Still in Sonnet 116 Line 6 "That looks on tempests and is never shaken" this is interpreted to be showing us that love can out last the storm/tempest, it can also be saying that love is a stronger force than the sea; the extra syllable in Line 6 can also show that love can deal with an unbalanced rhythm provided by the extra syllable, Shakespeare has also chosen to use the word "tempest" instead of storm to show the threat is much greater. In Sonnet 18 like in most sonnets Shakespeare talks of love, "When in eternal line to time thou growest" this line like Line 6 in Sonnet 116 talks of love being one almighty force yet again, it has an extra syllable to demonstrate love being longer, outlasting and eternal, and the love inside will grow.
Both of these lines of 11 syllables relate to love outlasting, love being "eternal", it is also showing love can overcome an uneven rhythm and 'impediments'. Both poems speak of love and how it is strong- stronger than death.