Re: [Paddlewise] chine and stability

Jed wrote:
<< Flatter bottomed or harder chined boats seem to
have a very clear righting effect as they are leaned slightly.  The force to
lean them further rises sharply as the angle of lean increases.  All this
boils down to the perception of stability for most novices. Also most people
understand this as primary or initial stability. >>

Matt responded:
<<Here you are confusing width below the waterline with chine. Chine is the
snip      This contradicts Jed's assertion that hard chines are more 
initially stable. 

Jed's response:
    Matt, I made an observation based on personal experience relative to the 
"feel" of one design feature versus another. Although I will never know as 
much about boat design as you have already forgotten, I do understand the 
definition of chine. My reference was to "harder chined boats" as if 
comparing two boats of similar design but with different chine features. I 
don't know if the chine is responsible for my perceptions or if it was other 
design issues that were packaged along with the hard chines, but my 
perceptions are what they are just the same.
    In my limited experience, harder chined boats exhibit stronger initial 
stability than similarly designed and dimensioned boats with softer chines. 
All things being equal, which of course they never can be.

Matt wrote:
<< snip . . . . . flared sides will have the greatest secondary stability by 
just about
anybody's perception or "feel". This contradicts what Jed just said about
the more rounded hull having the better secondary stability. >>

Jed's response:
    I certainly agree with your statement regarding the relative secondary 
stability of a flared hull, but that was not the issue. The original poster 
asked about chines only. My actual statement regarding secondary stability 
follows:
    "snip . . . . A softer chined boat may seem to 

have less initial stability but the secondary stability will rise smoothly up 

the point of capsize. . . . . snip . . . . . A hard chined boat will seem 

to have greater initial stability but reaches it's point of capsize more 

abruptly and with little notice."

    Again I do not attribute the effects relative to stability to the chines 
alone but rather to the boats that have hard chines versus boats that have 
softer chines. Certainly I see now, thanks to you post, that the degree of 
flare has the greatest effect on secondary stability. I do however maintain 
my perception that boats with harder chines tend to have a "notchy" feel when 
they are leaned aggressively. And that boats with softer chines tend to feel 
less "notchy".

Matt wrote:
<<I want to be clear here that I'm not trying to claim
the opposite of what Jed wrote just pointing out that the chine is not the
determining factor in primary or secondary stability (but it can sure help
dynamic stability compared to a rounded hull by decreasing the sideways skid
on the face of a steep wave--as can a V-bottomed shape). >>

Jed asks:
    Matt, is it not possible to back away from the forest a bit and make some 
generalizations about the current commercial offerings of boats with hard 
chines versus boats with softer chines? Given that we are addressing this to 
a novice curious about researching her first boat.
    I understand that your post seeks to correct inaccuracies from my post, 
and I welcome the correction. But please remember that I was referring to 
complete boats not the chines themselves.

Jed wrote:
<< Softer chined hulls tend to lack this strong initial stability but
instead offer a smoother transition as you move from an even keel to tilting
the boat.>>

Matt wrote:
<< How would one show or detect a smoother transition? What should I look for
on a static stability graph? All the graphs I've looked at seem to be
relatively smooth. >>

Jed responds:
    One could easily determine the rate of change of the righting force 
relative to the change in attitude. This then could be compared relative to 
similar stability data from other designs. The combined data then would allow 
a reasonable person to make a judgment about one design having a relatively 
smooth or less smooth transition of forces as the boat is tilted off an even 
keel. Smooth or less smooth in this case would refer to the relative 
steepness of the stability curves.

Jed wrote:
<< Past 10°'s or so of lean takes us into the realm of secondary stability >>

Matt wrote:
<< Has this been agreed upon somewhere and I've missed it all these years?
Please let me know your source or sources for this. >>

Jed responds:
    John Winters, in his essay on "Stability and Seaworthiness" offer a 
definition for a metacentric height. As part of that definition he refers to 
initial stability as stability at small angles of heel. He later refers to a 
small angle of heel as usually less than ten degrees.
    I used tranductive reasoning to (incorrectly) arrive at the definition of 
Initial stability as being limited to angles of heel less than 10°. I 
apologize to all involved for my incorrect interpretation of Mr Winter's 
writings. Certainly there is some range of heel that we can agree to refer to 
as the realm of initial stability and another range that we can agree to 
refer to as in the realm of secondary stability. Please tell me what these 
ranges are so I will not misspeak in the future.

Jed


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