How high will it float.

Why test with air able to escape ? They won't be that way in use.

 

Drum Layout

18 205Litre drums under 6m x 2.4m pontoon.

 

If I want to test a special property of a specimen (so as deformation resistance in this case), I put the boundary conditions to worst case.

The inside air pressure supports deformation resistance if it is higher than the atmospheric air pressure. But: if the drum is not perfectly tight a long time, the inside air pressure will not increase if outside temperature goes higher and there will be no such support. The plastics will be weeker at higher temps and the deformation resistance of the not supported hull material should be efficient also in this case.

Opposite case: the temp decreases and if the plug is perfectly tight the inside air pressure is lower than the outside one. In this case the air pressure difference ist not a support but a additional load to the drum.

Sometimes it occures, that a plug is undesired tight or is it not - depending on the direction of the air stream (pressure difference). Maybe fluctuate temps make an evacuation pump effect.

 

This plastic drum flotation business is not sound engineering, full stop. Maybe if they get filled with PU foam and sealed against water ingress at the cap, but it is really an el cheapo type idea and not worth the cost of the foam !

 

This is possible, but it will create more buoyancy at this end (and upwards trim).
Anyway you will have to pay attention to the distribution of masses on the deck. Goal is an overall center of gravity perpendicular above the geometric center of the deck (the pattern of floats). This includes the predominate positions of the people while at office.

 

Thanks for your help guys. Hemfried sort of got the general diection, but then got lost in the small things, like caps and construction of drums, following this up by "where will people stand". Thanks for trying, mate. Much appreciated.
At the end of the day, it's a simple maths equation with the only tricky bit being the round shape of the drums.
And speaking of drums, I could have sworn this whole post was about maths, not opinions.
The heading was "how high will it float" not "give me your opinion".
The opinion offered appears to be based on zero research, just the presumption that if something costs less it must be no good. The info available online about drums, strength and pontoon use is extensive. In fact, the floating office next to me in the Marina has had 3 trouble free years using the system. That's why I was asking for help on the maths side, not opinions on if the drums work or not.
I have always watched this site and marvelled at the way you guys love to attack a maths calculation. Not to be this time, so I have had to go the paid option - not so hard, really, as I used Elance and now have Naval Architect, with a masters degree, from overseas who is is working it all out at the princely sum of $11 an hour, and estimates 3 hours. We'll also do Stability and Metacentric height at the same time.
So thanks for your help, and if I need to know how much thread tape to put on the bungs, where to make sure a hypothetical 8 people stand at any one time, or simply an opinion I didn't ask for, which appears to be offered just to start an argument, I' ll be back.

 

Cjtatv. Sure enough any number of us can do the math and also explain it if that is your desire.

The responses that criticized the use of plastic drums were given in good faith. It is a matter of indisputable physics that a cylindrical hull is inferior to a flotation element that has a more appropriate shape. Please do not take offense to comments about reality.

If you are still interested enough to pursue some math exercises, I will, later, attempt to post enough math information for you to use at your own convenience. The truth of the matter is that calculations with respect to righting moments and buoyancy figures at different levels of immersion for cylinders is not particularly difficult but it is tedious and time consuming. Some might consider such exercises, equivalent to self flagellation. It's a damned site easier to calculate these things when the flotation member is in the form of a rectangular or trapezoidal element. That part is easy. But if you have the drums laying down, the calculations become very messy because each centimeter of immersion has different displacement from the last one or the succeeding one.

It would make the math far simpler if you stood the drums on end rather than laying them flat. Safer too. With the bung end up, a tiny hole in the bung would equalize any thermally induced internal/external pressure differential.

The drums have a capacity of 205 liters so they have a potential flotation of 205 kilograms. Eighteen of them have the potential to support 3690 kilograms. Suppose the height of the drum is 100 centimeters. That means that for each centimeter of immersion of a single drum it would have the potential for 2.05 kg of support. If eighteen of them were loaded equally, each cm of immersion would become 2.05 times 18 = 36.9 kg.

The ball is in your court.

 

Chris, sorry for telling you things, you don't want to hear.

 

Thanks

Thanks Messabout
It was great to have somone explain it such precise terms. Thanks for that. Sorry if i appeared smug in my replies, I could just see it spiralling into something other than maths, and i really needed that figure, which was way beyond my math capabilities. I now have my water height, metacentic height and centre of gravity from another source, so I can start putting it all together without the fear of hurting people and knowing where she sits in the drink.
Hemfried's original Calculations were pretty close, and I am comfortable with drums having read up extensively on them.

 

Chris, I read the original post once again....fortunately. If the dimensions of the drums are 48 cm diameter and 94 cm long, they are not 210 liter drums. They are closer to 170 liters.

Calculate the volume of the drum by taking the square of the radius (one half the diameter) multiplied by pi. (3.1416) That gives us 24 x 24 x 3.1416 which is the area of the ends or cross section of the drums. That will be 1808.56 square centimeters. Multiply that by the length, 94cm, to get 170,098 cubic centimeters. The volume formula is pretty simple it is Pi x Radius squared X height. One liter is 1000 cubic centimeters so 170, 098 divided by 1000 gives us about 170 liters. Therefore each drum, if fully immersed, would provide 170 kg of flotation. 18 of them 3060 Kg of total flotation.

Yes I know that we have not arrived at the information that you wanted. There are too many variables in this proposition. For example, if the floating office is to stay reasonably level with the dock structure then that means that the office cannot be in a tidal basin. The term "office" is not clear enough to know what your are actually describing. Those privileged with knowledge of Aussie speak may have a better grasp of what an "office" is. To Yankees, like me, it implies a place where there will be desks, phones, computers, and most importantly, people walking around. If people are walking around, that will complicate matters greatly. The floating platform is after all a sort of boat. Boats bob and weave under the influence of water conditions and passengers who move their weight from one place to another

Please note that the post in which I offered some displacement figures was only if the drums were standing on end, not lying flat. Lying flat configuration, as you have indicated with your drawing, requires far more complex calculations. We can do those calculations but even though the math is sufficiently accurate, it will not account for all those variables.

Shall we continue?

 

Hi
I must have done a typo, as rhe drums are 92cm high and 59cm wide. I have had a Naval Architect provide me with reams of calculations and it all works out in the end. Office means the same in Aussie, just there are only so many people you can fit in a 4.5m x 2.4m room.
As it is an office for booking tours, then 1 to 2 people behind a counter serving 4 at most. No more could physically fit. The area for customers is about 2m x 2.2m.
So, no need to continue, but thanks for taking the time.

 

Hi Chris, not about the numbers, but i assume your NA is not based in Aus from what you said. For insurance and liability reasons you may want to consider getting your paperwork done by someone certified in your state. In the unlikely event of an incident your helpful and understand (not) OSH investigator won't pay much attention to calculations done by anyone overseas that can no longer be contacted. It is a workplace open to the public rather than a hobby venture as far as i can make out.

 

I get that the drums are cheap, but xps foam (closef cell) glassed with epoxy (mayve plywood frame) wouldn't cost THAT much assuming a business setup. And with such structure you could get much smarter shapes for the use.

 

I plan to get it checked bt an AU NA after, I just needed to know if it would float at the right height and was stable . Sort of knowing the answer to a question before asking it. I have just sourced 2 fibreglass 2.9m by .6 pontoon tubes, so that takes most of the plastic drums out of the equation.

 

Talking to naval architects about a thing made of old drums is ridiculous, the first thing he should have been told is to abandon the idea and make a rectangular pontoon filled with a suitable flotation foam so it cannot sink. And your public liability insurance doesn't get tested.