Introduction: Mini Dredge
This is a project built to aid a PhD student to do research on small marine snails.
It needed to be small so a small boat could pull it easily and made of stainless steel, total weight ended up at around 6 Kg unloaded.
All up the stainless (304 grade) cost about $70 NZ dollars, welding rods (304) $15-20, shade cloth bag Free, canvas bag $80, My time.. well work paid for it.
4 of 100 x 400 x 6 mm Stainless plate (top bottom and wings)
2 of 100 200 x 6 mm Stainless plate (sides)
4 of 8 mm Diameter x 700 mm Stainless Rod (arms)
2 of 150 x 5 x 20 mm Stainless bar (dredge attachment point)
2 of 8 mm Diameter x 30 mm Stainless Rod (bracing for arms)
2 of 8-9 mm x 185 mm internal diameter Stainless tube
Catch bag attachment:
8 of 50 x 5 x 20 mm Stainless bar ( attachment points)
2 of 450 x 13 x 3 mm Stainless bar (Pin to hold net on)
2 of Coulter pins Stainless if possible (lock pin in place)
5 m heavy chain
50 m heavy duty rope
Welder (I only had Arc)
Hacksaw (electric and hand)
Milling machine (or cutting blade on grinder)
Step 1: Cut the Bits and Prep for Welding
Cut the bits of the body and wire brush the ends.
Using a grinder take the outside edge off so as to create a groove on the outside surface for the weld to fill.
Put in vice using the set square to ensure finished product is square, spot-weld the corners.
Note Stainless is notorious for warping when welding, bracing it with wood and G clamps can help.
Check it's square then weld inside and outside seams, then flatten outside welds with grinder as they will get in the way of the bag attachment points.
Step 2: Attaching the Flaps and Arm Attachment Points
I made two dredges up so was able to play with the angles of the flaps, one at 35 degrees and the other at 45, at the moment they haven't been used enough to know which angle is better.
The flaps were clamped to the approximate angle then spot welded at each end, this allowed the angle to be fixed by tapping the flap with a hammer. They were then fixed in place with 4 welds top and bottom. It was at this stage the worst of the warping occurred as you can see in the picture but I was happy with it bulging out like this.
Clamp the two 8-9 mm diameter x 20 mm pipe to the front face of each vertical side of the dredge body, this will be where the arms attach. Put a good solid weld down each side of this tube. At this point you will notice there is no way of getting the arms in, I wanted to have them detachable, you could have already made them and put them in place but this makes storage a little bit harder.
To make the arms detachable you will need to cut out some of the flaps, do this until you can only just get the 8 mm rod into the pipe from the top.
Step 3: Bag Attachment Guides and Arms
Cut the 8 bag attachment guides then shape them, note the rough outline of the intended shape drawn on the end one. I clamped 4 together at a time (four making one set top then bottom) I had a milling machine to cut the groove 5 mm deep for the 3 x 13 x 450 mm pin to go through but it could be done with a grinder-which would be faster but not as neat, note there also needs to be enough room for the bag loops to fit underneath the pin. Then cut/grind the angles into them and wire brush edges to get rid of sharp angles which could cut the bag later. Take the two end guides and clamp them 2-3 cm from each end with G clamps and spot weld at each end, put the pin in place and clamp the two remaining guides in place checking that the pin can be easily removed and installed, Once you are happy weld these in place too. Then repeat for the underside of the dredge body. The pin is easy using the two 3 x 13 x 450 mm bars, mark them 40 mm from one end and bend at 90 degrees. Slide this through the guides, at the point where it pokes out the other side mark it and drill a hole of suitable size for the coulter pin to fit.
Cut the arms, bracing and bar for attachment to chain. Mark the arms at 70 mm and bend at this point to about 80 degrees making a '7' shape. Lay the two pieces out on a flat surface with the two '7's facing each other forming a rough triangle with the short side being 190 mm (note there should be a little gap in this side). Once your happy spot weld the pointy end, once cooled test it's fit by putting it in place. This requires it to be held out to the side of the dredge body putting one end of the arm in place then the other you should then be able to swing the arm forwards thus locking it in place. About 80 mm from the pointy end of the arm add a bit of 8 mm round bar forming an 'A' shape, weld in place. Over this area we will weld the piece of 20 x 5 mm bar which will be the attachment point for the chain, make sure the piece you cut sticks out about 30 mm. Repeat this process for the other arm, once you have both arms in place with the 20 x 5 mm bar both on the inside you will notice they both need bending outwards until they come together (probably 15-20 degrees). With the two arms in place bring them together so they are centered, clamp and drill a hole which a 'D' shackle can hold the two arms together and a chain.
Step 4: Bag Construction
I organised for a canvas/tarpolan shop to sew a bag up to fit the dredge, unfortunately they ran out of time so I had to make one quickly for a trial run.
It consisted of heavy duty shade cloth sewn into a tube that would fit around the dredge tightly then sewed a tube on the leading edge that the retaining pin could fit through, it was put in place and cuts were made where the attachment points are. Theoretically the pin could then be passed through the tube of the bag and under these attachment points, once all the way through the coulter pin could be put in thus locking the bag in place. A piece of canvas was then sewn to the underside to reduce wear and the back end of the bag was simply tied up with rope.
Step 5: MODS
As this dredge was made to catch small snails for scientific research (down to 1-2 mm) it tends to fill very fast with sediment, using coarser mesh sewn to canvas would allow for most of the sediment to simply pass through thus allowing for longer runs and lighter loads when trying to haul the dredge up.
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