Yes, a “canon aux amarres rompues” would be a canon that has torn free of its moorings/ties.
However, in this sentence it is not the canons that are “crasse”-like.
I’ll translate “crasse” as “filth” as it seems more accurate to me, but maybe someone has a more accurate translation.

In this situation “comme” doesn’t mean “like” be “… as …” for a comparison between the two parts of the sentence.
Someone more skilled in gramar may bring a more accurate explanation as of how and why it is that way.

An attempt to translate this sentence would be :
“All the mountains seemed to the pilot like they were rolling in filth as these canons torned free of their moorings plowing the deck of old sailboats”.

My translation may not be the most accurate but I think it can help you understand the meaning better.
Basically, the author here talks about a dangerous flight and therefore wants to illustrate it with this comparison :
In the bad weather, the moutains emerging from the clouds looked as menacing as canon moving in the filth of an old ship.

Sorry for my potential misspelling or grammar erros in english as well as lack of technical explanation about the french gramar. It seemed however that you required just an help on understanding the meaning and I hope I helped.

Here,
“aux amarres rompues” means that the canons used to be moored to their places and that, probably following a storm, the moorings went torn, so the canons began rolling on the floor.

“labourer” is an image to express the fact that the wheels of the canons rolling on the floor of the ship were so heavy that they were digging a sort of grove on the boards (since they couldn’t turn, their va-et-vient was rolling always exactly on the same line, so that in the end you could see the marks of the wheels like a furrow on a corn field).

“semblent rouler dans la crasse” means that, to the eyes of the pilot, the mountains seem to be moving instead of the plane, as if the plane was still. Then, “dans la crasse” means that the landscape must be full of dust, probably because of the wind, and “St-Ex” sees the mountains moving through and above that dust, as if their movement was the cause of the dust.

I hope I’ve helped you make it clear.