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Technical Reports:

3 most recent reports added 05/14/07

Dredging with a New Type of Dredge

Getting The Best Performance From Your Dredge

The Hardships of Dredging

also below:

Venturi Jet Analysis
New Life for the Suction Nozzle
Specific Gravity

Venturi Jet Analysis
by Reggie Gould

To understand what the venturi is used for you have to know what it is associated with. The first dredge was a treadmill, and was first used by the Dutch in 1589, which was horse-powered, the ones that eat hay. The first bucket dredge was powered by steam and was invented by an American (Oliver Evans) in 1804. These were all bucket brigade type dredges; the suction dredge was soon to follow. The suction dredge was much smaller then the floating barges and requires far less equipment. The suction dredge required only a centrifugal pump and gas motor, a float pontoon system, a sluice box, a venturi jet generator, a dredge hose, and a nozzle.

This discussion involves the venturi jet generator that provides the suction for the dredge to operate. The Venturi principle is the key to making the suction generator work and the principle has been known and applied for many years.

The most practical usage was in the carburetor development. The carburetor would nor function properly without the venturi effect. Some bright miner picked up on this principle at an early age and decided to apply it to a dredge.

I am not sure what the first prototype looked like but the early nozzles I have seen, are the suction type nozzles. I have in the past done some extensive testing on various types of venturi jet generators and have written an article for the ICMJ, called New Life For The Suction Nozzle. I used the basic straight jet venturi as the baseline and compared it to the tri-jet, the suction nozzle and the infinity jet. The infinity jet produced the most vacuum, the tri-jet was 2nd, & the suction nozzle was 3rd, the straight jet came in last in performance. There are some strange dynamics going on inside the various jets. If one were to make each jet out of clear plastic, then he could study the fluid dynamics going on inside each jet. I have the interest in such a study but not the funds to invest in the project. In Canada, they have a national interest in projects such as these, and they invest money in them, as they did in the sluice box study.

To begin my study, I thought I would shorten the output tube on a straight jet venturi to save material but the owner of a Mining Supply told me that it would not work. He said that one of his manufactures tried to do that very same thing and it almost destroyed the performance of the device. I never gave it another thought until I was testing my infinity jet. I had constructed an infinity jet of my own design and I wanted to explore all the advantages and any problems. The design had an adjustable gap. Since this was a 4.0" dredge with a venturi jet generator, I based the jet opening to be the same square inch area of a 1.0" jet that was used on the straight jet. The area of the 1.0" jet was 0.785 sq.". The infinity jet had a diameter of 4.0", so I calculated the circumference of the jet, which turned out to be 12.56". If I multiplied 12.56" x 1/16"= 0.785 sq.". I filled a horse-tank with water and used a 5.5 HP motor with a 200 GPM pump for my power source. I put 15 ft. of 4.0" hose on the input of the device, for a static load. When I fired it up, the water was shooting out at least 5 ft., I was puzzled, the water shooting was hallow in the center. So I covered the output of the Venturi Generator to bleed out the air in the system and finally reached a point where the water was pouring out and I was getting some suction. I looked at the gap and it was around 1/16th.of an inch wide, which would be around 0.785 sq." in area. Something was wrong with the suction; it worked but not like the original one that I had constructed for the test in the article, New Life for the Suction Nozzle.

I had developed a special jet inside the device to improve the performance but it was just not doing the job. Then I remembered what the owner of the Mining Supply Store had told me about the length of the device. I measured the length of a 4.0" straight jet venturi and it was twice as long as the infinity jet. I got a coupler and installed another 18" of 4.0" pipe on the output tube. It worked; the suction was incredibly strong. This result only brought on more questions. Why does the output have to be a minimum length? What is going on inside that little devil? I started reading all I could find about Fluid Dynamics or hydrodynamics, and I could not find any material that directly applied to the venturi jet used in dredging. I did find all types of equations on Boundary Layer Theory, Laminar Flow versus Turbulent Flow, Bernoulli’s Law, Navier-Stokes equations using Reynolds stresses and a dozen or more equations.

