# Center Of Floatation Versus Centre Of Gravity

Can someone explain to me what the difference is or perhaps more importantly, how the \$%#* do you calculate it?

the center of flotation  is more commonly called the center of bouyancy (CB)

it is the geometric center  (balance pont) of the submerged volume (or displaced volume) of water.

The CB is the point where the sum of all volumetric moments (below the waterline) = 0

(v1*r1 +v2*r2 +v3*r3 ...=0)

the center of gravity (CG) is the point where the sum of all mass moments =0

or balance point for the complete object

(m1*r1 + m2*r2 + m3*r3...=0)

a practical explanation may go something like this:

hang any object from a string and the cg will be some point directly below.

so if you draw a vertical line below the attachment point it will pass through CG.

repeat by hanging  the same object from another point and draw a vertical line below that point .

the cg will be the intersection of the two lines.

if you toss the same object in water  and pull it down from some attachment point, the CB will be directly above (assuming it floats).

similarly, a vertical line drawn above the attachment point will pass through CB

repeat the process again  with the same force and from a different attachment point , you will have another vertical line passing through CB.

the intersection of the two lines will locate CB

oops, on edit:

the last statement:

"the intersection of the two lines will locate CB"

is mostly BS, since the submerged volume can quickly change shape as the force to submerge the volume ( ballast, rider, wave, etc) changes location.

I guess that's the whole point of comparing the relative location of CB to CG for stability estimates

this may help, but i'm sure there are lots more.

http://newboatbuilders.com/docs/stability.pdf

a quick google search for  "stability in ships/boats",  or "stability cg vs cb"  may have a much better explanation

-bill

Hi,

maybe my head is a little slow today (itâ€™s monday after all), but for a completely submerged surfboard (shortboard when paddling) the center of gravity/mass and center of bouyancy would be the same? CAD software would be my best bet for caluclating the center of gravity.

Some relavent similarities between yacht performance and surfboards ie.resistance to heeling(righting moment).......lateral movement of the boards centre of buoyancy away from the centre of gravity...........transverse moment of inertia                  it kinda goes out the window , because a mast can't think like a surfer or walk ( although someone would be workin on it !)   .... and  surfboard design continues to become more planing and less displacement...............the energy that drives a surfboard comes from underneath the  water and yacht power comes from above the water.............split the diference and its easy to see why windsurfing is such a high-tech advanced water sport.........now I'm confused.......if you balance your board on a tea-cup on the bench, this will show exactly where your centre of gravity is............should also be the biggest volume of flotation (foam) so should be your centre of buoyancy as well......................tip of the ice-burg ! considering the effects of different bottom contours....tail suction or release edge , soft rail , tucked rail, concave etc etc................good shapers and designers are worth thier wieght in gold bullion!!!!..............................................I wish I was one........my father had a saying.." wish in one hand and sh%t in the other , and see which one fills up first".......................

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Can someone explain to me what the difference is or perhaps more importantly, how the \$%#* do you calculate it?

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Here is a good description: http://en.wikipedia.org/wiki/Metacentric_height

It looks like it is mainly used to determine the stability of ships.  How would you use this measure in surfboard design?