What is the weight increase of a dredge when hoisting it out of water?

[Crane operator]

I am a crane operator and have prob, a very simple question for any of you. I set a dredge in a pond and when I set it in the water it weighed 80,000 lb. My question is when I'm hoisting it back out how much will the weight increase before it is free from the surface of the water.

 

[anorlunda]

You are not filling the dredge with water correct? If you are lifting it full of water, we need to know the volume.

I think you are asking about the forces created by water surface tension, correct? If so, as a minimum we need to know the dimensions of the barge.

 

[Chestermiller]

You are not filling the dredge with water correct? If you are lifting it full of water, we need to know the volume.

I think you are asking about the forces created by water surface tension, correct? If so, as a minimum we need to know the dimensions of the barge.

Do you really think surface tension will contribute significantly to the forces at the dimensional scale of a dredge? It seems to me, the main forces involved are buoyant force and weight.

 

[JBA]

I think the ambiguity of the term "dredge" is creating a bit of an issue in responding to the question because that term is used interchangeably to describe either the the complete barge or simply the suction head and piping. If you are asking about the suction head and piping as long as the system drains as lifted the effect of water surface tension should not significantly affect its lifting weight.

 

[berkeman]

Yeah, it would help if @Crane operator could clarify which type of dredge he is asking about...

https://c.o0bg.com/rf/image_960w/Bo...be.com/Metro/Images/kreiter_dredging1_met.jpg

 

[anorlunda]

Do you really think surface tension will contribute significantly to the forces at the dimensional scale of a dredge? It seems to me, the main forces involved are buoyant force and weight.

I'm struggling to understand the question. In my mind, a dredge is a bunch of machinery on a flat-bottom barge. Why should the barge be heavier lifting than lowering? Surface tension or contained water were the only things that came to mind.

Sometimes, lifting a flat object from the surface creates suction underneath. I was probably wrong calling that surface tension.

 

[JBA]

Here is another type of dredge and the cutterhead is also casually called the dredge as well as the entire barge depending upon the discussion at the time..

 

[JBA]

If it is a clamshell or drag bucket, as long as the bucket is in the water, the water will add no additional weight; but, as the bucket clears the surface of the water it is going to depend upon a number of factors: The volume of the bucket; how quickly water drains from the bucket; the amount of soil in the bucket; and the amount of water retained in the soil in the bucket are those that come to mind.

 

[anorlunda]

Yeah, we really need the OP to come back and clearly define what he is asking about.

This picture is from a dredging service company. It is easy to visualize that whole rig being lifted by a crane.

 

[Crane operator]

Thank you yes water surface tension, sorry I should've included more info, the dredge will not be filled with water and the dimensions are 50 ft long and 12 ft wide. It will break the surface almost level within 1 degree and the water temp is about 60 degrees. Thank you very much for your time and reply

 

[Crane operator]

This is a self propelled floating dredge with just an auger to cut a trough through the pond bed so water will drain out of the pond, the trough will be cut to a gate that will not be opened to drain the pond until the dredge is out

 

[CWatters]

Is the underside hollow? If so you might have to lift roughly the volume of the hollow in water as well as the dredge.

To see what I mean take a drinking glass and submerge it in a bowl of water so it fills up. Turn the glass upside down underwater then try to lift it out.

The solution might be to lift it at a slight angle to allow air in and water out.

 

[anorlunda]

@Crane operator , You tell us. Why do you expect it to be any heavier to lift than to lower?

 

[berkeman]

I am a crane operator and have prob, a very simple question for any of you. I set a dredge in a pond and when I set it in the water it weighed 80,000 lb.

the dredge will not be filled with water and the dimensions are 50 ft long and 12 ft wide.

Oh my goodness, you mean you are lifting the dredging vessel out of the water, not the dredge bucket. I thought that was a very heavy bucket!

The solution might be to lift it at a slight angle to allow air in and water out.

Or to blow air into any open spaces on the underside before lifting it out.

 

[Crane operator]

It will be at a slight angle but it has to come out almost level, yes the pontoons are hollow they flood them as needed to level the dredge in the water while cutting so there may be a little bit of water but shouldn't be much because they will pump them out before I lift it, I guess I'm just looking for an equation that I can use for the dimensions of the dredge to find out what the water surface tension might be in a worst case scenario

 

[Tom.G]

My question is when I'm hoisting it back out how much will the weight increase before it is free from the surface of the water.

Surface tension will be so small in relation to the gross weight that it won't show up on any instruement you are using. (roughly 0.7lbs, if I did the math right using 73 dynes per cm times the hull periphery)

Any additional weight will be due to the amount of:

• water left in the pontoons
• + water in holding tanks
• + water clinging to the hull
• + water in the bilge
• + fuel left in the tanks
• + mud sticking to the auger
• + any barnacles or plant material

Figure water = 8.35 lbs per gallon (fresh water, 8.6 for salt) and, to be conservative, Diesel fuel the same.

The only other thing I can think of that could happen is if there is a downward facing concave section in the hull. This could give a 'suction' action during the lift. (but it is unlikely the hull has a large dent in it!)

