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Home Science Physics Physics buoy

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Physics/buoy

Expert: Steve Johnson - 3/22/2009

Question

dear sir johnson

i want to know about total forces on a floating structure or buoy.i

want to calculate these forces (like wind,wave, current....). please

guide me.

best regard

Get the answer below

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Answer

Hello ameneh,

That would be very much related to the shape of the structure or buoy. First consider a boat at anchor as your floating

structure. Boats are shaped to minimize drag forces -- some more than others. These forces are minimized for

cigarette racing boats to a high degree, an oil tanker -- not so much. When at anchor, the efforts that minimize the

drag experienced under power would minimize forces in the same way when the boat is anchored. The boat would

naturally swing to face the current or wind. If there is both wind and current, then it will find the best compromise.

If the structure or buoy floats just barely above the surface, wind would not affect it much, but current would. If the

structure is light enough that it displaces very little water, the current wouldn't get much of a hold on it but wind

could. So, this would be hard to calculate.

I can see ways to measure the force. If you put a spring in the cable to the harbor bottom, you could measure the

stretch. And from that determine the force F. The horizontal force on the structure would be according to

Horizontal force on structure = F*cos(theta)

where theta is the angle between the cable and horizontal.

Another possibility would be to measure how much the angle of the cable pulls the structure down farther into the

water than when it is floating free. (To be able to measure this would probably require that the structure be a cylinder

or rectangular solid rather than the complex shape of some buoys.) That additional amount of sinking into the surface

would be equivalent to putting more weight on the floating structure. The additional buoyancy gained would be equal

to the vertical component of the tension in the cable. From that you could calculate the total tension in the cable and

then you could calculate the horizontal component of the tension.

In equation form:

Weight of additional water displaced = T*sin(theta)

where theta is the angle between the cable and horizontal.

Horizontal forces on the structure = T*cos(theta)

I hope this helps,

Steve

Richard J. Raridon

James J. Kovalcin

Steve Nelson

Steve Johnson

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Page 2 of 2Physics: buoy, ameneh, floating structure

http://en.allexperts.com/q/Physics-1358/2009/3/buoy.htm