Re: [Paddlewise] Stability

This is a response to an earlier post but that is the drawback of getting 
the digest.

Nick wrote;


> Lets take a really weird example: A box shaped hull with two fully
> submerged pontoons well down and out to the side.


I really good example of what happens below the surface. Which also shows up 
in Nick's "V" bottom and  Box shapes. However, it is the change at the 
surface that matters and if the hull shape is such that a change occurs at 
the surface then you get the differences I got in my examples.

Robert wrote;

> But for it to be a scientific concept it needs some kind of definition 
> that
> people can agree on.

How true. If one states that shape has no effect then it must have no 
effect. If it can bge shwn that it does have an effect then you have a 
different ball game and need to restate the theory. Maybe to, " Of the 
factors affecting initia stability CG and beam have the greatest effect and 
waterplane area and underwater shape have lesser effects.

Of course, I subscribe to Karl Poppers definition of a scientific theory. If 
a peson does not agree with that then we won't have much of a discussion. 
:-)


 > It is my contention that kayaks are VERY different from normal craft 
because
> the user shifts his/her weight constantly and this represents a 
> substantial
> part of the weight of the craft. Stability curves are based on the concept
> of the paddler sitting rigidly in the boat as it heels which does not 
> happen
> in real life.

Actually the idea of shifting weights is not unique to kayaks. Naval 
architects have to deal with shifting weights in their designs as well. when 
designing for shifts in cargo (like fliuids) and the effects of damageand 
sometimes these are rapid and severe. Kayaks are unique, however, in that 
the shifts are intentional and, apparently,  metaphysical in nature :-)

You can, with a little effort calculate the stability at any angle of heel 
with any shift in paddler CG. I don't do it much anymore because I think 
("THINK" is the operative word. I certainly don't know) that I can tell what 
my target paddler would like in stability characteristics.

> My contention is that the square cross-section kayak feels initially more
> stable than the round kayak because there is much more dynamic resistance 
> to
> the boat rotating around its long axis. It is NOT because of its stability
> curve which at 0-2 degrees might be identical to a boat with a rounded 
> hull.

That is certainly one reason. There is a difference between roll resistance 
and stability, however. The one measures the buoyant forces vs gravitational 
forces and the other measures the resistance to rotational motion. The two 
are interrelated but are often treated as separate. You can calculate the 
roll resistance (period) but I don't know if anyone does in the kayak world. 
(Having said that I just know some one will tell me how they not only do 
cross curves of stbility for every boat but roll period studies as well for 
nboth male and female paddlers)

Robert later wrote;


> Therefore, I would state that there is no commercial kayak out there with
> any arbitrary waterline width (X) that has a "primary stability" greater
> than any other commercial kayak with a waterline width of (X+1). And that 
> is
> irrespective of shape of the cross-section or the waterplane.

OK, so if I find one you will retract? :-)

The issue here was not one of degree but whether shape has any impact on 
initial stability. It's a nit but then science (and paddlers) picks at nits 
all the time.

> As the debate is currently framed with an actual definition of primary
> stability out there, reviews that talk about "poor" or "weak" primary
> stability are a little weird in that they say no more than one can 
> basically
> judge by knowing the waterline width. Sea Kayaker reviews contain the
> numerical data so why would the reviewer be making these statements about
> the primary stability at all?

Good point. As I mentioned earlier the hard part is fitting the stability 
characteristics to the paddlers perceptions. However, once you know how a 
particular paddler responds to a boat with specific stability 
characteristics you have a good idea how to design the boat. In this case, 
however.the discussion started about effects not degree of effects. When I 
used numbers it is to show that an effect exists. If we shift the discussion 
to the importance of degrees of effect we have real problems because people 
react differently. I would contend that people can compare the stability 
curve to their experience and draw useful conclusions. I can't prove that 
but from a practical viewpoint designers do it all the time even if it is 
unconcisious. I design my boats based on paddler responses to prototypes and 
comparing that to the stability curves and then make alterations to suit. I 
know of at least two happy customers so one can't say the method is a 
complete failure. :-)

Others may use a different method with equal, better or maybe even worse 
results.

Cheers

John Winters
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