2" or 2 1/2" dredge - is there much of a difference?
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  1. #1
    us
    Oct 2013
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    2" or 2 1/2" dredge - is there much of a difference?

    I'm planning on buying my first dredge, and I'm not sure whether I should buy a 2" or 2.5" dredge (at this time I don't want anything larger). I'm not too concerned with the weight difference between the two. I'd prefer to buy a 2 1/2" dredge, but if I bought a new dredge the 2 1/2" is about $500 more expensive than the 2". And if I look for used dredges, it all depends on what sizes are available out there. My question - is there really a large difference in the amount of material you can process between the two sizes, seeing that they're so close in size?

  2. #2
    us
    Jan 2012
    Chickasha,Oklahoma
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    2 1/2 will run twice as much
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  3. #3
    us
    Author of a book about finding gold in Colorado

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    2" or 2 1/2" dredge - is there much of a difference?

    The nozzle size is equal to 3.14x the square of the radius of the nozzle. Algebra!
    So a 2 has a suction area of 12.56 square inches and a 2.5 has 19.625 square inches. Bigger but only about 50%-ish bigger.

    A 3 would be 28.26 square inches and a 4 would be 50.24 btw. The size difference starts to be dramatic as we scale up.
    Last edited by KevinInColorado; Jan 09, 2020 at 01:10 PM.
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  4. #4
    us
    Jan 2015
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    Quote Originally Posted by KevinInColorado View Post
    The nozzle size is equal to 3.14x the square of the radius of the nozzle. Algebra!
    So a 2” has a suction area of 12.56 square inches and a 2.5” has 19.625 square inches. Bigger but only about 50%-ish bigger.

    A 3” would be 28.26 square inches and a 4” would be 50.24 btw. The size difference starts to be dramatic as we scale up.
    Math is great! Fantastic insight Kevin!
    Math is even better when anecdotal evidence is added to the story: in our gold club (Delaware Valley GPAA, now defunct) we used to take trips to Virginia and New Hampshire as a club, and everyone that owned a dredge would bring theirs. Members had 1.5", 2", 2.5" and 3" (I had a 3" Hydroforce nozzle, which I love). Just like Kevin's math predicted, my 3" would produce a much larger tailings pile than the others, but maybe the more important anecdote is that the owners of the smaller dredges would all look at the material my son and I had moved and say "wow, I wish I'd bought a 3" nozzle."

    I know you asked about 2" vs. 2.5", but my advice would be to buy the largest nozzle/dredge you can. (Though I'm very happy with the 3" and feel no need to go larger).

  5. #5
    us
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    There wont be much difference and your first dredge should not be a new one unless you have plenty of disposable income. A lot of people ask me this question and my answer is the same. Buy the biggest dredge you can afford that is small enough for you to handle and legal in your area.
    When people first get into this they tend to start small but ALWAYS move up to larger gear. My first real dredge was a 3" and that damn thing nearly worked me to death because if the rock wont fit up the hose, you have to move it by hand. It took us a full week to dig a 10 yard hole with the 3". Now with my 5" I can do that in a day.

    Rule of thumb on size difference is: with every inch of nozzle size you increase, you double your production capacity. So if a 2" will move 2 yards per day then a 3" will move 4 yards.
    A 4" dredge is a good balance between large capacity and compact size and can be moved and operated by one dude.
    The other drawback of a 2 or a 2.5 is that these usually are not equipped with air...which limits you to 3 feet of water or less

  6. #6
    us
    Mar 2012
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    Algebra? I'll never use that when I get older! wait...what?
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  7. #7

    Jun 2017
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    Quote Originally Posted by KevinInColorado View Post
    The nozzle size is equal to 3.14x the square of the radius of the nozzle. Algebra!
    So a 2” has a suction area of 12.56 square inches and a 2.5” has 19.625 square inches. Bigger but only about 50%-ish bigger.

    A 3” would be 28.26 square inches and a 4” would be 50.24 btw. The size difference starts to be dramatic as we scale up.
    You might want to recheck your math and use radius and not diameter.

  8. #8
    us
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    Quote Originally Posted by akflyer View Post
    You might want to recheck your math and use radius and not diameter.
    I was going to say the same thing. good catch.

    2" nozzle = 3.14"s
    2.5" = 4.9
    3" = 7.1
    4"= 12.56

    Even though going up in suction hose size means moving more volume, its true benefit is the ability to process larger gravels. Every rocks you can't "suck" is one you have to move by hand.
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  9. #9
    us
    Love Gold

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    2 no supplied air
    2.5 supplied air
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  10. #10
    Charter Member
    us
    Look at the Historical Gold Mining photo albums on my page

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    Circumference = 2πR, Kevin is correct.
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  11. #11
    Make America Great Again

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    my 5 inch dredge has a 4" ring on the nozzle and will move 4 times the materiel as my 4" mini
    of course my pump has a 4 inch intake and puts out 500gpm. better suction?

    what I wrote down from my files: water output from jet to sluice
    whatever the pump output to the jet, times the average efficiency of 1.7 of the jet
    at about 1.7 will give the gallons going to the sluice if the pump puts out 500gpm X 1.7 = 850gpm

    I would go with the 2 1/2" with the 3.5HP motor then you could put air on it later
    my old dredge
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    Last edited by winners58; Jan 11, 2020 at 06:07 AM.
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  12. #12

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    Quote Originally Posted by Reed Lukens View Post
    Circumference = 2πR, Kevin is correct.
    Agreed, but if you run the numbers he used diameter not radius. A 2" hole does NOT have 12.5 sq inches of area. Even using common core math you cant get to that number.
    Bonaro and KevinInColorado like this.

  13. #13
    Charter Member
    us
    Look at the Historical Gold Mining photo albums on my page

    Jan 2013
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    Quote Originally Posted by akflyer View Post
    Agreed, but if you run the numbers he used diameter not radius. A 2" hole does NOT have 12.5 sq inches of area. Even using common core math you cant get to that number.

  14. #14
    us
    Oct 2013
    Colorado
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    Thanks everyone for your input. I thought I would get mostly anecdotal stories, but it turned into a trip back to Algebra class, didn't it? Who said we'd never use it when we "grew up"?
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  15. #15

    Aug 2015
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    I converted a 1 1/2" Jobe combo to 2". My creek is pretty shallow and it seems to work fine. Sure, with a buttload more water, larger would be in order but for what we have, the upgrade is good enough.

 

 
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