In the most recent issue (June 1999) of American Conchologist, there is an
article on dredging, which includes general information about dredge
construction and dredging methods.  I won't comment on the quality of the
article, since I wrote it (hehe), but American Conchologist is a great source
of information for collectors.  If you are not a member of Conchologists of
America, you really should consider joining.  Then you will receive the
magazine (American Conchologist) as well as all the other benefits of
membership.
Regards,
Paul Monfils