Gravity Dredge Plans

Gravity Dredge Plans


By
Timothy McNulty
Copyright 1987
All rights reserved
Gravity Dredge Master Layout
My Experience with Gravity Flow Dredges
A Brief History of Gravity Flow Dredges
The Gasoline Dredge
The Gravity Powered Dredge
Available Head
Pipe Diameter and Type
Price for Various Types of Pipe
Length of Pipe
Suction Hose and Couplings
Number of Couplings
Number and Severity of Bends
Dredging Depth
Sample Survey Layout
Generating Your Site Plan
Sample Site Plan
Sample Site Plan Calculation
Designing Your Gravity Dredge
Efficiency Multiplication Factors
Graph for Determining Flow Velocity
Optimal Flow Velocity
Table for Converting Flow Velocity to Gallons Per Minute
The Watergate
Watergate Drawing
Sluice Design
Assembling Your Gravity Dredge
Operating Your Gravity Dredge
Closing Comments
Caution!


Gravity Dredge Master Layout

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My Experience with Gravity Flow Dredges

My experience with gravity powered dredges began in the spring of 1979 when my friends and I obtained a rather remote placer gold claim located on Grouse Creek, a tributary of the North Fork of the Middle Fork of the American River in the Sierra Mountains of Northern California. Just getting to our claim was an arduous task. From the town of Forest Hill we had about a twenty mile two lane highway, then about twenty miles of badly maintained dirt road, then about five miles of one of the most challenging stretches of four-wheel-drive road, and then finally about four miles of hiking over very steep an difficult terrain. We had considered bringing in our gasoline dredge, but I remembered a story my uncle Hank told me about some gravity dredging he had done back in the 1930's. I found that I was familiar with the principle, upon recollection, and thought that this location would be ideally suited for just such an endeavor.

Although my friends and I didn't strike it rich in those six weeks of fun and hard labor, we did manage to pay for all of our equipment, food, and supplies, even considering gold that summer was only fetching about $150 an ounce. In the summers since, I have had the opportunity to design, assemble and operate a variety of gravity dredges. My largest being an eight inch monster that required almost 270 feet of eight inch aluminum irrigation pipe. Some of my expeditions have been profitable and some have not, but they have all been richly rewarding.

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A Brief History of Gravity Flow Dredges

Gravity powered dredges have been used for mining gold for many years. Some of the first documented uses occurred during the Great California Gold Rush of 1850's, as evidenced by some U.S. patents of the period. These early designs were rarely very efficient or practical because of the limited selection of fabrication materials available to the mining engineers of that period. Modern materials, such as high strength polymer suction hose, efficient pumps, and light weight gasoline engines have greatly enhanced the efficiency of gold recovery for the small dredging operation.

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The Gasoline Dredge

Today, the typical recreational gold mining surface dredge uses a gasoline engine to power a water pump that forces a high pressure jet of water through a venture and into a short section of pipe that generates a negative pressure capable of pulling water through the pipe and thereby lifting the gold bearing sands and gravels from the stream or river bottom and delivering it to a sluice at the water's surface for recovery of the gold. This system, although very effective, requires the operator's frequent attention and maintenance of a gasoline engine, and pump. Frequently, for more remote operations, there can be almost as much time spent on maintaining the equipment and transporting gasoline as there is in the actual operation of the dredge. There may be additional considerations, such as environmental or risk of fire, that severely limit the practicality of the gasoline powered dredge. For some of these dredging sites, it is both possible and practical to eliminate the gasoline engine entirely.

