Yes, dredging can be productive for anyone who has a boat.  Even a
skiff with oars can be used for shallow water dredging - down to 50
feet  (15 meters) depth or so - if you have the stamina.  But a motor
- ANY motor - is far superior to rowing.  Dredging involves
considerable physical labor, even without the additional work of
rowing.  A workable dredge can be constructed without much difficulty
or expense.  The size of the dredge has to be suitable for the size
and power of the boat you are using.  A large dredge on a small boat
is a recipe for trouble.  Typically, an oversize dredge will collect
material until it gets too heavy to tow, at which point it is no
longer a dredge, but an anchor - and quite possibly too heavy an
anchor to haul up to the boat.  At that point, you have two choices -
cut it loose and go home, or don't go home.  I have used dredges of
two designs - one with a rectangular mouth 2 feet (60 cm) wide by 6
inches (15 cm) high, and the other with a triangular mouth, one and a
half feet (45 cm) on each side.  I prefer the rectangular model.  In
either case, the mouth frame is made from heavy gauge flat steel bar,
or angle iron.  Mine are made from the frame of an old dormitory-style
bed spring.  The pieces can be drilled and bolted together, but this
arrangement isn't very stable, especially for the rectangular model.
It is well worth the investment to spend a few dollars and have
someone weld the thing together for you.  The edges which will be in
contact with the ocean bottom during dredging (the two long edges of
the rectangular model - all three edges of the triangular model)
should have a lip about an inch wide, bent downward about 45 degrees,
which will dig into the sediment as the dredge is pulled along.  The
bag or net of the dredge is then attached to the frame.  The bag
should not be made of soft material like nylon net.  It gets snagged
to easily, and specimens get wrapped up and tangled in it.  A rigid
bag is better.  I make it from galvanized screen with a 1/4 inch
mesh.  Mesh size is important.  The larger the mesh, the more shells
you will lose.  The mesh I usually use won't retain all the smallest
shells.  However, if you use too small a mesh, the bag will fill up
with sand or mud in no time.  Ideally, the sediment should pass
through the mesh and out of the bag, while the shells are retained
inside the bag.  A ring of some sort should be attached at each
corner of the frame, for attachment of the tow lines.  A separate
line should be attached to each corner of the frame.  The four lines
(or three for the triangular model) should each be equal in length,
about 4 or 5 feet long, and the ends not attached to the dredge
should all be attached to a common, sturdy metal ring.  One end of a
4 to 6 foot length of fairly heavy chain is attached to this ring,
and the towline from the boat is attached to the other end of the
chain.  The chain provides the necessary weight to keep the biting
edge of the dredge mouth on the bottom.  The tow line should be of
good quality - remember, if it breaks, your dredge is gone.  However,
it should not be any bulkier than necessary.  Most rope is fairly
buoyant, and a thick rope will tend to lift the dredge off the
bottom.  Typically, you need about three times as much towline as
water depth, to keep the dredge on the bottom (150 feet of line to
dredge in 50 feet of water).  The speed of the boat is also critical.
 If you go too fast, the dredge will be swimming through the water
above the bottom, instead of dragging on the bottom.  After a little
experience, you can put your hand on the tow line, and tell by the
vibrations in the line whether the dredge is dragging or floating.
After dragging for a few hundred yards or so, the fun part begins -
getting the filled dredge back up to the boat.  This is where most of
the physical labor comes in.  If your dredge is of a reasonable size,
it can be pulled in by hand, but it's still a lot of work, and a tow
line of less than a half inch diameter doesn't provide much of a
handhold.  A small, hand-operated mechanical winch is a big, big help
here.  Something like the winch used to pull a boat onto a trailer -
however it has to have a much greater line capacity than a trailer
winch.  If your boat and your pocketbook are big enough, a small
electric winch could be used.  Once the dredge is near the surface,
raising and dropping it a few feet, several times, will often wash a
lot of mud and silt out if it, lightening the load - but if you are
looking for the tiny shells, you might not want to do that.  The
dredge has to be lifted out of the water and into the boat.  This is
another reason the dredge should not be excessive in size.  Not only
might it be impossible to get it into the boat, but if your boat is
small, you could capsize in the effort to haul a hundred pounds of
dredged material aboard.  Emptying a dredge into a boat can make a
heck of a mess of the boat, unless you are using a good-size boat
where the mess can be spilled on deck, and the deck later hosed down.
 Still, trying to pick through material that is on the deck can be
pretty exhausting, and tough on the back.  The solution is a sorting
board.  This is a board, perhaps 2 feet wide, that spans the entire
width of the boat, overhanging each of the boat's sides by a few
inches.  A block is fastened to the underside of each end of the
board, so that the blocks fit snug against the inner surfaces of both
boat sides.  This prevents the board from sliding back and forth.  The
two long edges of the board have a "retaining wall" consisting of a
2-inch wide board attached upright along each upper edge.  The
contents of the dredge are dumped onto the board.  The retaining
strips prevent anything from falling off the board into the boat.
People can sit or stand (depending on the size of the boat) along the
board, and pick through the material in comfort.  Rubber gloves should
be worn, to protect against possible spines, stings, etc.  You never
know what will be in the dredge.  The two ends of the board are open,
and overhang the water, so that material not of interest can just be
pushed off the ends of the board, back into the water.  After picking
through each haul, the board can be swept or rinsed clean in
preparation for the next haul.  I have had better luck on some trips
than on others of course, but have never come back empty-handed.
Using this method, you can comfortably dredge down to 50 or 60 feet,
even with a small boat.  The method is applicable down to 100 feet or
so, if you have enough boat, enough tow line, enough weight to keep
the dredge on the bottom, and enough energy (remember - dredging in
100 feet of water requires at least 300 feet of line, and possibly
additional chain or other weight leading the dredge - don't try it
with a rowboat).  Dredging in water over 100 feet deep becomes more
complex.  Steel cable is usually used instead of rope, to avoid both
the buoyancy problem and the excess bulk (500 feet of rope takes up a
lot of space).  This cannot be pulled in by hand, so a good winch is
necessary - and a boat large enough to accomodate all of this
paraphernalia.
Unfortunately, my dredges have sat on the shelf in my basement for
the past three years or so, but writing this is giving me the itch!
I'll have to get out there again soon!
Paul Monfils
Providence, Rhode Island, USA