Soda bottle flotation

[GreenLake]

Why complicate your life? Once you have holes in the cap, your soda-bottle "foam" becomes "open celled" instead of "close celled". I thought the hole idea was to put close celled foam in to be sure that flotation can never be compromised by retained water.

After you add holes, you have to go through all sorts of mental gymnastics to prove to yourself that this peculiar coarse-pored form of "open celled" soda bottle foam would not trap water under any conditions. (Condensation?)

So much easier to give each bottle a gentle squeeze and a dollop of caulk while you screw on the cap.....

[hectoretc]

Thanks GL,
I guess my frame of thinking at the time was, compressing 10-20% of the bottle volume and sealing it with the expectation it is going to expand (physically), putting as many compressed bottles as will fit into a confined fiberglass space, thereby allowing said bottles to expand said 10-20% in volume "could" result in a combined increase of of 10-20% volume and might, maybe stress (or fracture) the bulkhead seals. Could - might, maybe... but as you said, why complicate my life. Maybe my thinking of 10-20% is excessive.

As far as getting moisture in the bottles, I doubt there will be much condensation (in the bottles) as they basically exist at ambient (in the hold), therefore no differential in temperature, therefore no dew point condensation issue. The only moisture I would expect (barring a sinking event) would be standing water in the bilge which would only become an issue for any bottle oriented not spout down (which was my intent to drive toward), and only where the standing water was deep enough to reach the spout (of each bottle).

My line of thinking was more of maybe baffles? that's probably not the right word, but using it as a sub-partitioning of the air pocket rather than a stand alone front line cellular flotation system. Maybe that doesn't make sense to anyone else, and your thoughts are well accepted.
Thanks again (as always) for the input. - Scott

[kokko]

You are making unneccessary work for yourself by either compressing the bottles of drilling a hole.
Those of you not asleep during chemistry will remember Charles' Law - the volume of a gas is proportional to the temp. Let's not forget Gay-Lussac's law - that the pressure of a gas is equally proportional to the temp.
For a 100 degree increase in the temp, the pressure will increase about 1/3.
Normal atmospheric pressure is about 100kPa, so the increase will be to about 130 kPa. The burst strength of soda bottles is about 700 kPa. so there is little danger of them bursting.

As for drilling to equalize the pressure, no need as the bottles are unlikely to burst.

[hectoretc]

As for drilling to equalize the pressure, no need as the bottles are unlikely to burst.

Thanks Kokko... that works best for me addressing the concerns I had about expansion.

And yes... I was asleep during most of chemistry (if I even took chemistry)... and algebra, and English, and social studies... I tried to sleep during gym, but those instructors notice quickly if you lay down on the floor...

Thanks,

[adam aunins]

On some home built aircraft they use a foam buck to lay fiberglass up on and once it's cured they pour acetone in through an inspection port and slosh it around to dissolve the foam and then just pour it out. I know sloshing acetone around inside of a boat would be hard but it would work. The only reason to remove the foam is if it's waterlogged and adding weight. If that is the case then your hull is leaking, a lot, and you need positive floatation to counter act the water getting in. This is a common problem on Sunfish sailboats and the added weight handicaps you in races.

I've also blown up a lot of 2 litter bottles and they can take a tremendous amount of PSI before they let go, so I also would not worry about that issue.

[kokko]

On a model airplane this might be ok but do not try this on your boat.
The amount of acetone required would be excessive and there is no way to slosh it around. At best your would have a volume of acetone and dissolved polystyrene sloshing around that is both toxic and flammable. Removal of this solution would be difficult and could properly disposed of only as hazardous chemical waste

[adam aunins]

They use it on full size 4 passenger airplanes. It does not take much acetone to dissolve the foam. I'm not saying it's how I would do it but it is a thought if someone was going whole hog.

[GreenLake]

I'll keep that in mind for when I convert my DS into a ground-effect plane.