Is this a simile or metaphor?"the sled held as though it were an anchor."
This is a simile. In this case, the phrase "as though" means "like". A metaphor would say, for example, " The sled was an anchor in the deep snow."
That is a metaphor and not a simile. This is because for a statement to qualify as a metaphor, it has to make use of the word 'like'. A metaphor on the other hand, uses phrases to compare two situations. Go to http://www.dailywritingtips.com/what-is-the-difference-between-metaphor-and-simile/ for details.
Joeenns123 is wrong. A simile because that means it uses like or as.