It sounds like you maybe bought the wrong kind of rope. West Marine sells many different kinds of rope, including 3-strand, polyester, dyneema, spectra, technora, kevlar and vectran. Some will twist under strain, some wont. Some float, some sink. Some are skinny and strong, but hard to hold in your hand.
Here is a link to West Marine's "Guide to Choosing Running Rigging", which will help explain the differences, so you can make a better choice:
https://www.westmarine.com/west-advisor ... gging.html