150mm pvc pipe

[jgfish]

just wondering if anyone can work out much weight 4, 160cm lengths of pvc pipe should hold, given the diameter is 150mm as i am planning on building a raft from pvc pipe and plywood

[Trevlyns]

Sorry I don’t have all the bells and whistles on my keyboard for the correct symbols, but here goes anyway!
First you need to establish total volume in cubic meters and then multiply that by the density of water. The formula is pretty simple
Total volume of the pipes would be Pi times radius squared times length times the number of pipes, where…
Pi = 22 divided by7 (aprox.) and radius would obviously be half the diameter.
So your calculation would be
{[22/7] X [.075 X .075] X 1.6} X 4 = 0.1125 cubic meters approximately.
Now multiply this by the density of water (1000 is a fair average for fresh water) and you have about 112 KG
Provided the weight of the craft and its load don’t exceed that, it should float. Best of luck!

[jgfish]

cheers, i worked out about 28.3kg per tube, i guess i was close, any way to increase the buoyancy? would pressurising the tubes do anything? I have had experience with pressurising pvc before, unfortunatley it wasnt for anything lawful and i had it confiscated but luckily for me i didnt get fines, i built an air cannon btw

[jgfish]

say i double the weight it can handle, will that just mean that the raft is submerged to the diametre of the pipe?

[DanishBagger]

Pressurising it won't do a thing. In reality, it will weigh a little more, meaning it will carry just a tiny bit less.

It's not the air, that makes the tubes float, it's how much water it displaces - how much weight of water it displaces.

In reality, when it comes to buoyancy (spl?) you are comparing the weight of the water with the weight of the thing that displaces the water. If the pipe is weighing less per litre than the surrounding water, it will float.

If you want it to float at the widest point, it should weigh exactly half of the water it displaces. But keep in mind, that the weight of water changes, depending on temperature and salinity.

The only thing, you might get out of pressurizing the tubes is that it could become a little stiffer. In theory, that is, because a PVC-pipe is rather stiff as it is (if it's the sort I'm thinking), and you would need some rather high pressure going on, in order to acheive fiurther stifnees.

Pressurised containers can be dangerous, so please don't experiment with high pressure, unless you know _exactly_ what you're doing.

(I am not a naval architect, nor an engineer, so caveat emptor)

[Trevlyns]

I totally agree with Danish Bagger. What we are dealing with here is Archimedes Principle – think of the old fella getting into his full bath. The water that overflows (is displaced) is equal to his own weight. So pressurizing won’t increase the volume of the tubes - it is constrained by its physical shape.
At that figure of 112kg, the craft would sit with its waterline at the top of the tubes.
The only way to gain more buoyancy is to increase volume. Eight tubes (double the volume) would displace 225kg so a total load of 112kg would place the waterline at the diameter.

[jgfish]

bump..

[DanishBagger]

You already have the answer to your edited question. Subtract the weight of the materials from that answer, and there you go.

Edit: Assume you want them to float submerged to the diameter, they will hold 111/2 = 65.5kgs (or rather, they will displace water to that weight (in freshwater). Weight of the pipes, plywood etc is 12,5kgs (I don't know, you haven't told us), you subtract that, and up with 43 load carrying capacity to get to that waterline.

[DanishBagger]

I figured it might be easier to ask you how much weight you want it to carry?

[jgfish]

250-300kg is what i want to hold, i dont know that i will use pvc now as the cost will be more than i want to spend, any other suggestions of what i could use. i dont care what the materials weigh, as long as 2 people can lift it, im used to heavy lifting also

[DanishBagger]

then you will need at least 500 litres of flotation, plus what the floation containers weigh, if you want them to be submerged to the middle.

So, with the pvc-pipes, all being the length and diameter you mention, you would need at least 18 pipes for it to carry 250kgs to the widest point on the pipes. I write "at least", because I have no idea what a single pipe weighs. Do you have a weight?

Speaking of which: It's not that we want you to carry about weight, but if we don't know the weight, we cannot calculate how much flotation is needed. Theres quite a bit of difference, if the whole thing weighs 30kgs, or 130kgs.


Btw, if you haven't gotten hold of the pipes yet, it is good to know that the moment you double the pipe diameter, you quadrouple the volume. So all things being equal, with the same wall-thickness etc, you will get a whole less weight, which translates into more carrying capacity.

What about some plastic barrels? Can you get hold of them cheaply?

Basically, anything that is waterproof will do. The less it weigh, the higher it will float, thus being able to carry more.

[DanishBagger]

Oops, I miscalculated that. You would need at least 36 pipes, not 18.

[jgfish]

im not sure yet, ive been thinking of using 10 litre water bottles, the ones you sit on the bench with the tap on them, my girlfriend buys them occasionally cos she doesnd drink tap water so it might be a good option. As for the weight of the pipes, i have no idea at the moment as i dont have any. it seems quite expensive to buy also

[DanishBagger]

Sounds like an idea - the bottles, that is. Remember to glue the lids/openings, though.

They're also very lightweight, so you might be able to make something that floats, and can indeed be manhandled by two people.

If you do make them out of the bottles, then you will need to somehow protect it from being ripped when handled.

If I recall correctly, PVC pipes are fairly heavy. Sturdy, but heavy. But then again, it rather depends on how thick the walls are.

[jgfish]

the jugs are quite solid, i will need to patch up the ait holes made from usage, they wont rip, here is something that i quite like which is a suitable design... i would have more than one seat though : http://www.habeeb.com/images/funny.p...funny_0009.jpg