Re: [Paddlewise] Group Paddling - folders stability

-(SNIP)

>I appreciate all the input on this subject, especially from the boat
>designers.  I need to go back and reread the posts about the flooding
>question, because I always assumed that the sponson would increase edge
>buoyancy and thus righting moment, beyond that contributed by hull shape,
>when flooded.  I reasoned that, since there was water surrounding the
bottom
>and sides of the chamber, the chamber is displacing water toward the
center
>and ends of the craft.  Should this not add buoyancy and righting moment
to
>the edge of the craft, or does the entrapped air know its inside the boat
and
>refuse to do its duty?  I would love to see a clear analysis of this.
Matt,
>John, guys?  Inquiring minds of limited reasoning (mine) need to know.

I hope this helps.

First, consider a boat heeling say 10 degrees. As it heels the center of
buoyancy (CB) shifts outboard. If the Center of Gravity (CG) remains
constant and the CB lies outside a line drawn vertically through the CG a
righting moment results that acts to  bring the boat back to level.

Now, take the paddler out and replace his weight with water (Ignore for the
time being the relative heights of the center of gravity) . As the boat
heels the water flows downhill seeking its own level and shifting the
center of gravity thus reducing or even wiping out the righting moment
created by the shifting center of buoyancy and the boat now has less
stability

Now, add internal buoyancy tanks down the side. If the center of gravity
remains fixed as in the first example the righting moment will equal the
righting moment of the first example regardless of the presence of the
tanks since the tanks do not affect the shift in the CB.

Now replace the paddler with water of the same weight and heel the boat.
This time the water cannot flow as far downhill and seeks its own level
closer to the centerline of the boat. If the tanks encompass enough volume,
the CB will lie outside the new center of gravity providing a positive
righting moment. Of course, the tanks might also reduce the amount of water
to enter the boat with a further positive benefit.

Now let us repeat the example but this time we will use a bulkhead running
fore and aft at the same location as the air tanks. The bulkhead though
will have 3/8" holes in it. As the boat heels the water will flow more
slowly through the holes than if unconstrained and the boat will actually
have more stability even though there are no sealed tanks.

Now we will replace the water with ball bearings equal in weight to our
paddler or water. This time when the boat heels the ball bearings are held
in a similar position to the water in the boat with tanks. The result is a
boat with the same stability as a tanked boat.

By this experiment we can see that the stability effect of tanks has to do
with its restriction of water flow in the boat . The material of the boat
or tanks matters not so long as it does its job in restricting the flow of
water to the down side of the boat. Anti roll tanks on ships use the
principle of the holed bulkhead to reduce roll in ships.

Now, imagine another boat. with no water in it that has "x"righting
moments.
The righting moment is the product of the righting arm (distance for the
vertical projection of the CG and the CB. If we add water and it sloshes to
the low side we get both a shift in weight and the effect of the shift
(inertia). This we call free surface effect. The farther the water has to
travel the greater its inertia and so we can see how restricting its travel
reduces the capsizing moment of water in the boat.

I have simplified this enormously but hope it helps. Entire books devoted
to stability and damage stability in particular have been written.


Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



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