Ok hear goes.
18" rears are way too small for soft sand! I used to get stuck just looking at the sand...
20-21" are much better, and if you feel the bike slowing down, gear up and get the wheels spinning faster. Trust me, when you hit the power band on sand, you fly over the sand. hehe (o well, thats just my experience)
If you come to a complete stop, you are screwed! especially with the 18s. I had to physically pull my bike out of the sand trail by hand, as it was going nowhere!!! Got stuck in the first 10-15meters.
Im not afraid of sand anymore, but had some fun in very sticky mud last week
Im not so sure the blasters are that great dune machines though, sand is hard on a engine, and the water cooled machines are better suited for that. (not that im saying it cant be done, I will go anywhere with my blaster where any other atv has gone)
Next year I am planning another trip to Namibia, there are really nice dunes there, but will rent a bike though...