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Thread: 4" Trevs Subbie dredge.

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  1. #16

    Oct 2013
    417
    88 times
    Tim. . I have a 4 stoke gxh 50 engine. .

    On my 6 " subbie i have just a straight 1 inch pipe with no reducion on the end. .

    I was running it with a 6.5hp p180 pump .. i ran it about a bit less the 3/4 throttle..


    I was thinking of making the pressure inlet 5/8 because i think 1/2" just might not have strength too create enough suction..

  2. #17

    Feb 2016
    76
    51 times
    All Types Of Treasure Hunting
    A few years ago I saw a sub dredge on craigslist that used a jet-log design but put the Venturi jet after the sluice box. It kept the turbulence caused by the jet down-stream of the sluice where it wouldn’t effect the stratification of the material. That design would need to have a pretty good seal around the sluice to keep from loosing efficiency, but seems like a pretty good design.

  3. #18
    us
    Rob

    Oct 2012
    Kingfield, Maine
    316
    240 times
    All Types Of Treasure Hunting
    It has been my experience that going from a surface to a subbie reduces the power required to run to about 1/2. This means that a subbie dredge can use the same engine/pump combo to run a dredge with twice the hose volume of a surface dredge.
    In the case of the 6" subbie you used a pump/engine combo that runs a 4" surface dredge.
    4" hose has 12.5 square inches of volume
    6" hose has 28.2 (just over twice the volume)
    8" hose has 50.2 (4x the 4" hose)

    In the case of the 4" subbie you are trying to build the pump/engine combo is generally rated for a 2 or 2.5 dredge.
    2" hose has 3.1 square inches of volume
    3" hose has 7 (just over twice the volume)
    4" hose has 12.5 (4x the volume of a 2" hose)

    So stepping up from 2" to 3" is basically the same as 4" to 6". The problem of trying to go from 2" to 4" means some other system improvement like gaining efficiency from a better jet and/or reduced hose length is probably necessary to get good suction.

  4. #19

    Oct 2013
    417
    88 times
    Wow JS. Never heard of that kind of subbie pretty interesting.
    In my case i would be building a square sluice box which would make addapting a jet at the end pretty complicated.


    And Tim .. i was actually thinking the same thing..
    I was'nt even running the 6.5hp engine at full rpms. Just a bit higher from 1/2 throttle.
    Meaning i had to be running about 180 to 210 GPM. .
    Calculating from the specs of what keene says there p180 produces which is 300gpm . .
    What im going to do is change the hose to 1.5" and probably i could get away with 13' of hose cause i dont want to be too close to the engine and pump. .
    And ima try to go with a 3/4 " jet at the nozzle which i believe is half of the 1 inch jet that my 6" has. .


    My plan is trying to get a good suction around full throttle because i know i wont be able to get a good suction with anything less. I know is not so good for the engine but carrying a heavier engine with more things aint good for me hehe. .

    So isnt a suction nozzle that is pointing straight down the pipe more effiecient then a power jet ?? .
    A single log power jet sends the rocks to the other side wall of the jet; while the suction nozzle sends it down the pipe. .

    The only down side i see about a suction nozzle is the inlet pipe bend and pressurizing a foot or so before entering the jet which causes a pressure lost.

    This is why ima try to remove the suction nozzle bent inlet to a straight short one where all the water is going to do is focus into the jet. . This is the best way i could think of making the suction nozzle more efficient..

    Well today is monday let me see if i could get some things done.

    Thanks everyone for ya help. .
    Hopefully this time i could better results.
    Last edited by principedeleon; May 13, 2019 at 08:27 AM.

  5. #20
    us
    Oct 2009
    9,049
    8303 times
    All Types Of Treasure Hunting
    I dont think the suction nozzle loses much (if anything) due to the curve. That kind of thing comes down to radius of the bend (and quality of the bend). If the bend is too tight, then you are going to have problems with turbulence. Also, if the bend if poorly made and has wrinkles in the inside bend, that is going to cause issues. Looking at the wide bend of the typical suction nozzle, I don't see either problem.

    Another consideration, you are talking about basically using a jet-log as a nozzle but as you said, that input blasts water across the pipe (turbulence). The shape of the suction nozzle pushes the water straight back down the pipe so you will have much less turbulence in that very short section before the box.