One of the big problems with all these equations, is that they do not take into consideration the introduction of various sized rocks, gravel and sand, that would introduce more variables then the equation could handle. Most of the equations are designed to have certain constants and just cannot have these types of variables. I decided to use logic to solve the conundrum, there were just too many variables and too many complex equations to solve the problem.

There is one way of solving the problem, that is to instrument the device with flow meters and pressure transducers, unfortunately even after the data is entered into a data acquisition system and analyzed with a computer, it will probably still not be able to factor in all the dynamics going on inside this device. There are some basic rules to follow, the output tube length, which we have discussed, the laminar flow has to be maintained without introducing any turbulence. The turbulence can introduce a type of cavitation that causes the water to turn to vapor. Once this happens, the efficiency of the jet is lost. How do we reduce this problem? The input supply of water, from the pump, has to have a smooth flow to the jet. If the flow were to encounter an extreme right angle turn, without a smooth flow, then turbulence would be created. I usually tackle this problem by making the input supply much larger then the jet orifice. If a casting were used in the design, then it would have to be polished, and deep enough to allow the water to turn the corner, on a right angle turn. I decided to make my feed port come in directly into the supply cavity. I increased the size of the cavity to reduce any turbulence that might have been generated.

I remember early in my career as a young engineer, at North American, that we observed many liquid and gas behaviors, by building clear plastic models and observing the fluid or gas materials while in a testing mode. By this method, you can observe the dynamics going on in real time but to model them into a math model, is a real task. The incoming water pressure is varying with the load that it is being applied to. Some rocks are heavier then others so the load to the pump is changing in a dynamic situation. As the pressure is varying, the GPM varies accordingly, which introduces another dynamic variable.

What are the dynamics going on inside this device? The jet has an important function; it provided the power to run the device. The diameter of the jet has to be the correct size in relation to the diameter of the tube (venturi jet generator) that the water is flowing through. The action is fairly simple; it is the effect of a high-pressure stream of water impacting upon the column of water and material in the venturi generating tube. As the column of water moves away from the high-pressure stream of water, it leaves a vacuum behind it. This vacuum that is created is the workhorse for the dredge. It sucks up the material and water and delivers it to the sluice box.

The one important factor in the venturi system is the column of water that the jet is pushing against. The column of water is supported by a tube of sufficient length to create the load necessary for the venturi effect to function. I found by experimentation, that if I increased the length of a commercially produced straight jet by 12.0", I could increase the suction by a given amount. The column of water acts like a water-piston. The high-pressure jet is trying to tear a hole in the front of the column of water and is creating extreme turbulence. Therefore the column of water has to be of a sufficient length to maintain the structural integrity of a water-piston. By extending the length of the tube, the water column is lengthened, which makes it much more efficient.

I know of a least one dredge manufacture that locates his tri-jet at about 5 ft. from the sluice box. Now I understand why they did that., the extra length provided a hydraulic load for the venturi.

What is the other factor that effects the performance of the venturi generator? Tilting the output of the jet slightly skyward would also help to provide a better static load. The object is to have a hydraulic piston moving out the end of the tube, leaving a vacuum behind it. All these type of jets have to be operated horizontal to the earth’s gravitational force or be tilted up slightly. If the tube were pointed towards the earth, the water would run out the end so the head or column of water could not build up. When you hear, "I lost the prime on my jet", you will now know the physics involved in the process.

I once got this brainstorm to make the jet lighter, so I cut away 2/3 of the output tube and replaced it with a larger piece of plastic tube. I reasoned that this should work, the new tube I added is only slightly larger. Wrong, this new larger diameter pipe, destroyed the venturi action and only achieved ½ the vacuum. Apparently the venturi system likes a constant pressure throughout the structure. There are many equations that help to understand why this problem was formed but they are too complicated for this basic discussion.

So, what is all this discussion about the Infinite jet? What does it look like and how does it work? The design of the venturi generator is completely different from any other device. The actual jet is located at 360 degrees around the outside of the incoming water from the suction hose. The concept is to grab the outer perimeter of the water from the suction hose, and break the surface tension on the wall of the tube and accelerate the body of water and material. The losses in this type of jet are very minimal. Because the jet gap is so small, the jets are usually made adjustable for maximum efficiency. This has a built in advantage, if the gap was to get clogged, then it could be opened to self-clean.

