February 18, 2010 at 05:08 #33988Bryan HanselParticipant
Steve Killing has come up with a good standard for comparing the stability of canoes and kayaks. It would be helpful for canoe and kayak designers to include it in the next update.
We wanted to give our customers a reference for the stability of our boats. Our designer Steve Killing came up with the stability factor: the technical description is at the bottom of this page, but the real value is in comparing with other boats. Numbers range from about 70 to 120 – the larger the value, the greater the sideways stability. Many novice kayakers find boats below 95 to be just a bit skitterish, around 100 to be average and over 105 to be very stable. Your experience level will determine the stability you need. The Venture although it may appear low at 72 is for smaller people and because their percentage of body weight to boat weight is low and their center of gravity is low, the boat is comfortably stable.
These stability factors are calculated for solo kayaks at a common displacement of 250 lbs unless otherwise noted above. In the case of the True North this is slightly below the normal capacity, but provides a better comparison. The values are the metacentric height at 15 degrees of heel with 7.50″ taken as 100 percent for kayaks
We were striving for an easy way for paddlers to compare the stability of various canoes and kayaks before they start building, or before they buy a canoe. I now use this for all canoes and kayaks that I design, including BlueWater and Bear Mountain designs.
No single number will tell the whole story, but I think this number has guided many people to the correct canoe and avoided big surprises on launch day.
The calculation is done on the computer defined shape. I load the canoe to 400 lbs for a tandem and 250 lbs for a single and then heel it to 15 degrees. The software maintains the correct displacement as the boat heels. Once in this attitude the stability is related to the distance from the center of gravity (which I set at the waterline for all canoes as a base point) and the metacenter. The metacenter is the point (usually about 12″ above the waterline) where the vertical line through the center of buoyancy intersects the centerline. The higher it is the greater the stability.
The determination of the metacenter is not easy without a computer program and to do a physical test on an existing boat in the water is very difficult, so it is easiest to have the designer determine these numbers.
Once the metacenter at 15 degrees has been determined, the stability factor is simply the metacentric height in inches divided by 12.24 (for kayaks use 7.50) times 100.
A stability factor of 111 is comfortable for all novices – the Freedom 17’9
A stability of 100 is comfortable for most paddlers – the Bob Special is at 100
Some novices will feel a boat at 98 to be initially tender, but after a few paddles will be quite comfortable – the Freedom 17
The Huron Cruiser at 92 is not comfortable for most novices
Hiawatha at 88 is a challenging boat for some canoeists to paddle.
The marathon at 49 is well beyond my skill level.
February 18, 2010 at 12:11 #33989MarvenKeymaster
What you’re describing here (the distance between the transverse center of buoyancy and the center of gravity located at the keel) is already available in the extensions and is called KN Sin(phi)where K is the VCG at the keel and N is the N is the intersection of the center line and a vertical line through the center of buoyancy. In order to obtain the righting lever you have to extract VCG*Sin(phi). This is an exact measure of stability, but after that they fiddled with the numbers. Why is not very clear to me, because dividing a measurement by a constant value makes no sense when comparing two canoes of the same type if the KN*Sin(phi) of the second canoe is divided by the same constant. The ratios remain exactly the same, only the numbers will be smaller.
Comparing the righting levers is a good indication but not enough I’d say. Since the KN*Sin(phi) is mainly determined by the shape of the canoe you’re only comparing the shapes. Two canoes of exactly the same shape but different weights (lets say an ultra light canoe and a very heavy one) give the same KN*sin(phi) and therefore the same “stability number”. Better would be to include the weight of the canoe, which results not in a righting lever but a righting moment if you multiple the righting lever by the displacement. The resulting number are still easy to compare but by including the weight in the equation two similar shaped canoes of different weights would yield different stability numbers.
Of course you could also multiply the stability number calculated according to the method of Steve Killing with the weight of the canoe.
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