sheet blocks, provide attachment points for a deck awning, support an engine hoist and maybe even house some electronics.

With ballast, the situation is similar: why just use it as ballast when it can do other things as well; such as, for instance, form the entire bottom and bow of the boat—out of fiber-reinforced concrete. Fiber-reinforced concrete is portland cement with sharp sand as aggregate (something like 1:3 ratio, details TBD) with short-strand glass and polymer fibers mixed in. The result is a very tough substance, with good abrasion resistance, very resistant to cracking. It can be sealed with a penetrating epoxy sealer to be completely waterproof. The hull will still be classifiable as a fiberglass (FRP, or fiber-reinforced plastic) hull, but its core will be concrete below the waterline and plywood above.

In case you are thinking that this will make QUIDNON a ferrocement boat, it will not. Ferrocement boats were popular at one time, because of the extremely low cost of materials. They were constructed by making a “basket” out of steel rods and mesh held together by staples, which was then “plastered” by using a very dry cement mix. The cement has to be carefully skimmed to just barely hide the basket, or the end result is too heavy. What's worse, the entire plastering operation has to be done at one go, in a single marathon plastering session, so that the cement sets as a unit. This is too much know-how for most backyard builders, and so results varied from OK to awful. Nobody builds ferrocement hulls any more.

What I intend to do is pour the entire bottom and bow of the boat as a slab, using a mold. A cement truck will deliver a load of the right mixture, with the right ratio of cement to sand, and the fibers already added. This will be poured into the awaiting mold, which will already hold a structure made of rebar. I intend to be generous with the rebar, since it won't add much to the cost. The pour will be done upside-down, since the bottom of the slab will be very easy to smooth by hand, whereas the top of it will be very detailed, and include the following features (which would otherwise have to be built up out of other, more expensive materials):

1. Mast steps
2. Bottom half of the water tanks
3. Foundations for fuel tanks
4. Bottom half of battery compartments
5. Bottom half of the chain locker
6. Foundation for sewage holding tank
7. Chases for plumbing pipes
8. Chases for electrical cables
9. Ventilation ducts to draw in cool, water-chilled air for solar-powered air-conditioning
10. Drainage paths for condensation (a.k.a. “limbers”)
11. Compartment to hold the bilge pump
12. Foundation for cabin sole (floor)
13. Foundations for bulkheads and partitions
14. Lexan window for an in-hull depth sounder
15. Apertures for through-hulls (raw water in, sewage out for sanitation lines)
16. Aperture and foundation for the engine well
17. Bottom attachment points for the rails that hold the engine bracket
18. Shower sump and a well for its pump
19. Sockets for the rudder posts
20. Foundations for the centerboard trunks ...and, last but not least...
21. All-around lap joint with pre-cast perforations to hold the bolts which will make the concrete core of the bottom and the plywood core of the topsides and deck into a single, integral piece.

All of these intricate details will be drafted out using CAD, rendered in particle board using an NC mill, and then snapped or screwed together to make the mold, which will be back-filled with sand to make it hold the weight.

Once the concrete is poured, smoothed along the top and sets, it will be “hydrated” over a period of some months by keeping it covered with burlap and sprinkled with water. During the hydration period the concrete works up to full strength and stops absorbing moisture. The concrete slab will then be completely dried out and sealed with penetrating epoxy concrete sealer, making it impermeable to water. Then a fiberglass skin will be applied to its exterior surface, in effect making the concrete act as the core of a fiberglass boat.

Finally, the entire bottom surface will be covered with copper plate. This will keep it free from fouling by marine organisms for the life of the boat, so that it will never require painting. The plate will be attached to the concrete using self-tapping stainless steel screws. I do not plan to plate the sides below the waterline, because there the plating would deteriorate much more quickly and require periodic replacement. Instead, I will keep them painted with ablative paint, and scrape and paint them periodically when the boat is drying out at low tide. In this way, QUIDNON will never need to be hauled out, and will never need to have its bottom painted, eliminating a very large category of expense.

Finally, the slab will have a cage built around it, and flipped over using a crane. After that, the topsides and the rest of the hull will be built up, by bolting plywood pieces along the lap joint that will run just above the design waterline. Once the hull is built, the lap joint will be covered over with fiberglass along with the rest of the hull, faired to make it smooth and painted.

This approach achieves cost savings in the following categories:

1. Eliminates the need to form the bottom using several layers of plywood screwed and epoxied together, saving a lot of time and a lot of materials.
2. Eliminates the need to separately build in ballast: the amount of ballast is simply “dialed in” by adjusting the thickness of the poured slab.
3. Eliminates the need to separately form out of (expensive) plywood and glass in the 21 features incorporated into the bottom, which are listed above (and there will probably be more). The disposable plywood used to build the mold is only $2 for a 4x8 sheet.
4. Eliminate the need to ever haul out the boat and repaint its bottom.

In addition, this approach eliminates the problem of pounding. On certain points of sail, and sporadically whenever the sea state is boisterous or there are cross-seas running, square hulls have a tendency to pound. If the bottom were made of plywood (as it was with HOGFISH), the pounding would produce a resounding base drum-like sound that would reverberate throughout the boat. This is not dangerous, but it is not all too pleasant and interferes with sound sleep. With the bottom and bow made of a concrete slab, however, the pounding will be virtually unnoticeable. It will be like a rock hitting the water, and the only sound will be a loud splash.

Finally, the concrete slab surfaced with copper sheet would make QUIDNON go aground extremely well. Copper is quite tough, and, as Dave Zeiger's Triloboats have proven, is tough enough even when backed with just plywood, but is especially tough when backed with a slab of concrete. Thus, QUIDNON should have no difficulty with going aground and floating off again.