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I have an odd problem. Essentially I am working on fabricating a connection between two fiberglass pipes that are intended to be submerged underwater (depth 30ft). No water should make it inside of the pipe so I need the connection to be waterproof. I am imagining something with an O-ring and a screw like mechanism (similar to a toy submarine) but I can't figure out how to create this or hack something together. The pipes have an outside diameter of 4.74 inches and there are no threads, just a straight cross section. I can't seem to find any sort of coupling with this weird diameter, and I'm not sure how I'd attach one if I did. Any advice?

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If you dont need to undo do it.. use fibre glass to couple it – ppumkin Jul 21 '11 at 11:29
What is the inside diameter? – Tester101 Jul 21 '11 at 12:08
ID is 4.5 inches. I think I'll need to undo it eventually so I'd like something I can take apart. Would new epoxy/fiberglass adhere well to already set fiberglass? – William Jul 22 '11 at 3:28
Is the pipe pressurized (above 14psi, which is approx the pressure the outside of the pipe will have from being submerged 30ft), and if so, at what pressure? What is inside the pipe (I'm guessing wiring, but could be water, effluent,...)? How bad is it if the seal breaks, and water does get in (eg, do you want to really over-engineer this seal and have backups)? – gregmac Aug 18 '11 at 21:50
@gregmac Yes, the pipe is pressurized at about 1 atm (~14.7 psi). Inside of the pipe is indeed wiring and a few circuit boards, so a leak would be bad if the seal broke. I'm okay with not having backups if it makes the seal easier to create, but I'm hoping it will work reasonably well. Thanks for the help! – William Aug 21 '11 at 4:08

You might try using a no hub plumbing connection, it's like a rubber pipe with a metal band around it. I'm not sure if they come in 4.75 diameter though.

something like this

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