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The Gravity Powered Dredge

Requiring no gasoline, pump, and very little maintenance, the gravity powered dredge functions not by magic, but by basic physics. The gravity powered dredge uses the power of water and gravity to do the same job as that of the gasoline powered dredge. With sufficient head, a gravity flow dredge can have as much or even greater lifting power as the gasoline powered version. The gravity powered dredge works on the same principle as a syphon, that is, that the weight of water in the pipe down hill (at a height below the suction end) creates a negative pressure in the pipe that pulls the water and the gold bearing sand a gravel through the nozzle, down the tube, and delivers it to the sluice. Although the available head determines the pressure available, other factors limit the dredges efficiency at any given head. These factors include the length of pipe, the occurrence and severity of bends, the relative roughness of the pipe and fittings, the type of couplings employed, and the design of the watergate and sluice box. Although the accompanying velocity charts have been generated from both calculated and experimental data, they are based on the ideal configuration consisting of a smooth pipe without couplings or bends. To better familiarize my reader with the significant criteria involved in the efficiency of the gravity dredge, I have enumerated just a few with a brief explanation of each.

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Available Head

Head is a unit of measure used by the mining engineer to quantify the pressure of water under static conditions. Even though head is pressure, its units of measure are 'feet', i.e. the pressure of one foot of water is equal to one foot of head. A pipe filled with water to a given height is said to have that amount of pressure, or head, equal to the waters height in feet. The gravity dredge takes advantage of the natural gradient of a stream or river to develop the pressure necessary to move the water through the pipe and thereby deliver the gold bearing materials to the sluice. Although the feet of head will determine the pressure at the exit level of the pipe when it is full of still water, the other factors listed above should also be considered. To decide whether or not a gravity powered dredge would be practical for your particular application, a brief survey of the terrain and gradient (steepness) of the stream or river you intend to dredge must be made. You might think that your potential stream site has plenty of available head, or perhaps you merely hope that it does, but it is essential to sketch or draw your site plan so as to determine its practicality for gravity dredging before you begin. This site plan will also help you to decide which dredge size and configuration would be best, and, the plan will better help you to decide where and how to locate your sluice relative to your dredging pool so as to economically optimize the gravity dredge's performance. One such method for generated a site plan is discussed latter in this text.

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Pipe Diameter and Type

Pipe diameter is, as you might guess, the most important consideration for determining the overall performance of the gravity powered dredge. Most gold dredgers select the size of their gasoline powered dredge based on investment (cost), performance, and site practicality. These same considerations similarly effect the selection process for determining the size of the gravity powered dredge. In fact, any of the common sizes of gasoline powered dredges may be converted to gravity power by using pipe of the same diameter as the dredge size. For example, a 5 inch gasoline dredge can be converted to gravity power by using 5 inch pipe.

Another important consideration is the type of pipe to be used. The choices are varied in both material and cost. I have found, from my experience, that aluminum irrigation pipe is both durable and light weight, and it is also the most rigid, but, unfortunately, it is not the least expensive, and it is susceptible to denting. If you can locate a source for inexpensive used or scrap aluminum irrigation pipe, I highly recommend it, especially for diameters of 6 inches or larger. Aluminum irrigation pipe is available in several grades or wall thicknesses. If you plan on straddling (suspending the pipe over) long distances (more than a few feet), or walking on your pipe, I recommend the heavier grades of aluminum irrigation pipe that generally run between 0.075 to 0.938 inch wall thickness for the 6 inch and between 0.0938 and 0.125 inch wall thickness for the 8 inch aluminum pipe (not quoted).

The least expensive and the most readily available is the PVC (polyvinylchloride) pipe available from your local plumbing or irrigation supply. The durability of schedule 80 is great, but its probably to expensive and heavy for most applications. Schedule 40 would be my selection where durability is necessary, and where the cost would not be a significant burden. The class 100 and 125 PVC though, when handled and used carefully, offer the most economical options. I would not recommend any of the lighter classes or grades of PVC or ABS pipes or DWV tubing, such as the class 63 sewer pipe, because of their inability to handle the rigors and loads of the typical gravity dredging application without breakage. A look through your local phone book and the yellow page directory should list several suppliers in your area that stock almost all the possible grades, classes, and materials you might need for your application. Here below are some pricings based on cost per 100 feet.

