Re: [Paddlewise] chine and stability

On Wed 15 Nov 2000, 735769 wrote:

	snip

> However, most people will try to resist the capsize and the righting force
> obtained from a brace or body shift combines with the righting arm of the
> hull to act towards righting the boat. The greater the righting force past
> the maximum
> righting moment the less force one needs to exert to return the boat to an
> upright position.  Because the righting force involves both the boat and
> paddler we need to look at the system rather than the individual components.

So if I'm designing a boat for an individual that is going to definately 
expect to catch most instability events with a strong brace (even at extreme
angles), then I want to design in a smoother roll off of the righting force, 
so as to provide an easier catch for this specific set of individuals.  I 
have always been a little shy of understanding why an expert paddler wants 
such a damn tippy boat.  Now I think I understand, or at least I've found 
another piece of the puzzle.

The best explaination of "why a tippy boat?", has always been that they are 
actually less tippy when negotiating step waves.  But I never completed believed
this, and presumed even with solid athleticism you would be better off with 
(given relatively the same shape) a 22in+ boat than a 20-21.5in 18ft boat.
Now I see an important part of the trade-off is easier catch versus less 
stability.

So if I'm right about this the chine discussion has helped to identify some
of the strengths of the "tippy boat" for me, but I'm still at quite a loss for
understanding why you would want a hard chined boat.  It has been said that
the hard chines do a better job of resisting skid.  I would add that for this
skid resistance there is a price paid in reduced stability.

As a hard chined boat like the looksha (only hard chined boat I've paddled),
is leaned, you reach something of a discontinuity in the righting force curve
and at that point you sort of flop on to a side plane of the boat.  So if a
missed stroke and blast of wind or water combine to cause a rotational moment
towards capsize, in the critical first few hundred milliseconds of time (and
before you execute your brace), a smooth chined boat will be steadily applying
more righting force which will serve to decelerate the rotational velocity and
reduce rotational momentum...
...whereas a hard chined boat would hit that discontinuity in stability which
I spoke of.  At that point the righting force would more or less vanish and the
decelerating force would vanish along with it.  Since in a soft chined boat the
decelerating force would not vanish, it can be said, that relatively speaking,
the hard chined boat will act to apply an accelerating force towards, you guessed
it, capsize.  That doesn't sound like a good trade-off to me.  Even those damn
tippy boats wouldn't do that.

This is all arm chair postulating from a nonexpert, so take it with a grain of
salt.  But for now, this is my perspective.

-- 

Mike McNally		mmcnally3_at_prodigy.net

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