May thanks for the info.
In uk.rec.sailing I received this reaction about the Spade:

Yes, I see them in use regularly around the Med - and regularly dragging
on weedy bottoms.
It is as easy as teasing aussies to design an anchor that sets well in sand
and mud like the Spade. Indeed the Spade is so good on these bottoms that
it digs itself down almost indefinitely and you can have a hard job getting it
out. But it is as useless as a large frying pan in kelp.
-----snip

Anyone with a comment on the statement that the spade, when used in
sand or mud, keeps on digging down and can be difficult to get out ?
Thanks again,

Len
S/v Present