Re: [Paddlewise] Stability

On May 12, 2005, at 7:22 AM, John Winters wrote:

> Nick wrote;
>
>
>>  Primary stability has almost nothing to do with the cross sectional
>> shape of a boat. Chines, no chines, makes no difference. It is the
>> shape of the water plane and the height of the center of gravity that
>> will determine initial stability.
>>
>
> Hmmm. Out of curiosity I drew two boats with the same displacement,  
> same CG and same waterline area and shape. One had a "V" bottom and  
> one a flat bottom. The flat bottom boat had a righting arm at 5  
> degrees 38% greater than the "V" bottom boat. That seems like a lot  
> of effect. Granted this is an extreme example.
>
> Maybe what Nick means is that, given two similarly shaped boats,   
> having a chine or no chine does not produce a significant  
> difference. Am I reading you wrong on this, Nick?

I based my statement on several things. One being a similar analysis  
I did on 5 different cross sections each with the same displacement,  
waterplane shape and CG height: http://www.guillemot-kayaks.com/ 
Design/ConstWL.gif. It is also based on the standard naval  
architecture definition of Initial/primary stability which is the  
slope of the stability curve at zero degrees of heel. 5 degrees is  
much greater than zero

My "V" bottom sample has plumb sides and a chine below the waterline.  
As soon as the chine on one side leaves the water (around 10 deg in  
my example), the stability starts to fall off rapidly. A pure "V"  
bottom with chines or eventually a sheer above the waterline and the  
same displacement, waterplane shape and CG height will have the same  
initial stability by this definition, however the slope won't be  
maintained for very long, so 5 degrees is enough for a big change.
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