More of a raft really...

I'm looking to build a stationary raft for an art project. The raft will have to support a pendulum apparatus and pyramidal aluminum struts (all totaling about 300#), and be anchored in about 20ft of salt water. Right now, I'm leaning towards using food grade 50 gallon palstic drums, epoxyd shut/airtight. Can anyone guide me towards the number of drums I'll need to keep this contraption afloat?

 

A cubic foot of salt water weighs about 64 pounds, so every cubic foot of empty space you can submerge below the surface of the sea, will carry 64 pounds of weight.

For example, for simplicity's sake: an empty plastic drum measuring a foot in radius and three feet long will have a volume of 1*1*3.14*3 = 9.42 cubic feet.

It is very hard to push this drum below the water surface. In fact it would take 64*9.42 = 600 pounds to do it. A raft of six of them would all together carry about 3,600 pounds at the surface of the water. You only need to carry 1,800 pounds of weight?, then these drums would float half in half out of the water.

Don't forget to factor in the weight of the drums themselves.

Got the idea?

 

If the structure gets to high, you might want to look at stability too!!
The pendulum is moving, I suppose? You have a big chance that your raft will be rocking!

 

Thats part of the point. I want to have the pendulum crank a ratchet which will engage a small series of those perpetual motion balls that people put on their desks.

I'm going to fill the drums with some type of expanding foam. It'll be lightweight and helps the raft be slightly more risk averse.

 

Yeah, thanks a lot. Thats the first sensible explanation anyone has given me on this project.

 

That's including the weight of the foam you're thinking about!
Foam will prevent water coming in the drums, but with expanding foam you have to be careful not to damage the drums. It is incredible how much force is exerted while this stuff is expanding.
With making the raft wide enough you'll have no problems with the rocking motion.

 

Rafts are sometimes most stable upside down. Make it plenty wide.

 

is this the way you would do it?work out the weight that there will be (KG is easiest to turn into the buyoncy)turn that into buoyancy,add on the weight of the foam and then put in extra floatation.