Re: [Paddlewise] Stability

At 05:08 PM 5/12/2005 -0300, John Winters wrote:
>.
>John wrote;
>
>(SNIP)
>
>>Doesn't the "shape of the water plane" account for the differences you're 
>>seeing?  Maybe I'm missing what "shape of the water plane" means.
>
>I believe Nick used the same waterplane shpae for all his tests as did I.

Okay.  I assume that means that the tests were done on flatwater (i.e. no 
waves or current).  If that variable remains constant then what Nick is 
suggesting is that the height of the center of gravity is the primary cause 
for the differences in initial stability.  In other words, changing the 
height of the seat will have a greater impact than the shape of the 
hull.  Having built a couple of kayaks and carved seats for them I know 
that even shaving off a 1/2" makes a noticeable difference.  Similarly, 
those that add a foam pad to a composite or plastic seat are going to 
notice a difference in primary stability.



>(SNIP)
>
>>Having paddled lots of different boats with different hull shapes I've 
>>noticed a different "feel" in the initial stability in different.  Boats 
>>with very flat hull feel very resistant to tipping.  Boats with a shallow 
>>V have a gentle rocking feel. Boats with a deeper V start to feel 
>>"twitchy", Rounded hull boats feel "slippery" (sort of like sitting on a 
>>basketball). That's not very scientific but that's just how different 
>>shaped hulls *feel* to me.
>
>Hard to say what causes what when comparing different boats since it may 
>be more than just the shape that has changed. In Nick's examples and mine 
>we tried to keep all variables constant except the shape.
>
>"Feel" of course is the key word. This is why I think the stability curves 
>provide useful information since the shape of the curve seems to play a 
>major role in how the boat feels. I think Matt mentioned that in his  post.

When plotting the stability curves are the seat height and paddler height 
considered as variables?


Getting back to Matts post there was one thing I wanted to add regarding 
boats with very high initial stability.  A few years ago a local paddler 
let me try out the boat he had just built (one of Nick's designs, 
actually).  He hadn't finished the outfitting on it so the seat consisted 
of a thin foam sleeping pad.  It was so stable that when I tried edging it 
with just a j-lean I couldn't get it tilted over enough to get a good low 
brace turn.  I had to really lean over the side with my shoulders to get 
the boat fully edged.  I thought then that "what if I need to edge the boat 
in the opposite direction quickly?"  In a boat with lower initial stability 
I can do a subtle cock of the hips to edge the boat and it responds.  If I 
want to edge the boat in the opposite directly all I have to do is cock my 
hips the other way a little and keep my upper body stationary.  In the boat 
with very high initial stability I had to exert much more effort (and 
weight) to tilt the hull to one side.   If I want to tilt to the other side 
I've got to cock my hips more and move my weight from one side to the other.

For someone just learning to kayak they may not have developed the muscle 
control to make the subtle weight shifts so a boat with high initial 
stability so it's going to be more forgiving if they lean a bit too 
far.  To the more experiences paddler that has developed a good sense of 
balance and muscle control that high initial stability will be seen more as 
unresponsive.  Even though I've been paddling for 8 years or so I still 
come across boat that I feel are necessarily initially unstable.  I've 
thinking of one boat designed by a Brit whose last name is not 
"Dennis".  It wasn't too bad if I was moving but I never felt that I could 
relax if I was sitting still.  Of course, he can stand up on one leg in the 
cockpit, so maybe I still have a lot to learn.

John Fereira
jaf30_at_cornell.edu
Ithaca, NY
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