  6. #21

    Oct 2013
    417
    88 times
    What im thinking of doing Jason is... i dont know if it makes much a difference but instead of using a bent inlet which is basically smaller then the hose diameter.. is bending the hose on top of the nozzle instead of bending a piece of pipe.


    The difference from this and a jet log is that the suction nozzle haves a bent which you can direct the jet nozzle straight down the pipe. .

    While on a jet log with the lowest angle you will always force the jet nozzle to the side walls.

  7. #22
    us
    Oct 2009
    9,049
    8303 times
    All Types Of Treasure Hunting
    can you draw it out? I'm having trouble picturing your plan.

  8. #23

    Oct 2013
    417
    88 times
    Is like cutting the suction nozzle inlet pipe like 3 inches before entering the nozzle.

    Now the hardest part is in bending the hose and making it stay bend. .

    But im thinking of welding a piece of pipe to feed the hose through to act as a support for the inlet hose.

    This route just seems more simple with less connections just straight forward. .
    In a future if it becomes a problem ill change the inlet pipe in metal but for testing practice ill try to make it as simple as possible.

  9. #24

    Oct 2013
    417
    88 times
    Hii everyone. Ii just got back from the river. .

    and the suction got wayyyyy better..
    it was sucking harder even in lower rpms.
    I was surprised.

    Now ima have to build the inlet pipe on the suction nozzle in the same diameter to try not get any lost ..

    Thanks everyone..
    When i have the finished dredge i will post pictures.

  10. #25
    us
    Rob

    Oct 2012
    Kingfield, Maine
    316
    240 times
    All Types Of Treasure Hunting
    Good to hear P.
    You asked about the comparison of a suction nozzle vs a log jet.
    The suction nozzle has a jet that is centered and directed into the flow- yes this is a better jet configuration.
    The log jet has an angled jet which increases turbulence losses but in reality is always considered a stronger jet in operation.
    The real difference in strength comes down to where they are located in the hose system. Let's say for comparison that good suction flow speed in a hose is 10ft per second.

    A suction nozzle must force the suctioned flow volume combined with the injected flow volume of the jet down a long stretch of hose to the sluice box. To get 10fps of suction flow the total flow speed would have to be increased to accommodate the injected flow volume (5 fps) and the total flow speed would therefore have to increase to 15fps which takes energy and has more friction losses so even with a centered jet the overall design requires more power.
    A log jet has very little hose if any after the jet so the losses incurred with increasing the flow volume after the jet have very little effect. So it's not a better designed jet as much as where it is placed.

    As you know there are many other factors to consider for choosing a style of jet. If wanting to improve suction strength and allow for some hose settling, I would put the jet about 5ft in front of the sluice (mid-jet) I would also build a dual, tri, quad etc.. anything to improve performance of the off-center log jet.
    Jason in Enid likes this.

  11. #26

    Oct 2013
    417
    88 times
    Okay .. thanks for explaining this i would always keep this in mind even tho. .
    I dont think i would return to a surface dredge. .
    Im in love with the subbies. .

    Im getting ready to run a 4" with a engine that fits in a grocery bag.
    It cant get any better then this. xD..
    Timberdoodle likes this.

  12. #27

    Feb 2016
    76
    51 times
    All Types Of Treasure Hunting
    Quote Originally Posted by principedeleon View Post
    What im thinking of doing Jason is... i dont know if it makes much a difference but instead of using a bent inlet which is basically smaller then the hose diameter.. is bending the hose on top of the nozzle instead of bending a piece of pipe.


    The difference from this and a jet log is that the suction nozzle haves a bent which you can direct the jet nozzle straight down the pipe. .

    While on a jet log with the lowest angle you will always force the jet nozzle to the side walls.
    That 180 degree bent pipe on the suction nozzle should have the same diameter as the flexible hose that is feeding it. It looks smaller because the hose is so much thicker than the metal tubing. On the contrary not all nozzles are the same, so maybe you’re referring to one i havent seen before.

  13. #28

    Oct 2013
    417
    88 times
    I dont know i never seen one in person. .in the pictures it looks that way. But i made mines the same size as the inlet.

    It works okay but when i dredge down past 3 feet i feel a suction lost.
    I believe the 3/4" jet is too large and im thinking of trying it with a 5/8" .

    Maybe thats the correct sizing for best suction.

 

 
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