Cheers,
Tom

 

[256bits]

A flat bottom plate hull will also experience a negative hydrostatic pressure "pulling" up some water as it leaves the surface of the water.
After all, a column of water of 32 feet under vacuum can be supported by atmospheric pressure.
Since the barge does not have extended sides downwards one certainly will not have to lift an extra 32 feet of water.
Instead as the bottom breaks the surface, the perimeter of water underneath shrinks from the perimeter of the barge towards the middle as it "drains" away.

The density of water is 62.4 pounds per cubic foot.
Area under barge = 50 x 12 = 600 square feet.
Per foot rise from water surface for that area, weight of water = 37440 pounds.
Making the assumption that the water perimeter begins shrinking after only a generous 1/4 inch removal from the surface= 780 pounds of extra water weight.
Wave action, a rounded edges hull, and inclined lift should reduce that to even a lesser value.

As per the list in the previous post, additional factors can easily overwhelm the negative hydrostatics.

 

[CWatters]

When I asked if the underside was hollow I didn't mean are the pontoons hollow, i meant is the underside/bottom concave. Otherwise I doubt there would be any tendency to lift water with the dredge.

 

[Crane operator]

Thank you Tim and thank you ask for your input, it is appreciated very much

 

[Crane operator]

A flat bottom plate hull will also experience a negative hydrostatic pressure "pulling" up some water as it leaves the surface of the water.
After all, a column of water of 32 feet under vacuum can be supported by atmospheric pressure.
Since the barge does not have extended sides downwards one certainly will not have to lift an extra 32 feet of water.
Instead as the bottom breaks the surface, the perimeter of water underneath shrinks from the perimeter of the barge towards the middle as it "drains" away.

The density of water is 62.4 pounds per cubic foot.
Area under barge = 50 x 12 = 600 square feet.
Per foot rise from water surface for that area, weight of water = 37440 pounds.
Making the assumption that the water perimeter begins shrinking after only a generous 1/4 inch removal from the surface= 780 pounds of extra water weight.
Wave action, a rounded edges hull, and inclined lift should reduce that to even a lesser value.

As per the list in the previous post, additional factors can easily overwhelm the negative hydrostatics.

Thank you

 

[anorlunda]

I feel the need to explain more the role of surface tension on this problem, because the mechanism is obscure. It was 50 years ago when I learned about this, and I can't find references, so I ask for indulgence because without references this post can't meet PF's normal standards.

Consider a flat object (like a barge) being lifted from the water. It will lift some of the water with it, as @256bits described in #17. At the edges, this water column will have a curved shape. The curve is a variation of a meniscus; a word more commonly applied to liquids in vertical tubes. This is depicted in a 2D cross section below.

Now, the question is how is this suction broken? One possibility is that the radius of the menisci will increase until they meet in the middle. This is depicted below. But increasing the radius increases the surface area of the menisci, which is opposed by surface tension of the water.

So, because of surface tension the radii stay roughly constant, but the locations of the menisci move inward until they meet in the middle and the suction is broken. That is depicted below.

This transient requires a downward flow of water. But it does not flow uniformly throughout the whole water column, but rather the flow occurs in a thin layer adjacent to the menisci. In the diagram above, water flows from locations A to locations B. It could be described as A transitioning from wet to dry.

How fast this occurs depends on the height of the object, on the properties of water, and on the properties of the brown object's surface. It will happen more slowly if the brown surface is hydrophilic than if it is hydrophobic. Based just on personal experience (not science) I estimate that a barge the size of @Crane operator 's lifted 1/4 inch above the surface, that it will take 1 to 5 minutes for those menisci to propagate inward and completely break the suction. Tilting the barge will also make it happen faster.

So that raises the issue of how fast the barge is lifted. If the lift is rapid, the column of water might be much higher than 1/4 inch which needs correspondingly higher lift forces. But if it is lifted slightly, and then paused long enough to break the suction, then the lift can resume again without the extra force.

 

[256bits]

Further investigation and I found this,
https://link.springer.com/content/pdf/10.1007/s10665-015-9832-8.pdf
for a flat plate being lifted from water, and is quite complicated.
I'll have to read it about 8 times more to get a glimpse of its meaning.

But in the Reference section was the cat lapping problem ( Reference 1 ), which not completely directly applicable to the dredge problem, is still quite interesting.
Since cats can't suck ( something to do with their cheeks ), they lap, using the adhesion of water to the dorsal side of their tongue, rather than spooning it in from the top side of the tongue. The flick of the tongue results in water being uplifted, and due to inertia enters the cat's mouth satisfies its thirst after several flicks, which are too fast for the eye to see. Fig 1. shows still frames of a cycle.
Froude number and cat's lapping.

More interesting is Fig 3. An experimental setup of a disk exiting the surface of water.
The final height is 3 cm. Estimating frame B height as less than 1/4 inch, one can see contraction happening to the water.
Next frames show the water continuing to rise upwards with the disk as the disk ascends, and more contraction until pinch off.

Of course, the experiment is done in still water.
http://web.mit.edu/preis/www/mypapers/how_cats_lap_reis_etal_Science_2010.pdf

 

[anorlunda]

We must be zeroing in, although the leap from dredge to a cat's tongue would have been hard to foresee.

Here is the fig 3 that @256bits referenced in #22. Compare that with my sketches n #21. I think we're on the right track.

 

[Crane operator]

Thank you all, this is a great forum and I appreciate everyone's input. I'm sure I will be back for more active in the future.