Here are the stats from the [Corpus of Contemporary American English][1] (1990–2011) and the [Corpus of Historical American English][2] (1810s–2000s):

                 COCA   COHA

    propelling    425    363
    propeling       0      0
    propeller     782    981
    propeler        0      0

The general rule, [according to Wikipedia][3], is:

> The final consonant of an English word is sometimes doubled in both American and British spelling when adding a suffix beginning with a vowel [...]. Generally, this occurs only when the word's final syllable is stressed and when it also ends with a single vowel followed by a single consonant. **In British English, however, a final -l is often doubled even when the final syllable is unstressed. This exception is no longer usual in American English**, apparently because of Noah Webster.

Emphasis added.


  [1]: http://www.americancorpus.org/
  [2]: http://corpus.byu.edu/coha/
  [3]: http://en.wikipedia.org/wiki/American_and_British_English_spelling_differences#Doubled_consonants