Neccessary flotation

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Ernest

Member II
OK the Ericson is sold and now I bought an old Sonar. For the physicists . How many empty 1 gallon bottles do I have to put into a 2300 pound boat to guarantee that it will float when full of water? The Sonar people don't have a list like this. Thanks Ernie Schlesinger "ex E 35-3 True Love" and currently "Nautically Correct" Sonar.:confused:
 
Jim Mobley

Jim Mobley

Member II
Not too much Physics required

Ignoring for the moment the question of "Why?"; this is not to tough to figure out.

First off, we'll assume that the "bottles" are milk jugs and are not truly empty, but actually full of air and sealed. And we will also assume them to weigh nothing, though of course they actually weigh a little bit.

The law of buoyancy, often called Archimedes's principle is: "The buoyant force is equal to the weight of the displaced fluid." Good reference is:

http://en.wikipedia.org/wiki/Buoyancy

So, ignoring compressibility, each milk jug, when submerged, will displace one gallon of water. One gallon of sea water weighs about 8.5 lbs., so each milk jug will provide 8.5 lbs of buoyancy. (Minus the weight of the milk jug, which I've ignored, but I've also been rounding down.)

According to the Shumway Marine website,

http://www.shumwaymarine.com/sonar/,

the Sonar displaces 2,100 lbs., so 2100/8.5 = 247 milk jugs to float your boat if fully submerged. We are of course ignoring some things, the trapped volume in the mast if it's sealed, the hull is cored and this may provide a bit of flotation, the air-filled milk jugs will compress depending on depth, thus reducing their displacement and buoyancy, etc.

Put another way, sea water weighs 64 lbs/cubic foot, so 2100/64 = 33 cubic feet of sealed volume for flotation. Milk jugs may not be the most efficient way of obtaining this volume, but if you're a milk drinker, I guess they're essentially free.

So now the question, why?
 
E

Ernest

Member II
Flotation

Well, I discovered 18 sealed 1 gal jugs under the V berth (its not actually a berth) and the the two other Sonar owners I know state that this is the way it came from the factory. So why only 18 jugs if it takes over 200 to float the boat if holed? Core is 1/8 inch thick so I don't think it will help much. It seems to me either you put in adequate flotation or none, because either a boat sinks or doesn't. This is perplexing. I will query the importer for ideas. Thanks for the explanation. Doubt there is room for more than 200 jugs. Regards Ernest Schlesinger.
 
E

Ernest

Member II
flotation

I haven't actually sunk any of my past six boats (yet) so I think I will simply remove the bottles, use the space and just not have an unsinkable boat. Since I will sail this one on the Hudson River, risks are smaller than before. Thanks to all. This list still beats all the others and I think I'll lurk here forever. Ernie Schlesinger
 
Jim Mobley

Jim Mobley

Member II
Might be other flotation somewhere ...

There might be other flotation elsewhere in the boat; under the gunwales for instance or behind the cockpit. Or, maybe the bottles provide just enough flotation to keep the bow up high enough that air is trapped in the cabin, keeping the boat afloat.

This does seem to be a very inefficient way of adding floatation to a boat, why not just seal off the entire area under the v-berth? And doing the math makes it clear why ballasted sailboats don't normally have positive flotation, there wouldn't be any room for anything else--like people.

Maybe the builder had an excess of one gallon containers and this was the easiest way to get rid of them?
 
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