Setting the gap opening is a little tricky. I used an old mining method to measure the GPM output of the venturi generator. I have discussed this method in another article, for it is too lengthy to discuss here. The basic principle is to adjust the gap for a maximum amount of water flowing out the end of the venturi generator. The suction nozzle is the other strange device. I know it works better then the straight jet venturi but I really never gave it much thought. The one big thing that is obvious, is that the output flow of the jet is inline with the dredge hose, instead of being tangent to the flow of water, you know, coming in at an angle. The jet is pushing at the center of the column of water and no losses of energy are wasted on the side of the tube. The dredge hose now provides a long column of water and material to push on, this is an ideal situation for the jet. The water and material are now coming in to the jet at an angle and get blasted into the flow. I am amazed at the efficiency of this device; it is so simple in construction. After building an infinity jet, I now understand why the simpler jets are produced. I personally like the efficiency of the suction nozzle, the tri-jet and the infinity jet, but like anything that is more complicated to build, it is going to cost more.

The trade off to the cost of the more efficient jets, is their ability to do more work with less energy. What does this translate to the dredger? The benefit is operating with smaller motors & pumps or eliminating one motor & pump, where two are needed to complete the job. The extra motor and pump are also very heavy when you have to pack it in to a remote spot. Another benefit is you will use less gas, for those of you that had to pack in your gas a long way from the road. There is one conclusion that I have arrived at with all this testing, there is a minimum size pump & motor that can be used to produce the venturi, after that size is met, the improved jets just produce more vacuum for a given size pump system.

For questions or comments about this article can be directed to the author at gould@gouldeng.com.

Reference Materials:

Schmidt, F., Complete Manual of Dredging 1988.
Turner, T. M., Fundamentals of Hydraulic Dredging 1984.
Van Nostrands’s Scientific Encyclopedia Fifth edition page 1076.
Cheremisino, N. P., Encyclopedia of Fluid Mechanics, 6 vols, (1985-87).
Streeter, V., & Wyle, B.,Fluid Mechanics, 8th. ed. (1985).
White, F. M., Fluid Mechaanics, 2nd. Ed. (1986).

New Life For The Suction Nozzle

By Reggie Gould

The Suction Nozzle was one of the first Venturi Nozzle to be used in the dredging business. Years later it was replaced with the Straight Jet Venturi System, which is sometimes referred to as a Log. The one main benefit of the Log, was that the operator only needed a Suction Hose with a Short Nozzle, at the other end of the Suction Hose, it was attached to the Log, and the Pressure hose was then attached to the Jet. This only required a small length of Pressure Hose to run the Logs Jet, where as before, it would have to run the length of the Suction Hose.

Most Dredge Operators now use the Log type Jet Venturi System. The Log is a straight piece of pipe whose diameter is equal to the inside diameters of the Suction Hose. The Jet is welded on the pipe at a 45 degree angle or less, and at about 1/5 the distance of the Log, from the end of the Log.

The actual Jet varies in size, a 4.0" diameter Log would have a 1.0" diameter Jet. The Jet of course is connected to the output of the Pump. The Jet is the heart of the system, it pushes water and leaves a void or vacuum behind it. The vacuum that is created is used to draw the water and material to the Sluice Box. The size of the Jet, determines the size of the Pump Flow in GPM (Gallons per minute) and motor HP (Horsepower). The Supply Hose also has to be sized to the GPM of the pump and the Log Jet size. If too small a Supply Hose is used, there will be a drop in flow and pressure at the Logs Jet. This is caused by the surface resistance of the water rubbing against the inside of the Supply Hose. If you get a good book in Fluid Dynamics or Hydrology, you can calculate the exact diameter needed.

