types of buoyancy

[ribdriver]

Slightly boaty related..we are making some floats consisting of 2 x 45 gallon plastic drums, held together using a rectangular scaffold tube frame. The two sides of the frame pass through the centre of the barrels - top to bottom, so the barrels are no longer water tight. So effectively a catamaran.

To add buoyancy (given that water gets into the barrels) we have tried polystyrene beads - they are cheap and easy to pour in, which works but the floats sit quite low (quite a bit of weight upon them). Anyone got any ideas how elese we could do this? Is there a way we can force more beads into each barrel - thus reducing the air space? Or a better buoyance option? It needs to be cheap, and easy to pour into the barrels throught the standard 2" bungs at the top - the barrels do not have removable lids.

Thanks in advance

 

[ripvan1]

Pourable 2 part closed cell foam, builders foam - expensive tho' I should think

 

[boaterbaz]

Hi expansion foam, Two part mix,pour and stand back

 

[Seajet]

I can't really think of a better option than your beads, obviously bouncing / jiggling to get the max beads in.

Incidentally I'd be very sure the beads cannot get loose into the water no matter what happens, as it would be a bit of an environmental disaster if this is for outdoors use in a natural setting.

Remember the barrels won't produce their maximum potential buoyancy until submerged, but that would be awkward to achieve and probably lead to rather squirrely handling...

Things are significantly more buoyant in salt water compared to fresh; with sailing dinghies this can be felt and shows in the handling.

 

[maxcampbell]

Hi expansion foam, Two part mix,pour and stand back

It's polyurethane. DIY packs available. Much better than beads, 'cos it completely fills whole area - even if barrel later falls apart you'll still have buoyancy. (sp?)

 

[Seajet]

I think you'll find that type of foam does absorb water over a period; years ago it was popular in sailing dinghies, but not now people have some long term experience with it.

 

[maxcampbell]

I think you'll find that type of foam does absorb water over a period; years ago it was popular in sailing dinghies, but not now people have some long term experience with it.

You're probably right, but in case of damage you'd have weeks or months of buoyancy with foam slowly getting sodden, rather than seconds or minutes as the beads rush out into the environment. It's still used in production boats (eg Drascombes, probably many others) in supposedly watertight compartments, to give buoyancy in case of damage.

 

[prv]

How about not drilling holes in the barrels in the first place, and lashing the poles to the outsides as in conventional raft building?

Pete

 

[vyv_cox]

As Seajet says, no polyurethane foams that can be bought by the general public are 100% closed cell. The foams sprayed into boats for buoyancy use materials not generally available, either mdi or tdi based, where the 'i' stands for isocyanate.

Builders foam and two pack foams sold by grp suppliers can be a long way from 100% closed cell. They will absorb water relatively quickly, i.e. constantly immersed in a couple of months. Also, polurethanes are considerably more dense than polystyrene, giving a lot less buoyancy. If this is a one-off project, such as a raft race, you will probably get away with PU foam. If the floats will remain in the water for some time you might reconsider either the attachment method, as suggested, or increase the number and use polystyrene.

 

[lw395]

You could reduce the quantity of foam needed by filling the bulk of the volume with either an inflated polythene bag, or a polythene bag stuffed with polystyrene beads.
Put the bags into the barrel before filling or inflating.
Then inject the foam into a hole at the bottom of the barrel, let it expand through the barrel and around the bags.
Where the poles go into the barrels, the foam should ooze out.
Seal any exposed foam with some waterproofer such as gloss paint to reduce absorption.
If the paint attack the foam, use a couple of coats of water based paint first.

Ideally you'd get really stout poly bags, but decent bin bags might do?

 

[Seajet]

One thing we don't know is what this is for, which is usually quite relevant !

The pub near our club had their annual raft race last Saturday, no doubt when building their fine vessels people were thinking ' July - ahh, T-shirts and shorts, cold beer '; I saw them grimly paddling away into driving rain and a F6, more Shackleton than Robinson Crusoe !

 

[macd]

I recommend inflated wine box bladders. The best part is emptying them.

 

[status]

Weld a bit of plastic pipe in the barrel from top to bottom to pass the scaffold bar through but keeping the barrel water tight.

Easy when you know how!