"chains and nets may upset the stability of the submarine" is a statement. "chains and nets that may upset the stability of the submarine" is a noun phrase. So, if you insert the word "that", it ceases to be a complete sentence (has no main verb).
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Amir OghlowChains and nets may upset the stability...The main verb is upset.Yes.
Amir OghlowChains and nets that may upset...The noun phrase is "that may..."No, the noun phrase is "chains and nets that may upset ...". The head noun is "chains and nets". "that may upset ..." is a relative clause that modifies "chains and ne