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Price for Various Types of Pipe as of 12/16/92

Prices quoted 12/16/92 from:

Ron Davis, Agricultural Sales Manager
A & T Sprinklers Inc.
3267 Monier Circle
Rancho Cordova, CA 95742
(916) 635-7850


Schedule 40 PVC Pipe
Class 100 PVC Pipe
Class 125 PVC Pipe
Light Duty Aluminum Irrigation Pipe


Schedule 40 PVC Pipe
Size
(inches)
Wall thk
(inches)
lbs/ft Price per
100 ft
Stock
length (ft)
2 0.154 0.703 $43.22 20
2.5 0.203 1.115 $66.69 20
3 0.216 1.458 $86.37 20
4 0.237 2.077 $127.83 20
5 0.259 2.977 $166.88 20
6 0.280 3.883 $224.52 20
8 0.322 5.661 $352.89 20
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Class 100 PVC Pipe
Size
(inches)
Wall thk
(inches)
lbs/ft Price per
100 ft
Stock
length (ft)
3 0.085 0.597 $35.69 20
4 0.110 0993 $59.37 20
5 0.136 1.518 $90.76 20
6 0162 2.153 $128.73 20
8 0.210 3.633 $230.63 20
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Class 125 PVC Pipe
Size
(inches)
Wall thk
(inches)
lbs/ft Price per
100 ft
Stock
length (ft)
2 0.074 0.346 $21.22 20
2.5 0.088 0.504 $30.16 20
3 0.108 0.753 $44.55 20
4 0.138 1.238 $73.20 20
6 0.204 2.693 $159.23 20
8 0.265 4.555 $267.30 20
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Light Duty Aluminum Irragation Pipe
Size
(inches)
Wall thk
(inches)
lbs/ft Price per
100 ft
Stock
length (ft)
2 0.050 0.360 $75.30 20, 30, 40
2.5 0.050 0.450 $78.75 20, 30, 40
3 0.050 0.550 $80.00 20, 30, 40
4 0.050 0.730 $116.00 20, 30, 40
5 0.052 0.950 $153.50 20, 30, 40
6 0.058 1.270 $200.00 20, 30, 40
8 0.064 1.880 $296.00 20, 30, 40
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Length of Pipe

For many site applications the length of pipe required to develop the necessary head can be considerable, but don't let this fact intimidate you. On one occasion I assembled almost 270 feet of 8 inch aluminum irrigation pipe. If the longer pipe lengths will capture enough head to offset the loss of efficiency caused by the increased friction of those longer pipes, then the greater length may be justified. Pretty simple? Well if it doesn't seem so, try generating a few trial site plans to get a feel for the complexities of the problem. You will notice that the flow velocity tables in section 7.0 are given relative to the head, in feet, and the length of pipe, in feet. The number you'll use for the length of pipe, in feet, will have to be adjusted for other factors, such as suction hose length, and the number and severity of the bends. When these factors are taken into consideration with your site plan, you a better understanding of the relationships between the length of pipe and flow velocities.
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Suction Hose and Couplings

The suction hose should be both flexible and light weight, but it must also be capable of withstanding the powerful negative pressures encountered when operating the gravity dredge. The same type of suction hose that is used for most gasoline powered dredges should also be sufficient for use with the gravity powered dredge. Additionally, the same type of suction hose used at the dredging end may also be used in short segments to connect the lengths of pipe required to develop the necessary head. For dredge sizes between 2 and including 3 inches, you will probably require about 8 or 10 feet of suction hose for the dredging end, and then as many short 1 foot sections as is required to connect all the pipe segments and the sluice. Each connection will require a hose clamp to achieve an air tight seal between the hose and the pipe's end. It is important the these connections be air tight so that air does not leak into the pipe and reduce the dredges efficiency. The larger pipe sizes will require heavy duty hose clamps to avoid damage or breakage. Contrary to what you might think, a leak at any point along the dredge pipe or couplings will cause air, or water, to be sucked in, and not the case that water might leak out, because the pipe is everywhere under negative pressure. Also, for the larger 4, 5, and 8 inch dredges, you'll probably need a longer suction hose at the dredging end, between 8 and 15 feet, and slightly longer coupling hose segments depending on the angle of the bend required between pipe sections. A good rule of thumb is to use a variety of coupling segment lengths from 1 to 3 times the pipe diameter so as to allow for varying bends.

