Hull Speed

Hull Speed

This link takes you to a hull speed calculator that can be used to provide input to your first decision: How long should we make the boat?

Longer boats go faster. Or, less effort is required to attain a particular speed in longer boats. That is why rowing shells are so long and why the beautiful Roy Folland double kayak was 21 feet long. The multiple choices are: a) the same length at the waterline as the cheap canoe [12.6 ft], b) as long as practical from two four foot by eight foot sheets of plywood/cardboard [~15.4 ft is 10% faster than choice a], or c) 21 ft [20% faster than choice b!].

Thoughts:

Choice a has some boyancy issues already. It is just too small.
Choice b has general utility and is in keeping with the spirit of the event.
Choice c is over the top. A very large cardboard structure difficult to transport without breaking.

Facts: Choice b should beat every other contestant in speed except for choice c kayaks and rowing skulls.

Opinions: These should not be allowed to participate. Choice b if presented as a cardboard canoe made from two 4 by 8 sheets should be the fastest craft to pass muster with the judges.

Cardboard Canoe: A Pint a Pound the World Around

A simple experiment in design.

"Build a fast flat boat that slides over the water yet does not tip over."

Yogurt cup experiment.

1. Float a small empty yogurt cup in water. I used a large yogurt cup full of water to simulate the ocean.

Notice: The small yogurt cup is stable. If you try to tip it a bit, one side comes out of the water then falls back down to the water. All the support is coming from the part of the yogurt cup that is touching the water.

2. Fill the small yogurt cup halfway with water. Float it again.

Notice: The small cup no longer returns to flat if you tip it!
Notice: The level of the water inside the cup is about the same as the level of the water outside the cup.

Conclusions:

1) Design the boat, so that when it tips, part of the boat away from the low side comes out of the water. It is the this unsupported weight falling back to the water that keeps the boat upright.

2) How low a boat sinks in the water depends on how much weight it is carrying. Each pound of payload will make the boat sink one pint further into the water. The volume of the part of the boat that will be under water will follow this rule: A 300 lb boat will have to push 300 pints of volume below the surface.