The hydrobubble is junk IMHO, as anything that reduces the weight of the anchor in the water is going to reduce its effectiveness at digging in.
I am unsure what the displacement of the bubble itself is in relation to the rest of the anchor. In concept I agree with you sailingdog, although the emperical evidence would suggest it's not a terribly important factor - the HydroBubble did reasonably well in the SAIL testing (and that was hard sand). See the graph in the document linked to below.
Some clown on another board stated that this negative factor must get worse with increasing depth, as the floatation would increase, rendering the anchor useless in deep water
- now that's not
From Peter concerning the float: "I will make no harsh comment on this, save to point to possible issues concerning durability in the harsh conditions anchors are subjected to, and longevity if exposed to UV."
Peter has written a little something about the SAIL testing and also addresses copies and variations of genuine anchor types. We've put it online here
. Comments welcome.