I have had very poor results using the grapnel anchors. I find then fine for small inflatables but have had them drag on my Ribcraft 4.8 when it gets a bit rough, and that was with 5 meters of chain !.
They are great for space and much better than nothing but they don't seem to bite into the bed much, especialy in sand or mud, fine on rock or weed though.
I now use a 3.5Kg Bruce with 5 meters of chain and have never had it move.
I moved to this after leaving the RIB at anchor one night in a bay to find it have moved best part of 100M over night and it was not that rough maybe F3.
The only problem is space as Bruce's does not fold and are quite big. The flat folding, danforth, type anchors like keiths pic also work much better than the Grapnel's.
I also still carry the 3Kg grapnel and use it on a very light, 6mm line when i want to anchor in a narrow bay and not swing round.
Also on triping lines, I keep away from them the worst case is you loose your anchor, but the nice floating marker looks just like a great place for someone else to tie upto for a few hours or more .
Call me pessimistic but its often a sequence of events which lead up to your anchor being the only thing between you and the rocks, and bad weather is likely to be of them.