In my incredibly large ballast / support structure, the inside is made up of ribs which are a part of the ferro-cement form and which give the thing more over-all strenght. I've been fretting about this aspect of the internal shape of this area because at the top the ribs are a place where pockets of air can't freely slide out to where the vent valves will be ( I'll have a vent on each side) . So to eleviate this problem I was going to put small 1" holes at the top of each rib ( you can see the plugs at the top of the ribs in the photo). But now, in lew of the dissusion on the bubble riding factor, I'm thinking that that may not be nessesary. For one thing I don't like the idea of a hole right there because it will weaken the rib at that point, though I think there is enough steel to handle a hole at that point. http://www.frappr.com/?a=viewphoto&id=759383&pid=10308219 Brian
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