When calculating the length of pipe for your dredge, the total length of the suction hose and couplings should be multiplied by a factor of 1.5 before adding to the length of pipe.
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Number of Couplings

The hydrodynamics involved by the process of moving sand, gravel and rock through this pipe system dictates that each time the flow of material makes the transition form hose to pipe, and the diameter is reduced slightly because of the wall thickness of the pipe, friction is thereby created and a loss of efficiency results. To allow for this loss of efficiency, you should add a length of pipe equivalent to 1 times the pipe diameter for each coupling segment for aluminum pipe, 2 times the pipe diameter for class 100 and 125 pipe, and 3 times the diameter for schedule 40 PVC.
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Number and Severity of Bends

The more severe the bend at each coupling, the greater the loss of efficiency. Slight bends of 10 degrees or less do not appreciably effect efficiency. However, for a bend of 15 degrees, you must add an equivalent length of pipe equal to 1 times the diameter of the pipe, additional factors are, 20 deg. = 2, 25 deg. = 4, 30 deg. = 6, 35 deg. = 8, 40 deg. = 12, and 45 deg. =20. It should be noted though that these factors are based on a bend radius equal to 2 pipe diameters, and that these factors may be reduced considerably from those numbers given when the bend radius is increased even lightly.
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Dredging Depth

Dredging depth will have no effect on the velocity or power of a dredge when there is only water flowing through the system of a fixed pipe length. This is because the greater depth is also accompanied by an increase in water pressure at that depth. However, this is not the case for the dredge involved in lifting alluvial materials from increasing depths. Because the sands, gravels, and rocks lifted by the suction power of the dredge are considerably heavier than water, this action requires greater power to lift this material from greater depths. Another consideration is that any dredge, either gravity or gasoline, must have more power available for increased dredging depths. Although increasing the power of a gasoline dredge can involve a process of replacing the pump and upgrading the horse-power of the engine, the power of the gravity dredge may be increased by simply repositioning the sluice so as to gain an increase in head and thereby increase the power the necessary amount. Although one might consider assembling the gravity dredge so as to initially have the greatest power afforded by the site, it is best to consider the velocity requirements of the sluice so as not to initially operate the dredge at a power that would cause the flow velocity to exceed that required for the efficient extraction of gold for the specific sluice you are using.
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Sample Survey Layout

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Generating Your Site Plan

You don't necessarily require a transit and surveying chain to adequately generate a site plan. A long tape measurer and level will suffice nicely. See figure 1 for an example of the site plan layout.

Begin by placing a level, of at least 2 feet in length, near the pool of the stream that you intend to dredge and at a height above the pool that will be comfortable to sight along the top surface of the level (after you have it level of course). Then, measure the distance from the top of the level to the height of the desired pool. Next, measure the horizontal distance going down stream every 10 or 20 feet and place markers as you do. Have a friend stand at each of the markers with a tall measuring stick in hand as you site along the top of the level. If your horizontal distance is not extreme, you should be able to accurately estimate the vertical drop by counting the number of feet marked on your measuring stick. Just remember to subtract the height of the level above the pool. Continue this procedure until you have mapped a sufficient distance of the stream to allow for a adequate site plan.

