Our goals were to achieve a better finish on the hull and reduce the overall weight. Last year we encountered difficulty in working the concrete through the mesh, creating gaps over the surface. This year we provided a wire mesh that allowed the concrete to pass completely through the mesh, providing a smooth overall finish. To reduce the weight of the canoe we reduced the freeboard on a fifteen foot length starting at the aft. This was done by securing a 2" strip of plywood to the inside of the mold. The reduced weight is estimated to be 140 pounds. A female mold should theoretically produce a more uniform finish on the outside of the hull that will require less sanding and patching after the canoe is removed from the mold.
The female mold has a 8 inch square grid mapped inside it. Holes were drilled at the intersecting points on the grid. These are used to tie the reinforcing to the mold. The tie wire used is 20 gauge. To accurately position the primary reinforcement in the mold, one- eighth inch spacers were cut from one of our basic concrete mix design test cylinders. Reinforcement for the gunwale was then secured to the female mold using the one-eighth inch spacers and the 20 gauge tie wire. To provide extra lateral support, ceiling grid wire was secured to the wire mesh laterally every two lee;. This was done by using 20 gauge tie wire. Reinforcement for the thwart was made of the secondary reinforcement material. The thwart was formed using a wooden jig that was placed at the widest point of the canoe. The ceiling grid was used to reinforce the thwart. First the wire was wrapped around the inside of the canoe laterally. It should be noted that the ceiling grid wire was placed on top of the wire mesh and secured using the 20 gauge tie wire. A turnbuckle was used to create tension in these wires. The turnbuckle was placed in the middle of the thwart and was spaced off the bottom of the thwart mold. Separate pieces of ceiling grid wire were strung through the thwart form and the ends were spread out diagonally at a length of about 4 feet. The secondary reinforcement was tied into place with 20 gauge tie wire during the casting process.
The concrete was mixed in batches containing 20
lbs. of portland type I cement. All concrete was placed using a
standard masons trowel A rubber mallet was used on the outside of
the mold to help consolidate the concrete.
Last years canoe was very thick. so controlling the thickness was a priority. In order to ensure that the concrete was of uniform thickness., a trowel was used to screed the concrete to a depth such that the lateral ceiling grid wire was barely exposed. The result of this procedure helped to control the thickness of the canoe, thus reducing the overall weight of our canoe.
The boat was cured by draping it in moistened
burlap and then covering it in plastic to hold the moisture in.
The moistened burlap was left in the canoe for 28 days. Water was
added each day for the first 7 days to keep the canoe moist
during the initial curing.
When removed from the mold, we found that the finish of the outside of the hull was indeed of higher quality than last years boat. Some minor defects in the outside surface were filled with grout. Grout was used to fill in the air voids that were present on the surface. This was smoothed out using a rubber trowel. The partially dried grout was then sanded down to a smooth finish. The high spots on the inside of the canoe were sanded down using finishing stones. At this point, the canoe was ready for painting. Last year we considered placing the boat in a muriatic acid wash to reduce alkalinity of the concrete, but decided that this was probably not necessary. This year, however, we decided to use the acid. Two coats of black exterior semi-gloss latex paint were applied. This was done instead of using the Velspar Block Primer, which was used last year.
Click here for pictures of construction.
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