The Suction Nozzle on the other hand is a little different animal. The pipe is shorter and is bent to a 45-degree angle. The Jet is welded at the bend and the stream of water is directed down the center of the tube towards the Sluice Box. The original dredging configuration was to put the Suction Nozzle at the very working end of the Suction Nozzle. The material and water were then pushed to the surface to the Sluice Box. The only drawback was there had to be two hoses to make the system operate; a Suction Hose and a High Pressure Supply Hose. When the Straight Venturi (Log) came into use, the Suction Nozzle was abandoned by most Dredgers. I thought it would be interesting to test each type of nozzle in a controlled environment, using commercial test equipment. I used a 5.0' diameter cattle water tank filled with water for the test. I decided to test several Log manufactures to see if there was any difference in performance from one manufacture to another.

The test setup was very simple, a short piece of pipe with a vacuum port venmturi was installed at 90 degrees to the suction side of the Log and a 15.0' length of Suction Hose was attached to the other end. A Vacuum Gauge was connected to the vacuum port. A water pump, capable of producing 225 GPM, was attached to the water supply Jet of the Log. The pump was operated at half throttle and then full throttle, and each reading recorded. The full throttle data produced the best-controlled data, so that is what will be reported on. Why was a Vacuum Gage used instead of using a Calibrated Hand? I thought empirical data would best be derived at by using a Scientific Instrument, over the Calibrated Hand approach. The average reading was 4.0 inches of vacuum at full throttle. An interesting thing was found when I testing the Log, I decided to extend the length of the Log by various amounts and record the results. By extending the length of the Log by 12", the Vacuum would be increased by 1.0" of vacuum, for a total of 5.0" inches of vacuum, or an improvement of 25%. Why would increasing the length of the Log provide more suction? It is the nature of the beast, the Venturi System derives the vacuum or suction by various pressure zones. I did not attempt to calculate the proper length. The original length was probably determined by trial and error. It will be noted that there was no appreciable difference from one manufacture to another, they all did what they were supposed to do.

Why are vacuum measurements important and what do they mean? The various readings are used for a baseline to measure the differences from one device to another. The vacuum readings are directly proportional to the suction at the end of the nozzle.

The 2nd. Device tested was an experimental Tri-Jet, it tested at 7.8 inches of vacuum, at full throttle for an increase of 95%. I understand that there is a smaller manufacture of dredges that installs his Tri-Jet in the Suction Hose located about 5.0' from the Dredge. The people using his equipment say that this installation greatly increases the Suction.

The 3rd. Device was another Experimental Jet, called the Infinity Jet, that device measured 9.5 inches of vacuum for an increase of 137.5%. The 4th. Device tested was a real shock, it was the old fashioned Suction Nozzle and it tested at 6.7 inches of vacuum, for an increase of 67.5% more power. So why not replace the Log with the Suction Nozzle and locate it at the Dredge and attach the Suction Hose to the end of the Suction Nozzle? For my own needs I put the Suction Nozzle on the front of my device and used a 4.0' long piece of Suction Hose between the Suction Nozzle and the Dredge. I also operated my Dredge under water at the bottom of the river. It has far greater power when operated under water and requires a smaller pump to run the system. If you stop to think about how a normal dredge operates, you will see that the rock and sand, mixed with water, must be sucked to the surface by the Venturi System. The deeper you operate, the higher the lift required or better put, the more energy required. If the pump cannot compensate for this heavy lift, then the material will move slower and thus the efficiency will go down. There is also a tendency for rock jams when things start to slow down and the material and water start to pack up. There are Sub-Dredges available on the market today but they are not widely used for some unknown reason.

In my own experiments, using my own designs, I find Gold recovery can be achieved above or under the water with excellent recover of Fine and Flour Gold. Maybe it is time to re-think the Sub-Dredge and the Suction Nozzle. From my own experimentation, I found it is possible to double the efficiency of the Venturi System by operating under water at the bottom of the River. The optimum goal is to find a system that will require a pump and motor that is 1/2 the size of a normal dredge. This would drop the weight by 50% and make it a lot easier to pack in.

Jamming is another problem that most all dredges have in common. The jam usually occurs at the input of the Log. There have been great improvements in this area, the interface has been redesigned so there is a seamless flow from the suction Hose to the input of the Log. The ideal jet would be three jets spaced at 120 degrees from each other. This method would force the material to travel through the center of the tube rather than be pressed to one side of the Venturi Tube. The old Suction Nozzle Venturi however, pushes the material to the Sluice Box rather the sucking it to the surface. The pushing appears to be more efficient then the sucking type Venturi System.

