Go find a tent rental company, they will sell/give you some of those thick tent stakes, they make great boat/trailor tie downs. I have a set in the yard to tie the boat down when its home and a set that I take with me to regatta's and on vactaion to tie my boat down. You can pound one of these into just about anything, gravel, hard clay, etc, stuff that those corkscrew tie downs will never get in.

Bill