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Calculations for using recycled propane tanks for floatation

Calculations for using recycled propane tanks for floatation

Calculations for using recycled propane tanks for floatation

I am interested in building a 100' by 50' floating island out of used 500 gal propane tanks. They are 3' dia x 10' long and weight 800 lbs. I am planning on using 50 of these, the layout would be: 10 in a row, 5 rows spaced and braced. Total flotation of the system seems to be around 75 tons, I would like to add 50 tons of buildings and equipment. The unit will be self propelled by two modified dozer boats at the ends. Looking to move in fair weather at 1 to 2 knots.

My questions are:
Do you think I am correct in my flotation calculations?
Is the layout one that would serve best?
What would be the simplest and lightest bracing, ( 8'spaces between rows )?
What issues do you think I may run into or haven't covered.

Thanks for any comments and thoughts in advance.

RE: Calculations for using recycled propane tanks for floatation

Braces between the tanks will add a lot of water drag.
Diagonal 'V' braces from top of tank to (unspecified) structure above would be better from that perspective.

It will need a lot more paint than a monohull, and more zincs too.


Mike Halloran
Pembroke Pines, FL, USA

RE: Calculations for using recycled propane tanks for floatation

Your dimensions are coming up a bit short for me.  10 rows of 5 ea. at 3' x 10' winds up with 30' x 50', which is substantially smaller than your 100' x 50' stated requirement


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RE: Calculations for using recycled propane tanks for floatation

IRstuff, thanks for the reply, I live on the Queen Charlotte Islands, Haida Gwaii, in British Columbia. This "eco-island" would ply the waters surrounding the island, maybe only moved 50 miles at a time every month or so. Only on fair weather days and anchored in protected bays.

Bracing would be needed for spacing of about 8 feet on the 5 rows gives me 50', each row has 10 tanks = 100' I am adding the spacing because my structure is quite light and I am looking for more "ground area" Any further thoughts would be great.

Mike, good thought on the bracing. The structure above would be modules that will be placed on the float, so I was looking to basically have a flat surface of bracing above the tanks as the sub floor and bolt the modules to it. My main concern is structurally keeping the tanks together should it hit rough weather or ground.  Have seen some success with epoxy paints, properly applied, to stop corrosion, good idea on the zincs as added protection.Was not sure if I needed antifouling, since I am only doing 1 or 2 knots anyway. Wondering if growth would eventually be a problem?

RE: Calculations for using recycled propane tanks for floatation

TTFN I understand I am in a grey area of your forum. If my thread is totally inappropriate I will remove myself from the site. Could you and other users let me know.


RE: Calculations for using recycled propane tanks for floatation

TTFN, first saw this site as a gathering of engineers with resounding capabilities to solve problems posed by issues that surround us. Quality of life, capabilities to build structures that would not only support sustainability, but economical answers to marine and ocean construction of structures that could not only benefit the "industry" but also have some social value. Was my mistake to think that such a vast resource of some of the smartest people would not want to partake in an adventure as such. Is this an advertising site for engineering company's to develop new customer bases or is this a site where one can ask to share the resources of the great minds that we have in this world.

I have truly appreciated those who have questioned my prose, and look forward to responses that illicit critical thinking for us DIY people combined with the mastery of the engineering community.

RE: Calculations for using recycled propane tanks for floatation

This site is for work-related problems.  That's why it doesn't allow postings from student postings, even though the lazy one clearly need the brainpower available here.  

DIY and hobby postings aren't always reported for removal, so long as no one feels the urge to report them.


FAQ731-376: Eng-Tips.com Forum Policies

RE: Calculations for using recycled propane tanks for floatation

Few things spring to mind.

I get about 80 Tonnes (78 long) of buoyancy, bearing in mind that seawater is a bit denser than fresh.  It would pay know the dimensions of your cylinders to more than one significant figure - especially the diameter.  Your final buoyancy calcs will want to take account of the shape of the cylinders (which won't be cylindrical!)

The reason you need to know the buoyancy quite accurately is that by the time you add the structure and the payload, the amount of buoyancy you have in reserve is going to be much, much less than you started off with:  This works a bit like the Micawber principle.

You will need to think about how much you can tolerate losing a few tanks (either through flooding, or through detachment)

With buoyancy coming from tanks that are evenly distributed and each almost completely immersed, you are going to need to look closely at stability.  Unlike a traditional hull, this kind of arrangement doen't automatically add buoyancy in the right place when you load it unevenly.  Look at what happens if something makes it roll, if you don't load up evenly, while you're moving things aboard or if you lose a few adjacent tanks.

If you attach each tank individually to a platform above water level, all the structural strength and stiffness is going to have to come from the platform - which is probably something you're otherwise keen to keep quite thin.  Don't design for an evenly distributed load, because you won't get one.  Even if you can be sure the raft is never going to be exposed to a big swell (having your tanks mostly submerged actually works a bit to your advantage in a swell), you are going to have to think about what happens if you put the structure aground on a falling tide (imagine the whole structure supported only at opposite corners - or perched on a pinnacle somewhere under the middle).

Similarly, in the event of a collision or grounding, the attachment of the first tank in the firing line (probably one of the ones at the periphery) is going to find itself the weak link between getting on for a hundred tonnes of moving metal and something much bigger and is going to need to be designed to withstand that.

Mooring and tug attachment both generate quite beefy point loads which will need to be spread through the structure.

I think this project is unconventional enough to need the detailed involvement of a professional naval architect. This isn't me touting for business (I don't have the right skills and live in the wrong part of the world anyway); Remember that something this size is going to turn out to be environmentally hostile, not to mention personally inconvenient, if it ends up unevenly distributed along forty miles of foreshore.


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