Re: [Paddlewise] Stability Discussion

I agree with much of what you have written.
Some points that I would emphasize perhaps even more than you have

#1: The seat height is SO critical to the appearance of the stability curves
that it is easy to game the system. For many people there is a tradeoff
between seat height and comfort. Many stiff old types such as myself, find
higher seats more comfortable because we are not very limber. If a
manufacturer makes a kayak with a very low seat then his stability curves
will look MUCH better. His seat may be so low that everyone who actually
uses that kayak adds some padding to the seat. Another manufacturer may
offer essentially the same boat with a comfortable seat that does not
require building up the seat. His boat will look a lot less stable. I have
always thought that SK should do the curves from the bottom of the boat and
just add an explanation about the status of the seat. As it is, IMO, those
curves are just an elaborate way to tell you how wide the boat is and how
high the seat is.

#2: You give a definition of initial stability and dance around giving a
definition of secondary stability. (curve under area to the max height
point). If you accept these definitions, it is VERY hard to find a boat that
has "poor" initial stability but "good" secondary stability. Except for some
extreme cases (sponsons above the waterline) NO kayak will fall into this
category. And yet, many people seem to like the idea of having a kayak with
these characteristics and it is very common in the reviews to read reviewers
talking about "poor" initial but excellent secondary stability. The great
thing about there being no mathematical definition of secondary stability is
that these claims can be made without fear of contradiction.

#3: Knowing three numbers (waterline width, width of seam line, and height
of the seat) you can very closely "predict" the stability curves. I venture
to guess that there is no commercial boat with a waterline width of 22
inches that has more initial stability than a commercial boat with a
waterline width of 23 inches assuming that the seat height is the same.

#4: By your definitions there is no magic out there. People seem to want to
believe that some clever underwater design can make a skinny boat more
stable. Using your definitions, I think not. At least, the influence will be
minor. But see the below.

#5: You make a good point about how the flare influences your definition of
secondary stability and it does. However, the initial slope (initial
stability) also has a great influence. Therefore in the "real" world, where
there is not all that much difference in flare between the common kayaks,
secondary stability can be pretty accurately estimated by only looking at
seat height and waterline width and not even bother with seam width. I have
long thought that I should just go through all the old Sea Kayaker magazines
and plot your definitions of initial and secondary stability against only
TWO variables (seat height and waterline width) and see if there were any
real "outlyers". I suspect there are very few.

#6: I have a little trouble believing that the paddler particularly
perceives just starting down the slope after the maximal height has been
reached. Throughout this region, even after you have slipped "down" away you
are getting a good righting force. I am not sure that you are particularly
concerned or appreciate that that force is a little less than it was a few
degrees ago.

The above addresses refinements of what you have written.

-------------
Having said all the above, I think that there is an important issue that
your do not address. You have neglected something. This is that the paddler
changes his position in the boat by bending at the waist. Therefore the CG
moves and moves rapidly. This has nothing to do with bracing etc. Beginners
often do it TOO much. They almost "vibrate" back and forth as they feel the
boat tipping under them.

If you get a chance to make a corrective lean, you will feel comfortable.
Imagine the two copies of the same kayak. One has a daggerboard that hangs
down three feet. Imagine that the daggerboard is very thin from side to side
so it has no real influence on the stability curves. (Actually what
influence it has will be to make the boat less stable using the traditional
stability curves) That boat with the daggerboard will be perceived as being
much more stable than the other boat. It resists, by dynamic forces on the
water, rapid tipping. As the kayak starts to tip, the paddler starts leaning
the other way to compensate. He will hardly even know he is doing it. That
kayaker can take pictures etc from his boat without worry. The daggerboard
acts like a built in "brace". With a big enough daggerboard, a beginner
would feel comfortable in an 18" wide boat.

Now, of course, kayaks do not have daggerboards though I suspect even a skeg
could be perceived. But they do have different underwater shapes. A
completely rounded shape does not push any water around if that kayak is
rapidly tipped. A square shape, however, does. A square cross-section
resists rapid tilting and thus gives the paddler a chance to react with
leaning the body (or if more experienced) with setting up a paddle brace. I
believe that this factor is very important in the perception of how stable a
given kayak is. This is NOT captured in the stability curves. IMO, chined
boats feel more stable than a rounded hull with exactly the same stability
curve.

This is certainly true in calm water. How it plays out in rough water is
more complicated because those shape can give waves and currents something
grab.

As a kid, I used to paddle logs around. Round logs 20" wide were tricky.
Flat planks 18" wide were no problem. (I had a low CG in those days) These
were simple shapes. You could spin that round log in the water around its
long axis with no problem. There would barely be a ripple. The plank would
splash and kick up a huge wave system if you tried to spin it the same way.
If these logs are virtually submerged, they don't really have ANY stability
by the traditional curves and yet the rider can easily tell the difference
between the two.

Robert Livingston
rlivingston_at_mac.com
livingston_at_post.harvard.edu





----- Original Message -----
From: "Nick Schade" <schade_at_guillemot-kayaks.com>
To: <paddlewise_at_paddlewise.net>
Sent: Thursday, February 15, 2001 7:34 AM
Subject: [Paddlewise] Stability Discussion


> I've put together a discussion on stability and would like to get a
> critique. Check
> http://www.guillemot-kayaks.com/Design/StabilityArticle.html and let
> me know what you think.
> Nick
>
> --
> Nick Schade
> Guillemot Kayaks
> 824 Thompson St
> Glastonbury, CT 06033
> (860) 659-8847
>
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