Dog screw will not be enough even in solid ground, better use something like that (sorry I have no clue how you call this in english):


However in sand, it will not hold. You have to take an old tire fill it with concrete, tie a line around, dig it in the sand and knote it arount the mast step/beam (not the doplhin striker alone). I use one of those blue IKEA bags filled with sand and digged it in (line around the bag, not the handles). It is about 50 liters of sand.
It is good idea to prevent the boat from rotating as well (e.g. a line from the rear beam to the trailer).

I was thinking about storing the boat mast down. But I think the risk that somebody drive in the mast (on the parked trailor) at night is higher.

Cheers

Klaus