: I went SCUBA diving in 1971 at a (usually) closed lake in Danville
: IL. The park rangers opened it a fw times a year so local divers
: could get familiar with the bottom, just in case they needed to
: call on volunteer divers to recover a body from a drowning
: victim. When i surfaced I was in the middle of the annual
: concrete canoe competition between the University of Illinois
: and Purdue. Danville was about midway between the campuses and
: the colleges took advantage of the one day that the lake was
: available.

: I had never before seen a concrete canoe on the water, but I
: recognized these things immediately. As i swam to shore i did
: some mental calculations. During my previous summer of canoeing
: I had gathered the information from a 17 foot Grumman aluminum
: canoe's ID plate that it had a capacity of 900 pounds. figuring
: the weight of the paddlers at just under 300 pounds I calculated
: that the canoes must be a bit bigger--maybe 18 feet-- and would
: weigh around 600 pounds. At the time I thought they would be
: pretty heavy, but at that weight they would certainly float.

: About a decade ago this topic came up on this board and there were
: links to college teams from (as i recall) Kentucky and Michigan.
: The boat design had evolved to include sealed floatation
: chambers--which in some case doubled as the canoe seats and
: cross bracing, replacing of thwarts. Putting a deck on these
: designs helped with the floatation chambers and structural
: strength. It also allowed them to float lower in the water
: without swamping. about that time some of the colleges were
: making some extremely light boats from modern
: "concrete" mixes. I read here about a concrete kayak
: which weighed under 20 pounds, but i was not able to confirm
: that elsewhere. I think it might be possible to do something
: that would weigh about the same as a rotomolded plastic kayak,
: or roughly 45 to 70 pounds.

: Rob's comments about using fiberglass fabric as a reinforcement,
: rather than a steel mesh, seems like a sound idea. I'd worry
: about the fabric flexing and the concrete flaking off--but that
: can be resolved by using small sections of fabric surrounded by
: stiffer bracing. think of a typical SOF with a space of 6 to 10
: inches between the stringers in some places. Add chines between
: the existing stringers to reduce the unsupported space the
: fabric spans to 3 to 5 inches and you reduce the amount of flex
: the fabric will have. So the solution would be simple--just add
: more chines.

: In the 1980's I got a book from the library on concrete boats.
: Sorry, but i can't remember the title. The author was mostly
: concerned with larger commercial and sport motorboats and sail
: boats, but he included a story of a rowboat he built for a
: summer cabin he had. As i recall the story, he brought 3 bags of
: high strength cement to the beach by his cabin and mixed this
: with sand from the beach. The mold for the boat was made from a
: pile of beach sand (think about the sand castles built on the
: beach and imagine an ugly one which looks like an upside down
: rowboat). Over this mold he put 3 or 4 layers of metal screen,
: tying the layers together with twist ties of iron/steel wire.
: When he had the form completed he used a squeegee and plastered
: the frame with his concrete mix. The build took a day, and he
: kept the plastered shell on the form, watering it for a week so
: the concrete would cure to its proper strength. After that he
: flipped it over, fitted seats to the inside and oarlocks. At the
: end of the summer he sank the boat so the concrete would simply
: stay wet and unfrozen all winter. the next few years he would
: refloat the boat and enjoy it.

: I think the low cost and minimal upkeep would make concrete canoes
: and rowboats interesting camp projects--but I can see a lot of
: concern for safety issues.

: PGJ

: There is a product called gypsum concrete, lighter that conventional concrete that is used in renovations where they want add radiant heat in the floors of an older building, this might be worth a try? Kevin