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Dry testing of kayaks
Nothing can replace the testing of kayaks on water. Only then can you get a feel for how the kayak works for you. But there is a limit to how many kayaks you can test before you buy or make one that shall meet your needs. Most people do not get to wet testing, they rely on the vendor's recommendations. Tjelmeland Kayak has developed a program for testing essential characteristics of a kayak.
Outline of the method
Graphs and tables produced by the software are not given here, but can be found where specific kayak models are mentioned.
Mathematical form for the hull
For our own kayak models we use a mathematical description using typically a dozen parameters. Aside from practicabilities when it comes to producing drawings and dxf-files, the big advantage is that it easily enables systematic testing of hulls by varying parameters. Then we can calculate the hull that gives the lowest resistance (i.e. the highest speed potential) for a given stability or the hull that gives the highest stability for a given resistance. Usually length and width are constraints. Optimal hulls are conditional on the weight of paddler, load and kayak. The fastest kayak for an untrained beginner is not the fastest kayak for a strong paddler.
For other kayaks we sometimes can fit the hull within the model, but usually we fit the model locally for each lateral section and interpolate in the length direction. Then the hull can be tested, but we cannot optimize the hull further. For this we make a full model as close to the hull as possible. This has been done for some kayaks, and indeed existing hulls can be improved.
For calculations of stability and resistance the kayak is divided into sections, typically 300-400 depending on the length. Each section is divided into typically 200-300 points, resulting in a total number of hull points of typically 20000 or more. The hull points are used to build the hull using triangles, which is the fundamental unit when stability calculations are carried out.
Mathematical form for the paddler(s)
The dry tests rely on a realistic placement of the point of gravity for kayak and paddler, and for some tests also for cargo and/or ballast. The point of gravity for the kayak is simply calculated from the hull model. The point of gravity for the paddler is calculated from a human model. Two different paddlers are used, one large of about 80 kg and 185 cm and one small of about 45 kg and 160 cm. A realistic human form is also needed in order to calculate realistic moments of inertia when dynamic tests are carried out. Such tests include the behaviour of the kayak in waves from the side to investigate capsizing issues.
Testing stability
The kayak model is first tilted a given angle around the horizontal longitudinal line through the gravity point. For a given height of the water line in relation to the kayak the bouyancy (volume of hull beneath the water line) is calculated. The water line is systematically varied until the bouyancy equals the total weight. Then the vertical force on each hull triangle is calculated and the torque around the gravity point is calculated and summed over triangles. This gives the righting moment for the given tilting angle. This procedure is repeated for a range of tilting angles.
Testing resistance
One should bear in mind that a kayak that is fast for one person is not necessarily fast for another person. The resistance of the kayak to forward motion can be divided in two: surface drag and resistance due to the kayak making waves. Surface drag is proportional to the wet surface and is the dominant resistance at low speed. As the speed gets higher, the importance of wave-generating resistance becomes greater. The kayak with the smallest wet surface is not necessariy the kayak with the least wave-generating resistance. So, what is the fastest kayak depends on your paddling ability. The program Flotilla developed by Leo Lazauskas at the University of Adelaide is used. This program has had a wide range of uses, including design of olympic sprint kayaks. The program is a standalone DOS version which is started repeatedly from our software, each time varying design parameters for the hull in a systematic way to find the hull with the least resistance, given length and stability constraints.
Flotilla requires the hull offsets to be defined at regular intervals along the kayak. The internal division of sections and offsets is not used directly, rather a resolution better suited Flotilla of fewer sections and points have been used. Experiments have been performed to ensure accuracy is not lost.
Unfortunately, Lazauskas' web site cyberiad has been hacked and displays wrong information, so the best we could find in way of documentation is INLINK http://www.boatdesign.net/forums/design-software/flotilla-6-2-released-50116.html here OUTLINK.
Testing wave stability
Stability in waves (danger of capsizing) is experimental yet.