Another factor in the Suction Jet is the alignment of the actual Jet. In the Suction Nozzle the Jet is exactly centered in line with the final flow of water, where as in the Log design the water flow is at a tangential angle to the water and material flow. In the final analysis, 67.5% more power is achieved and this is nothing to sneeze at. If the Suction Nozzle design reduces jamming, it would be worth it to change back to the old design. If you have an old dredge you probably have one of these old nozzles in your garage gathering dust. Put one on and try it out, what do you have to lose? The installation is quite simple, remove the Log and attach the Suction Nozzle. The Dredge Suction Hose will attach to the bent or input of the Nozzle. The Log was much longer, so the difference in length will have to be made up by adding a short piece of Suction Hose, with a short piece of pipe to interface with the Flare, that is if your unit has one. This of course is a new roll for the old Suction Nozzle, to be placed at the Sluice Box end of the Suction Hose. It may be a new configuration for the Nozzle but the results will be greatly appreciated.

Since the ban of 2 cycle engines in many States, the 4 cycle is muck heavier. To make it easier for the Small Miner to dredge, it would be great if a 4-cycle engine capable of 100 or 125 GPM, could be developed to run the 2.5" Dredge, with an improved nozzle design. If you think of it, it can be built, it is only a matter of time tell someone comes up with it.

Specific Gravity

By Reggie Gould

What is Specific Gravity and how does it effect the mining world? First we have to look at the guy who came up with the idea. His name was Archimedes of Syracuse, I guess his parents forgot to give him a first name handle. He was born in Dsyracuse, Sicily in 287 BC. One of his many discoveries was a basic principle that we still use today. If he placed a 1.0 once of silver into a beaker of water, it would displace more water or raise the level in the beaker, to a greater level then when he placed a 1.0 once of gold in the same beaker. The science seems simple, gold weighs more then silver so it must be geometrically smaller in size the 1.0 once of silver. The obvious conclusion is that the silver displaces more water then the gold.

A formula was derived, many years later to calculate the specific gravity. Dry Weight / Dry Weight minus the wet weight or SG = D/D-W. So how is this done? First we get a set of scales to cover the range of the item to be weighed. Then we tie a piece of thread to the item to be weighed and fasten it to one end of the scale. First we weigh it dry and record the weight, then we weigh it submerged in a beaker of water and record the weight. The beaker is not being weighed; we just want the weight of the item as it is suspended in water. Now plug the measurements into the formula and determine the specific gravity of the item. If the item were gold, then the specific gravity would range from 15.3 to 19.3. As we all know there are other trace elements in gold that would vary its weight. So lets assume that gold is a constant SG (specific gravity) of 19. How does it stack up to other minerals? Platinum is 14 to 19, silver is 10.0 to 12.0, mercury is 13.6, lead is 11.4, copper is 8.9, nickel is 7.0 to 7.8, pyrite is 5.0 to 5.2, arsenic is 5.4 to 5.9, magnetite is 5.2, hematite is 5.2 to 5.5, diamonds are 3.52, and quarts is 2.65. So what is the pattern here? The precious metals weigh more then the other minerals. The diamond is more precious then gold, but it is not a metal.

The SG of the precious metals works to our advantage, especially in a common gold pan. Ever wonder why we submerge the pan under water to clear off the overburden? SG is the key to separating the lighter material from the precious metals. When panning, all the material is submerged in water and the gold will sink to the bottom of the pan. As you are working the pan the riffles on the side of the pan keep the gold from being tossed out of the pan. The quartz and lighter minerals will float out as you work the pan. Who would ever have thought that a guy that lived over 2200 years ago would contribute so much to the gold miner? I don't think that most people give it a thought, they just know that the pan has to be underwater for the panning procedure to work.

All miners know that when panning, there is another mineral that likes to stay in the pan, it is magnetite and hematite (black sand), which have a SG of around 5.5. If you try to pan out all the black sand, then you will start losing the fine gold. It is better to save all the black sand with the gold and process it at home with specialized equipment.

