: concrete is essential as it is for a concrete kyak contest...
: i just recieved the dimensions: 500 cm give or take 3 cm and max
: width 80 cm give or take 3 cm.
: im assuming my biggest problem is figuring the thickness

Thickness may not be a relevant issue.

There is probably no requirement that the hull be of a uniform thickness, so you can start with very thin material and increase the size and strength in areas where such reinforcing is required.

Is weight a factor? Some concrete canoe contests involve a race of a certain distance. if the paddling time is adjusted for the weight, or if additional points are awarded for having a light boat, then you will need to consider light aggregates, such as polystyrene beads instead of gravel, and very thin reinforcing wires, such as fencing material for containing chickens.

Get a good look at a skin on frame kayak design from Yostwerks.com or klepper.com. From this you should be able to see that the stringers-- the long pieces of metal or wood which run the length of the kayak --are secured by frames or bulkheads. You can make such frames from bent 3/8" rebar, and use more rebar as the stringers. Cover this with several layers of wire mesh, tie the mesh together to keep it thin, and plaster the mesh with your concrete, working inside and out to keep the thickness as thin as possible.

your concrete may be fairly porous. several coats of exterior house paint will seal it. Don't try to increase ater resistance with a thicker or heavier application of concrete.

make some test panels (12" by 12" is a good size) from your wire reinforcement and your concrete. Weigh them and use that figure to determine the expected weight of your boat.

This will also give you some idea of how thin and light you can make these things.

Ballpark estimate is that the boat should weigh well under 300 pounds. Some weigh under 50 pounds. Add to that the weight of the paddler or paddlers. Determine the sidewall height of the boat by figuring the displacement. If we are talking 2 college kids weighing about 180 pounds each, and a 250 pound boat, the displacement must be greater than 610 pounds. that would be a minimum displacement of roughly 10 cubic feet.

Qick and dirty way to estimate displacement: Assume the hull of the kayak is a long diamond. The area of the diamond is half the length times the width. So with a 5m long boat (roughly 16 feet long) with a 80 cm width (roughly 2.5 feet). you have half of 2.5 x 16, or 20 square feet. If you submerged that area 6 inches (half a foot) into water it would displace 10 cubic feet of water. So, you will need to make the sides of the boat at least 6 inches high, but preferable a couple inches more--say 8 inches. Dome the deck a bit so that it stays above the water and the total height of a 250 pound boat will be under 12 inches high.

Most concrete kayak contests I have read about specify that the kayaks have enough built-in floatation compartments (either filled with foam, or sealed air chambers) to support the boat when it is fully filled with water. That means you would want to have at least 5 or 6 cubic feet of floatation to support the boat. Don't worry too much about supporting the paddlers weight should the boat flood. The paddlers will float in whatever water is in the boat, and effectively weigh maybe 30-50 pounds each!

most of the strength of the kayak is found in the gunwale and keel areas. make those strong and you can basically fill in the rest with lighter material. If you have 1000 pounds weight on that 20-square-foot kayak bottom, that is just 50 pounds per square foot or just a bit more that 0.35 psi which the concrete needs to support.The paint film on the outside will be strong enough to support that tiny pressure.

Good luck with your project. According to the last book i read on the subject, concrete boats have been popular for decades in Australia and New Zealand. Not so popular in the US

The exception would be those concrete canoes and kayaks built by college engineering students.

Hope this helps

PGJ