At this point you should have a good idea of whether or not your site will have sufficient head available for your dredge to operate with adequate power. If you have less than four feet of head, you probably don't. If your head is less than what you require, don't panic and give up just yet. In some cases it is practical to build a small dam with the overburden from the working area of the pool you intend to work and raise the water level a foot or two. Though the work involved in raising the level by a foot or two may be considerable, it may still be worth it. After all, you had planed on moving a lot of overburden anyway. I frequently build a dam for the working pool about a foot higher even if there is adequite head, so that I can have the pipe exit the pool at a level below it's surface. This is a great help when it comes time to prime the system and start the dredge (to be discussed later).
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Sample Site Plan

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Sample Site Plan Calculation

The goal of the site plan calculation is to determin the overall efficiency of the gravity dredge components and assign a value equivalent to the length of perfect pipe for any given dredge size (diameter). For example:

Given 8 inch diameter aluminum irragation pipe:
Total length of pipe = 12' + 6' + 12' + 16' + 6' = 63'
Total length of suction hose, water gate, and couplings = (12' + 1' + 1' + 2' + 2' + 4') X 1.5 = 37.5'
Added length due to coupling transitions = No. of couplings and hose segments = 7 X 2/3 {8"/12"} = 5'
Added length due to coupling angular bends = [{15 deg}(1 X 1) + {25 deg}(2 X 4) + {40 deg}(1 X 12)] X 2/3 = 14'
Total equivalent length of pipe = 63' + 37.5' + 5' + 14' = 119.5'
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Designing Your Gravity Dredge

With your site plan drawn, and after you have determined that you have adequate available head to power your dredge, you should next determine what size dredge you intend to assemble. You might just want to design a gravity dredge for your existing sluice box, I highly recommend this. I have built sluice boxes from scratch and have largely have been dissatisfied with their performance. Considering the time and money it took to built them, I would have been better off just buying one. You will next try to determine where the sluice should be located so that when the dredge is assembled you'll have adequate power. For now, just pick a location with about 6 to 8 feet of head and begin to draw in your suction hose, pipe segments, couplings and sluice. See fig 2 for just such a layout. After you have your layout, use the tables in the next section to calculate the resulting flow velocity. If its not enough, redraw your layout with the sluice place far enough further down stream to develop a foot or two more of ead.
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Efficiency Multiplication Factors
Item              Mult.

                  Factor

Length of Pipe    1.0

Suction Hose      1.5

Couplings         1.5



Length of Pipe to Add

(Pipe diameter times...)



Each Coupling     2

10 Degree Bend    0

15 Degree Bend    1

20 Degree Bend    2

25 Degree Bend    4

30 Degree Bend    6

35 Degree Bend    8

40 Degree Bend    12

45 Degree Bend    20

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Graph for Determining Flow Velocity

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Optimal Flow Velocity

The optimal flow velocity for a gravity flow dredge depends upon the specific requirements of the sluice being used. Most sluice manufacturers publish specifications for flow in gallons per minute. In general, you will find that most modern dredging sluices will require a flow velocity of between 10 and 15 feet per second to operate at their designed flow volume specification. Sluice designs tend to vary however, and it is recommended that you check the flow requirements for your specific sluice and adjust the layout of your gravity dredge so as to operate within the manufacturer's published specifications. Converting velocity (in feet per second) from the graph above to gallons per minute is relatively easy given that there are 7.481 gallons per cubic foot of water, and the volume of water in a one foot length of pipe is equal to the crossectional area of the pipe. This should roughly be pi multiplied by the radius squared. Volume multiplied first by the velocity, and finally, by 60 seconds per minute will equal the predicted flow of the gravity dredge in gallons per minute. I've calculated out conversion factors for the common dredge sizes in the table below. Just multiply the conversion factor from the table below with the velocity deturmined from the graph above to yield the flow in gallons per minute.

