Hull Design
by Joshua P. Keatley, 1998
These are just a few short notes on the design of a canoe hull. You can apply them to any style of construction. A little further down the page you can find some notes on the design procedures used at Queen's for our hulls.
The waterline of a hull is just what it sounds like - the line along the hull where water meets air. This is a major reference point for several fancy calculations which aren't covered here yet. The shape of the waterline and its length will help to determine the performance of the canoe. In general, a longer, slimmer hull will be faster than a short, wide hull of the same displacement. This has to do with the laws of hydrodynamics, mostly with drag and wavemaking.
Rocker is the departure of the ends of the hull from a flat plane at the lowest point of the boat. Imagine a canoe hull sitting in a parking lot. Rocker would be the distance of the ends of the hull to the pavement. A hull with lots of rocker looks like a banana. In general, more rocker means more manoeuverability, but at the expense of directional stability. In other words, it is easy to turn a boat with lots of rocker, but not so easy to keep it going in a straight line. Many whitewater canoes have extreme rocker.
The cross-section of a canoe hull usually looks like a wide U. The bottom is more or less flat, the sides are more or less vertical, and the space in between is where people and cargo fit. If the slope of the sides changes near the top, or gunwales, the boat may have either a flare or tumblehome shape. Flare means the top edge bends out, which is good for keeping water out, but means you have to lean over farther to paddle, and tumblehome means it bends in, which makes it easier to paddle.
Thwarts are braces that cross the open top of the canoe hull, adding stiffness and structural support to the hull. They can be made out of some other material than the hull, usually, wood or aluminum.
Unlike fibreglass and thermoplastic hull materials, concrete is a brittle construction material with very low tensile strength. This means that the structural system must be carefully designed to keep tensile stresses in the hull below the level that cause cracking. In regular concrete construction, cracks are acceptable, and can even indicate that the reinforcement is doing its job, however in boat construction cracks are not your friend! They let water in, which tends to make you sink. For this reason, a good understanding of the strength of the materials used as well as the expected stresses in the hull are necessary. The stress patterns in the hull can be controlled by changing the shape and the loading in the hull, and with luck, a non-cracking hull can be built.
The performance of a hull can be anticipated by using rules developed by naval architects and by using computer models and towing tank models, but until the whole thing is actually built, it can be difficult to say for certain if the design is good. In concrete canoe building this is a major problem, since concrete can take the better part of a month to harden to a useful strength. Testing tends to be done right before a race.
This is a work in progress, and reflects the state of knowledge of the writer, who is mostly self taught in these boat-like topics.