Wednesday, May 14, 2008

On the stability of small boats, part 1

This is not going to be a detailed discussion on the stability of small boats. I will actually write something for my website and it will be concise and definitive.
My intent here is merely to commit myself to writing this new page for my web site.
What is prompting me to do it is the persistent mention of secondary stability in kayak and canoe magazines. Three things about this concept.
1) All kayakers use the term as in, "Yeah, I tried that boat. It has low initial stability but good secondary stability."
2) Attempts to find a definition of secondary stability on the internet come up with only a few definitions
3) Such definitions as you can find seem to be subjective and conflicting.
So I will tackle this topic as best as I can.
More to come; stay tuned.


Anonymous said...

I consider primary stability to be the tendency of a boat to float completely level. (on calm water of course)

Secondary stability is the boat resistance from rolling over past 90 degree mark on either side, thus keeping you from bobing upside down :-)

I am interested in you insight, into the whole matter.

Anonymous said...

I sure wish i could edit my writting on the previous post. I guess watching euro cup, and typing with one hand isn't condusive to effective writting.

Wolfgang Brinck said...

Even though everyone has a subjective idea what kayak stability means, the ability of a kayak to resist capsizing is still something that can be measured. My reason for griping about the concept of secondary stability is that nobody seems to be willing to define it in terms of standard measurements.
Basically, static stability is plotted as righting torque vs. angle of heel. Kayaks generally generate more righting torque as the boat is heeled over up to a maximum, after which righting torque decreases. Usually righting torque becomes 0 and then turns negative at some angle short of 90 degrees of heel.
So when we look at a plot of righting torque vs. angle of heel, we typically see a curve that looks like a cross section of a hill with a rounded peak.
The question about secondary stability is, what aspect of this plot can we call secondary stability? Is it the maximum righting torque? Is it the angle at which maximum righting torque occurs? Is it the angle at which righting torque becomes 0 and the boat capsizes? Is it the total area under the curve, which is a measure of the total amount of work that is needed to make the boat capsize?
Unless we define secondary stability in terms of some aspect of the static stability curve, we cannot really compare the secondary stability of two boats, something which we should be able to do.
The definition of anonymous tends to favor angle of heel as a measure of secondary stability. The greater the angle of heel at which a boat capsizes, the greater the secondary stability.

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