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Table for Converting Flow Velocity to Gallons Per Minute

Converting Flow Velocity
to Gallons Per Minute
Size
(inches)
Conversion
Factor
2 9.791
2.5 15.30
3 22.03
4 39.17
5 61.20
6 88.12
8 156.7
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The Watergate

The watergate is a very important device. Without it, it might prove very difficult or impossible to prime the dredge. Priming the pipe is necessary to start the dredge going. The watergate should be constructed of materials and in such a way so as to close the pipe, and the flow of water, at a location sufficiently beneath the level of the pool so as to allow the dredge to start when the pipe is filled with water. The watergate must stop the flow of water, but it doesn't necessarily have to seal water tight, it just has to seal well enough to allow the pipe to be filled with water and for the air in the pipe to be minimized. The watergate is usually located close to the sluice, but this may not always be necessary or practical.

If your are building an 8 inch dredge with 10 feet of head (a very powerful dredge) you might find it difficult to close the watergate and fill the pipe with water without having a catastrophic disassembly of your couplings, pipe and watergate. This is because 10 feet of head is a lot of pressure for the hose clamps to withstand without allowing the flex couplings to slid off the pipe. A better location, perhaps, for the watergate in this case might be at a position where it would be at about 5 or 6 feet of head. This should be sufficient pressure and volume to start most gravity dredge configurations. My experience with designing and building watergates, and the numerous resulting catastrophic failures that have resulted, has led me to a rather simple and yet effective watergate design. This design consists of a few feet of pressure hose and a hinged clamp. In principle, this design can be used for any size of dredge and should withstand the resulting pressure upon closure for all but extreme heads. You'll need a section of pressure hose about as long as 4 to 6 time the diameter. The type of hose that I recommend is goes by the brand name of Nylobrade Pressure Hose and is available from Keene Engineering Company, part numbers NH-2, NH-3, NH-4, etc... Sizes larger than 4 inches are not normally stocked by Keene and may have to be ordered. Or, you may try your local rubber products supply distributor. Nylobraid Pressure Hose is a fairly common and readily available product and your phone book's yellow pages should list a supplier that carries it.

The flow of water and material is stopped and the pressure hose forms a seal when the two hinged 2x4's are closed on the center of the hose. This method works well and is almost maintenance free, unlike various gate valves that I have constructed in the past. The greater the diameter of your dredge, the longer you'll want to make your hinged 2x4's so as to allow you to close them down easily on the hose. But be sure to allow a small gap between the 2x4's when they are closed that is slightly less than the thickness of the hose when fully compressed. The 2x4's, once closed tightly on the hose, may then be tied, bolted, or latched together to keep the watergate closed. See fig 3.
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Watergate Drawing


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Sluice Design

As mentioned before, I highly recommend that you use a sluice box that was designed for a gasoline dredge, and merely hook it up to the end of your pipe and couplings. Converting a gasoline dredge to gravity power is a easy as assembling the pipe, couplings, and watergate to replace the gasoline engine. If you don't already have a gasoline dredge that you can convert to gravity power, I suggest that you purchase a ready made sluice box, or a kit that can be easily assembled. Ideally, the only difference between a gasoline dredge sluice box and the gravity powered sluice box is the angle at which the flow enters the box. Most gasoline dredge sluice boxes are designed to accept flow from about a 30 degree angle below the horizontal. Though this is not ideal for the gravity dredge, it will work only slightly less efficiently than the ideal. The ideal for the gravity dredge is to have the flow entrance angle nearly horizontal and without a deflector baffle, but instead a flow dispersal flap. Interestingly, this is the same design used be the latest generation of Keene Engineering Company gasoline dredges. These dredges are designed to operate more efficiently by allowing the sluice to be placed very close to the surface of the water, and so avoid the efficiency loss associated with the deflector baffle and abrupt flow direction changes of up to 300 degrees. I recommend the Keene Engineering Company catalog as a source for the sluice box, Nylobraid pressure hose, and suction hose, and other gold dredging specific hardware, and yet the local hardware store will probably be a less expensive source of many of the various mechanical hardware, such as hose clamps, that you may require. You can write or call Keene for their latest catalog:

