Concrete Bottom Rethought

I haven't studied concrete to any great extent—up until now. It is ubiquitous, and is one of the most ancient and well-understood construction materials, right after mud brick and plaster. It has a bad reputation as a boatbuilding material because of all the failed ferrocement projects, but that's not concrete, that's cement plaster over wire mesh. I was planning to do something different: create a steel-reinforced concrete slab for the bottom. And so I delved into the details on engineering concrete slabs, and came up with an answer that didn't please me at all.

Concrete has excellent compressive strength, and unreinforced concrete blocks can be stacked miles high before the bottom-most blocks gets crushed. But its strength under tension is more or less nonexistent, and to avoid placing it under tension ancient builders had a rule that the compressive force has to be concentrated within the middle third of a column. Modern builders work around this problem by making concrete into a composite, by embedding a rebar cage or mesh in a concrete slab, with enough thickness on either side so that when, under load, the armature stretches, the slab bends hardly at all. Because, it did bend, cracks would instantly open up on the convex side, letting in moisture, causing the rebar to corrode, expand, and cause “spalling” (meaning the concrete structure falls apart). What's more, this is bound to happen eventually in any case, and so reinfoced concrete slabs are engieered for eventual failure by being over-reinforced and under-cemented, because then they give warning of impending disaster in the form of cracks, as opposed to failing catastrophically.

Neither “eventual failure” nor “failing catastrophically” sounded good to me, and so I set out to calculate the required concrete slab thickness for QUIDNON's bottom, and came up with 6 inches. That translates to 16 tons of weight, not counting the rebar, the sides, and all the other structures I wanted to cast into the bottom. With all of that, the weight would push 20 tons of ballast, and that's just too much.

Also keep in mind that nobody has ever tried to join a concrete bottom to a plywood top, so I would be doing something rather experimental, if not to say adventurous. And adventurousness is, to me, akin to incompetence: I like my engineering tasks to be as boring as possible. The fun part comes after I do my boring engineering work, build it, and hand it over to other people to try to destroy. And find that they can't without trying really really hard. In general, there are two approaches to solving engineering problem: look it up (best) and guess the answer (not as good). In this case, I would only know that I guessed right if I manage to sail QUIDNON in all sorts of conditions and observe that nothing catastrophic happens, so I'd rather adhere to the much safer “look it up” strategy.

And so I decided to backtrack, and make the bottom out of plywood and fiberglass. The boat still needs ballast. There will be 5.8 tons of water ballast, which is good, but most of it is forward of the center-line. It needs to be balanced by about as much ballast aft. It works out to a 1-foot-thick slab of reinforced concrete located aft of the centerboard trunks, between the trunks and the aft cabins, under the galley, the heads and the companionway ladder. It will incorporate a rebar cage, located about 2 inches up from the bottom of the slab. That's because this concrete slab will serve as the mast step for the mainmast, taking a compression load from it, which will stretch the rebar at the bottom while compressing the concrete at the top. Here it is, shown in purple:

In addition to providing a counterbalance to the water ballast and serving as a mast step, the concrete slab will provide thermal mass. I will pour it over a few layers of dry fiberglass cloth encapsulated in plastic, to thermally insulated it from the hull and from the seawater below, and I will provide a couple of air conduits through it. One of them will be used for the exhaust of a rocket stove, to heat it up; the other will be used as part of the interior ventilation system, to keep the cabin warm. I will cover the rocket stove design in a future post.

As for the foremast step, that will just be a fat stick of wood spanning the width of the hull. I'll fiberglass the bottom of the stick, to take the tension load, so that the load on the stick itself is purely compressive.

As far as joining the bottom to the sides, I intend to follow the procedure that Chris Morejohn used on HOGFISH and his other designs: screw, glue and tape. I don't have his drawings with me, but from memory it looks something like this:

The edges of the plywood are screwed together, the joint is saturated with thickened epoxy with a high-strength adhesive filler, then fiberglass tape is applied over the joint, and then the procedure is repeated, screwing and gluing each additional layer of plywood until the right thickness is reached. Then the whole structure gets covered with fiberglass mat, which is nailed down using bronze annular nails, and saturated with epoxy. Then three layers of fiberglass cloth are applied over that. Then the topsides are made smooth using fairing compound, primed and painted. In the case of QUIDNON, the bottom will receive a layer of copper cladding, so that it never needs painting.

"What a boring design!" you might say. But that's how I like it. The fun part will be in seeing how it performs in big waves and lots of wind.