Re: [Paddlewise] Stability

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

--[____________]----  < waterline
  /              \
O                O

Stability occurs because the CB moves as a boat tips. This is due to  
the lifing side of the boat losing buoyancy, and the sinking side  
gaining. The effect of the pontoons on the stability curve will not  
be evident until one of them comes out of the water because until  
that point the pontoons have little relative effect on the CB. The  
primary factor changing the buoyancy will be the box at the surface  
of the water. At some large angle a pontoon will lift free of the  
surface. At that point its effect will be very noticeable. The  
buoyancy contributions of the pontoons puts the center of buoyancy of  
the pontoons halfway between the two pontoons.

Now imagine moving the pontoons somehow:

--[____________]----  < waterline
   \            /
    O          O

Notice that the buoyancy of the boat does not change. The CB of the  
pontoons does not change so their contribution to the overall CB does  
not change. The only time you will notice any change is when one of  
the pontoons comes out of the water, at some large angle of heel.  
Moving the pontoons in or out will only effect the angle of heel at  
which they emerge from the water.

Placing the deeper will have more effect:

   --[____________]----  < waterline
    /              \
   /                \
  /                  \
O                    O


Lowering the pontoons lowers the overall CB of the boat. This can  
make the CG move out beyond the CB more quickly, but at small angles,  
the difference will be small.
Nick


On May 12, 2005, at 3:07 PM, John Fereira wrote:

> At 07:22 AM 5/12/2005, 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?
>>
>
> Doesn't the "shape of the water plane" account for the differences  
> you're seeing?  Maybe I'm missing what "shape of the water plane"  
> means.
>
> I also assume that "height of center gravity" is in relation to  
> center of buoyancy (yes, I've read Nicks excellent stability  
> article).  Would the center of buoyancy change for differently  
> shaped bottoms?   Having paddled lots of different boats with  
> different hull shapes I've noticed a different "feel" in the  
> initial stability in different.  Boats with very flat hull feel  
> very resistant to tipping.  Boats with a shallow V have a gentle  
> rocking feel. Boats with a deeper V start to feel "twitchy",   
> Rounded hull boats feel "slippery" (sort of like sitting on a  
> basketball).  That's not very scientific but that's just how  
> different shaped hulls *feel* to me.
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