Re: [Paddlewise] stability curves--Matt

From: "Jolie Smilowicz" <jolie_at_rockisland.com>

> Am I the only dumb bunny who can't figure out how to read those damn stability
> curves in SK?

No.  I wouldn't be surprised if a majority of SK readers find them of limited value.

Without going into a comprehensive study of the graphs, allow me to offer how I 
look at them and make a few comments.  I will advise you that there may be 
exceptions to the generalizations I make.

The graphs show two sets of curves, one for a 150lb paddler and one for a 200lb
paddler.   Both sets are for paddler alone and with 100lb of cargo.

If your weight is not one of these, mentally interpolate or extrapolate the curve
to what would seem to be yours.  Don't worry about being exact.  I don't think 
that these curves are useful for absolute values, but are useful in comparing 
two different kayaks, say one you are familiar with and one you may wish to 
purchase.  Remember that the upper loads may be irrelevant for a kayak
designed for a small paddler - they may excede the design loads for the 
kayak (where design load means useful paddler range, not ultimate strength).

The reason that they are of limited value on an absolute scale is that they 
represent a rigid paddler - one who doesn't flex at the waist when the
kayak heels.  Any real person with a modicum of experience will keep
their torso upright as the kayak rolls.

The first thing to consider is the curve near the origin, where the angle of heel
is near zero.  The curve will either rise or fall from this point.  A falling curve
(negative righting moment) represents a kayak with no initial stability.  A deep
V hull can exhibit this behavior.  It will flop to the side immediately unless the 
paddler actively balances it.  This is not a beginner's kayak.  

A slowly increasing curve represents a kayak with low initial stability.  A 
roughly circular hull can show this, as do some moderately Ved hulls.  
This is not a bad thing, but takes getting used to.  If the curve rises quickly, 
the kayak has high initial stability.  A beginner will consider this to feel 
comfortable, since the kayak isn't tippy when you first enter it.

The next point to look at is the topmost part of the curve.  Two things to 
consider:  how high is it and how far to the right?  The height reflects how
solid the kayak feels on edge and the position to the right indicates how
far over you heel the kayak before the resistance really kicks in.  Now 
since the curve reflects the rigid paddler, don't put absolute faith in the 
numbers.  If you like a kayak with high secondary stability, you want more 
height.  If you want to be able to set on edge and cruise comfortably, look 
for a peak further to the right.  There's probably such a thing as too far to
the right, but I'm not sure what that is.  I haven't seen any examples in SK
that make me scratch my head.

A high peak to the _left_ would leave you with a kayak that is stable, but difficult
to get on edge.  This is a consideration when you want to really push the limit
on edged or carved turns.  My Solstice GTHV is like that.  I have to really lift
to get it on edge; many Solstice paddlers I know rarely bother.  My Ellesmere
is the other way.  It has little initial stability and its secondary is firm at a high
angle of heel.  Hence: a bit tippy in calm water and very comfortable on edge
or in rough water.

Consider as well that a kayak with high stability that is difficult to roll is less 
stable in rough seas and can flip on a breaking wave quite readily.  It will be 
less comfortable on beam waves - rolling with every one.  You want a balance 
between stability and the ability to keep the kayak upright in a beam sea.  
Also, if you do go over, rolling the kayak upright will be more difficult and is 
a liability.  I have paddled wide stable kayaks that are difficult to push to 
the limit, narrow tippy kayaks that are a pain to keep up and some nice
solid narrow kayaks that have both good stability and good edging 
characteristics.   The Ellesmere, Pintail and Romany Explorer are examples 
of the latter.  The Slipstream (for a guy my size) is too tippy.  The Solstice GT
for smaller paddlers are in the first category.  

If you compare the curves on a single kayak you will notice the following trends.
Increasing the cargo weight increases the stability considerably.  This is 
because the added weight sinks the kayak and moves the center of gravity (CG) 
lower while not lowering the center of bouyancy (I think if you overload the kayak
significantly (i.e. up the tumblehome or over the gunwale), this breaks down 
- I stand to be corrected).  If you increase the weight of the paddler, you reduce 
stability.  This is because bigger paddlers tend to have their weight up high 
and the CG is raised.

We've discussed these curves before and the general feeling seems to be that
if the curves could account for a real, moving paddler, they would be more 
useful.  I think that is true, but we can get some info out of the existing curves.

Any comments on my comments would be appreciated.

Mike





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