Keene Engineering Company
9330 Corbin Avenue
Northridge, CA 91324
(818) 993-0411
Fax (818) 993-0447

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Assembling Your Gravity Dredge

Depending on the size of the pool that you'll be working, you may need the ability to easily move the first section of pipe after the suction hose at the dredging end. If you do allow for this, just be sure that you can still have the pipe exit the working pool at a level below the surface. Although you may have the pipe exit at a level as much a several feet above the pool, the problem is that this will require much more effort to initially fill and prime the pipe before starting the dredge. When the pipe is placed so as to exit from the pool below the surface, all that is required to prime and fill the pipe is for the watergate to be closed and the suction hose brought up to the surface level so that the air may escape as the water flows into and fills the pipe. Once the pipe is full, and most of the air has been allowed to escape, the dredge may be started when the watergate is opened and the weight of the water in the pipe will start the dredge going.

To continue with the assembly, the sections of pipe after the suction hose are connected to each other with a short section of flexible coupling hose. These coupling hose sections should be cut from the same type of hose as is the suction end. It is a good idea to vary the lengths of these coupling pieces between 2 and 6 times the pipe diameter so that the shorter pieces may be used where very little bend is required between the adjoining pipe segments, and the longer pieces may be used for the joints that require the greater bends. As previously mentioned, try to avoid extreme bends between segments, or, where highly angular bends are required, allow for much longer coupling segments so as to increase the radius of the bend. These considerations should allow the gravity dredge to operate more efficiently and also help prevent the likelihood of an undesirable catastrophic disassembly. So, from this point its fairly simple; assemble the watergate and sluice box at the positions previously mentioned, connect the nozzle to the suction end, prime the pipe, open the watergate and start gravity dredging.

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Operating Your Gravity Dredge

The gravity dredge, when set up properly, can be a dream to operate. Its quiet, doesn't require constant attention, and allows you to switch modes of operation, from dredging the streams sands and gravels, to moving large rocks and boulders, without having to worry about shutting off and then restarting the engine when your ready to go back to dredging.

Cleaning your sluice box is just about the same procedure as with the gasoline dredge, except that instead of shutting off the engine, you merely close the watergate. Just as with the gasoline dredge, the gravity dredge will sometimes have rock jams. Here, the procedure for clearing the jam is about the same as it is with the gasoline dredge, disassembly and removal of the obstruction. Although rock jams may be more common with the gravity dredge, because of the increased length of the pipe for which the rocks must travel, these jams may be minimized when the operator is aware that most jams occur when he allows a great number long slender rocks to enter the nozzle. Here, it is prudent for the operator to be selective about what he allows to enter the suction nozzle. Some rocks you just know will cause a jam, so be you will learn to take the time to carry them away instead of being tempted to suck them up. With a gravity dredge, you are not under the pressure of not wasting gasoline while you go about clearing the obstruction. Just close the watergate and disassemble the segment that is obstructed. It helps to have spent a little more money initially, by investing in quick leaver hose clamps, so as to speedily execute these procedures.

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Closing Comments

This writer has constructed and tested a variety of gravity powered dredges with both surface and subsurface sluices. Some of the preceding information and tabular data for certain dredge sizes has been extrapolated from experimental data gathered from other dredge sizes. Some of the data contained in the velocity flow tables has been calculated from formulas and tables from published sources (Peele, 918).

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Caution!

The power of a gravity dredge can, under certain circumstances, be very dangerous. The reader must understand the potential dangers inherent in using any powerful suction device under water. This author and publisher takes no responsibility for injury or financial loss incurred as the result of following these instructions, assembling, or using a gravity powered dredge.

Thank you for your interest in gravity dredging. Please write me about your gravity dredging experiences. I'll be glad to answer any questions you might have, and I would very much enjoy hearing from you.

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