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netmaker
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Join Date: Mar 2007
Location: Cajun Country , USA
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Friction Coefficient

09/21/2012 3:25 PM

Formula for Friction Coefficient .

Recently one of my competitors reported that certain trawl nets had a Depth/ Friction Coeffcient of -150m/ .60 Another had -300m/.40

Now I have used Bollard Pull to decribe the tension on a net at such and such a depth, but it was easily determined by attaching an Electronic Load cell to the 45mm dia. steel cable Tow warp ( cable holding everything in the water as it is pulled). A number came up and it was recorded. We went faster, the number increased. We went into a heavy current, the number increased. We went down current, the number decreased. We picked up a load of shrimp and fish and the number went back up again.

I am curious as to how a Friction Coefficient can be obtained on a net that can span 20-30 Meters across the bottom touching the sea bed?

This is not the first time I have seen this, but now its bugging me as to how they determine something like that at 150m ( 480' + PLUS of water). In 3 decades, I've never been asked for this but it is NOT going to hurt if I know how this number is obtained on such large, non-linear gear running over a most uneven bottom.

* can it be determined under laboratory conditions witha scale model in a Test tank ... and then correlated (estimated) onto the larger gear???

Comments please.

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passingtongreen
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#1

Re: Friction coefficient

09/21/2012 3:35 PM

Not exactly an answer but your increasing resistance is drag not friction. If they slowly drag a piece laid across the bottom at those depths they could find the friction because the drag would be non-existent. That sounds a bit dicey given the buoyancy factor.

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Joshi
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#2

Re: Friction coefficient

09/21/2012 3:39 PM

Please study these for clue: Measurements of groundgear friction on sand and mud

DNV-RP-F109: On-Bottom Stability Design of ... - DNV Exchange

DNV-RP-H103: Modelling and Analysis of Marine Operations
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netmaker
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#5
In reply to #2

Re: Friction coefficient

09/21/2012 7:00 PM

Namaste,

I am not smart enough to understand these formulas. However, I see the tractor pull method in the picture and that looks more like what we would do on the water with the Load Cells. It is too pristine of a condition for me . However, I am not an engineer.

I suspect that this Friction number can be very variable depending on depth, bottom, load on the nets themselves and what type additional gear is loaded on to the net.

Thank you for the references. there is some information there that might come in useful later on. The formulas are over my head. Thank you again.

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lyn
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#3

Re: Friction coefficient

09/21/2012 3:54 PM

I think they're blowing smoke.

Friction (drag) increases with both water depth, and speed of the object moving through the water.

Drag Coefficient - Engineering ToolBox

I think your method is more descriptive. So many pounds of pull per square foot of net gives a clear picture of the forces in play.
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netmaker
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#7
In reply to #3

Re: Friction coefficient

09/21/2012 7:14 PM

I have used the DRAG coefficient in years past. The test tank guys love it... and write bible and verse by the results. I am more convinced by actual real world testing.

I do not think the Friction thing is something you can just drop a number on. The bottom is never flat, there are always cross currents and junk in the net ( detritus) and the whole rig is hardly ever fine tuned so that each section is pulling the same.

IN the pictures provided by Joshi, they were dragging a weight with a tractor over a muddy or sandy stretch of tidal land.... Excellent analogy and demonstration but without water I can not see the correlation to Real World. However, it made me think of a method to test other issues I deal with... thank you Joshi.

I think I am just going to stick with what works for me ....Bollard Pull / sq/ft of net/ speed and depth.

Of course, if someone comes into the discussion with some hard evidence I will be open to suggestion as always.... never too proud to learn something new.

Thank you for the comments.

FYI

Winter hit Louisiana this week...it officially dropped below 70F at night ...Gumbo weather!

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rashavarek
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Join Date: Jul 2012
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#4

Re: Friction Coefficient

09/21/2012 5:13 PM

Formula for Friction Coefficient

(X #greenpeace supporters + weight of POS boat they are in) x number of hypothetical Tuna in the net / (government regulatory affairs agent payoff+donation to dophin safe whatever)=the amount of friction percieved by NBC nightly news

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netmaker
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#6
In reply to #4

Re: Friction Coefficient

09/21/2012 7:03 PM

Ha Ha! Actually, you might want to revise the TUNA perceived option....There are a lot less Perceived tuna these days. Dogfish or Asian carp might fit in better.

Thank you for the humor.

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