A lot of people will ask if diamonds can be recovered with commercially available gold recovery equipment? I would probably suggest using a diamond magnet, which is a woman with a sharp eye for diamonds. It is easy to see that diamonds have a SG of 3.52 and quarts has a SG of 2.65. It is like separating fly poop from pepper. There are commercial dredges that mine for diamonds but I don't have a clue as to how the separating process works. I have seen natives work in the riverbeds in Africa, looking for diamonds and they appear to be using their eyesight more then SG.

So now we know that SG works in a gold pan, where else is it used? The sluice box, either in dredging or high banking, uses the principle of SG. The force working against SG is the air in the water, created by the turbulence; the other force is the forward motion, which adds inertia to the gold flowing down the sluice box. The various riffles decelerate the forward energy but tend to flip the gold in an eddie current pattern. If the gold is lucky it will sink to the bottom before it is washed out of the sluice box. This is a good case to present for the sub-dredge. It is submerged underwater so there is little air for the small gold to attach itself to. The gold is suspended in a water environment, so the principle of SG is at its best. For the sluice box to work underwater, it would have to have a cover over it to work properly. The cleanup would also be a problem; the box would have to be removed from under the float assembly, the cover removed and the sluice box cleaned out.

There have been sub-dredges on the market but they had a severe limitation on gold recovery. The recovery unit was just too small and inefficient. So why haven't the dredge manufactures put more effort into designing and making an underwater dredge? That is the $20 question, why? So getting back to the underwater design, do we now need a riffle system? Not really, all it would do is stir things up. Once the gold has settled to the bottom of the device, that is where we would like it to remain until it is recovered. At this point we need to come up with a trap that recovers the all the gold, flour, fine and course. The best trap I have seen, is just a series of slots for the gold to drop into a compartment surrounding it. The cylindrical dredge, with the slots, equally spaced apart, seems to work the best underwater.

The real trick with gold recovery is to allow the gold to settle without churning it back up. Gravity will take care of recovering the gold. Lets look back at the SG of gold of 19 and compare it to a SG of quarts of 2.65, it does not take a Rocker Scientist to figure out that gold is heavier then quartz. So what happens when the overburden is traveling down the cylindrical dredge? The gold sinks to the bottom of the dredge, where the traps are located and the quarts and other lighter minerals are swept out to the end of the dredge. The only unwanted element captured is the black sand with a SG of around 5. It may be possible to adjust the GPM (gallons per minute) to a much higher flow to overshoot the slots but in the process you will lose much of the fine gold and most certainly the flour gold. You might say, so I lose some fine gold and all the flour gold, so what? That is your choice of course but personally, I like to recover all the gold. Course gold is getting harder to find more and more, as more people are dredging.

New dredges will have to make adjustments in their designs to recover the fine and flour gold if they are to stay in business. I like to dredge the tailing piles left behind from the bigger dredges and pick up their fines and flour gold. It may be small potatoes to them but any gold I recover is big potatoes to me.

So wrapping it up, is specific gravity important? If you think it isn't, then you missed the point of the article. I helped build an experiment for my Grand Daughter for a school project. The project was designed to demonstrate specific gravity; we took 6 large test tubes and filled them with a very heavy viscosity liquid. Clear liquid soap was the final medial that we selected to use. We then put in a sample of various minerals. Gold, pyrite, silver, lead, quartz and talc. We then filled each test tube with the liquid soap to the top of each tube and put a cap on each tube. We built a wooden rail to hold all the test tubes in place and would invert the entire assembly. The gold would race to the bottom of the tube, where the other materials would descend according to their SG. The quartz and the talc were the slowest to descend. The talc almost floated on top of the liquid. The demonstration was simplistic but it demonstrated the specific gravity principle. I also built a clear plastic cylindrical dredge with a re-circulating pump to demonstrate the trap recovery system. Gold and overburden was placed into a hopper at the top of the assembly and far left of the dredge. When the material was poured in to the hopper, the gold and black sand could be seen dropping into the slots into the recovery unit, where the lighter material would be swept through into a bucket.

Questions about this article can be directed to Reggie Gould at  or www.gouldeng.com.


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