If secondary stability does not exist
then all slopes of all stability curves are equal when the boat is heeled at 35-45 degrees
I do not think so.
The slope of the stability curve in that range will correlate with the perceived secondary stability from most experienced paddlers.
Also the rate of change for that curve in the range of 30-50 degrees is very important.
Hold any opinion you want, ( I certainly do). I also realize when mine is way outside of general consensus.
I have only read one respected designer with opinions like yours on this issue. There may be a couple of others (ain’t that many designers I really respect) but it’s got to be a small minority.
lee moyers
is his name, I think. He designs kayaks and was a navy boat designer. After scratching my head over this secondary stability feeling stuff, I read a short disertation by several designers on the subject. They all blew it off as myth and then went into detailed descriptions of hull designs and stability issues. It sounded right and correlated with my own experience in leaning kayaks.
Do some hulls hold a lean better than others? Yes. Is that a definition of secondary stability? Why not. Whatever floats your boat, so to speak. For my money, that boat which can be leaned and held signifies a high volume of water being displaced.
that too is incorrect
– Last Updated: Jun-01-04 9:26 AM EST –
I have an 18" wide surfski with excellent (for a surfski) secondary stability (Custom Kayaks Mark 1). I can hold this boat with the seat edge basically underwater, while maintaining a good sweeping low brace. This is quite different than another ski I had that had next to no stability once the hull was leaned. Both are 18" wide, which for most of us is considered fairly low volume, yes? Another good example in the WSBS Thunderbolt, which has nearly no primary stability, but can be held on its side quite nicely by an experienced paddler (it has relatively flat sides, compared to its very round bottom).
the discussion on the mariner site is excellent and i have copied it here (apologies to mariner):
What is initial and secondary stability?
Initial (or primary) stability is how hard the craft resists being tipped from the upright position. If you are looking at a graph of stability (see the XL review's stability graphs or any recent issue of Sea Kayaker magazine) the steepness of the angle off of zero is an indication of the primary stability.
Secondary stability is a lot harder to define. Most experienced kayakers will tell you they know it when they feel it (sort of a seat-of-the-pants thing).
One designer used to claim there is no such thing as secondary stability and any kayak that is more stable initially will be more stable at all angles of lean. I'm not sure he still claims this because I once showed him some stability graphs (Chinook and Puffin in the Winter 1986 issue of Sea Kayaker magazine) where the less initially stable kayak, the Puffin, had not only higher relative stability at higher angles of lean than the Chinook but also a higher maximum stability and a greater total area under the curve). Personally, I define secondary stability subjectively as how secure you feel when you have leaned the kayak well to one side.
A kayak whose maximum stability is five times as high as its stability at 5 degrees of lean will "feel" more secondarily stable than a kayak with a much higher maximum stability that is only three times as stable at maximum as it was at 5 degrees. This defines secondary stability as being somewhat in an inverse relationship to initial stability. I'll make an analogy with rocking back in a rocking chair (low initial stability/higher secondary stability) and compare that to rocking back in a regular four legged chair (high initial stability/high total stability/low feeling of security as you teeter at the balance point). It is hard to lean back on a standard chair (most of my teachers frowned on my practice of balancing my desks on the back legs when I was bored, especially when I would draw their attention to what I was doing when I would almost lose my balance backwards and --in a desperate attempt to recover--crash back down loudly to the "initially" stable position. I don't remember ever going over backwards but that was always the risk I flirted with--a capsize to the rear). Looking at the static stability graphs I would define a kayak with good secondary stability as one whose stability curves show a relatively shallow angle off of zero (so you don't have to put a lot of energy into leaning it) but which has the point of maximum stability (the top of the curve) at a greater angle of lean (and maybe, but not necessarily, at a higher maximum point) than a kayak with less "secondary" stability. (See the Mariner XL, Arluk III, and Solander stability comparisons in the XL review in the Spring 1987 issue of Sea Kayaker magazine).
Even this doesn't totally account for the difference in "feel". A smooth progressive increase in stability out to near the maximum allows a trustworthy "feel". Any abrupt changes would be like putting a speed bump or flat spot on the rocking chair rockers. The above definition is from my own observations and guesses regarding secondary stability in kayaks. I make no claim to scientific validity or even at a valid definition of the term (which may exist somewhere in Naval Architecture). I and most experienced paddlers prefer lower initial and good "secondary" stability. Easy to lean yet secure while leaned.
af
BEG TO DIFFER SOME, STEVE-SALLY IS 5-7
around 145, and I’m 6-0 around 200. She paddles the Isthmus nice and light. I paddle it doing the hula and perfecting my entry form. We both have the same inseam -our legs are the same length…
That extra 5" waist-to-top of head for me, versus Sally, DO make a diff, friend -and when combined with denser mass, makes ME comparatively VERY top heavy. (Sally sez it’s my dense skull, but…)
On a boat like the I, THAT elevated center of mass makes a BIG difference!
And it was IMMEDIATELY noticeable when Greyak fell off the I 1-2-3 boom (as opposed to my 1-2-3…-23-24-25 splash) but where both he AND I ran his QCC 700 through the same stuff and stayed up just fine: we lowered our respective centers of mass relative to the roll axes of the two boats.
Sally, BTW, has not only an improved CM for yakking, she also has a better balance adaptability I do think… She has NEVER swum off the I, in the same stuff I’ve gotten wet in multiple times the same trip…
So: long-waisted/short-legged v. short-waisted/long legged certainly DOES make for a difference, especially in the (more) tender boats! So if a Paddler’s long-torsoed, I suggest they get a yak with a nice, low seat relative to the waterline & roll axis.
Their paddling life will be MUCH nicer, a LOT easier, and probably a WHOLE lot drier as they
Paddle On!
-Frank in Miami