Dampening Cable Vibration

Change the resonant frequency of the cables. Sleeves, dampers, weights?

...or spiral strakes?

I don't think it is wind induced, not at 2.75 -3 knots, it must be induced by the boat and/or the frame. If you clamp weights on the cable, you could move it's position for optimum effect.

The frame seemed to vibrate at a, more or less, regular frequency; I wonder if that is the frequency of vibration for the cables. That might need a different solution. Try changing the natural frequency first.

The cables are partially in and out of the water.

They are very tight.

How would you use a weight?

A sleeve would be how long and/or how would it connect ?

Would a heavy and short SPRING work if we attached it to the boat side and had the cables shackle into the spring?

Are the cables always used in the same position (so much in the water, so much out of the water)? Are the conditions of its use always the same, except for the boat speed?

Several ways can be used to change the dampening of the cable system and have been suggested here. Two other things that you might try are 1) put an automotive shock absorber in line on the cable or 2) attach some loose hanging quarter-inch line to the cable (perhaps two feet in length) every foot or two.

You could try a larger line size, say 1/4", to change the frequency...You could mount rubber gasket under cable connection mount on boat...

I see two possible sources of the vibration, the cable vibrating at it's natural frequency or completely controlled by the relative movement of the frame and boat. I doubt that springs would help except that they would change the natural frequency, autos have dampers to absorb the energy but I think you don't have enough movement for that type of damping. You could try a weight clamped on near, but not at, the middle, the middle would yield a harmonic. You could try a spring loaded wire from the boat going more or less to the middle of the cable as a spoiler, or you could try putting a garden hose over the cable.

The cables are always in the same position.

The frame has to fold back for travelling from point A to point B, so there can not be too much in the way.... as the netting will collapse with the cables as the frames are folded back for travelling. A tangle means a lot of wrassling with the net either out on the water or driving the hull against the bank and getting into the water to undo the mess.

several things here sound very do-able :

Rubber snubber gaskets at some point between the thimble and the shackle.

Spring loaded ( like a smaller version of a shock, that is used to tie a boat down on a trailer....) attached so as to give some play but NOT enough to collapse the fame work back onto the boat.

* The frames must stay within a few degrees of perpendicular to the boat..... the Top outside cable will always puller the hardest. The bottom outside, no. 2 and the bottom inside the least.

A weight on the cables if we could find one that is smooth.

Covering with a rubber hose would actually dampen the effect??? That sounds the easiest for sure. I am curious as to how that works?

Thank you all for the suggestions........

Please let us know the results of your efforts.

Using a hose cover will change the total mass of the cable, and also the resonant frequency and add some mechanical damping.

Might just work.

....I just called them and told them to head for Lowes or Home Depot and pick up 50' of Garden Hose.

I know I ask you all a ot of dumb @$$ questions and y'all are so very good about answering back in plain language.

But what does the hose actually do? I have no idea about harmonics except I think its wave lengths ? ? ?

Thank you

I'm new to this discussion so forgive me for repeating anything that has already been already covered.

Your scaffolding and guy wires in your net supports are like the strings in a piano. When an impulse hits them they want to vibrate a different frequencies. If you add weight to them or change the tension then a different frequency will be changing the preferred resonant pitch to possibly a frequency not being generated by boat and water. If you wedge a soft material (flexible hose) at key points in the rig you will be adding a dampener like the dampener pad that rests on the strings of the piano wires that prevents them from resonating when the pianist is not playing. This will prevent things from vibrating even if the boat and rig are at the same frequencies.

In the realm of thinking out of the box, maybe the fish are responding to the sound from your propeller and not this rigging. Despite living on a big island I have zero nautical experience. Maybe a propeller with a different blade pitch or number of blades could answer this. Maybe the net could be closer or beyond the bow of the boat so the carp are already in the net.

Thank you Red for the explanation.

We are now looking into rubber hose and possible some kind of rubber 'connection' in the cables. The frames and nets have to remain fixed where they are for hull stability.

Long island is a BIG island..... i actually did some work for one of the Power and Light companies back when they were looking into those turbines in the East river.

I think Hurricane Irene messhed up their schedule though for using the sampling nets I supplied.

Thank you.

The cable vibrating at its own natural frequency is like a violin string, it is continuously excited by the bow rubbing over it. In your case, the continuous excitement would be from the vibration of the ends, the boat and the frame, or from the wind and water going by. If you change any of the parameters, weight, length, tension you change the natural frequency.

Imagine two boats linked together in the manner of a two car train, give them both masts and tie a line between the mastheads. When the stern of the lead boat and the bow of the trailing boat are in a trough, the mastheads move toward each other and the rope goes slack. When they are on the crest of a wave, mastheads move apart and the wire is pulled tight. In this case, the vibration (the stretch then relaxation) of the wire would be at the frequency of the waves, The equivalent would be the frame vibrating relative to the bow of your boat.

The hose adds weight but not strength, it changes the shape in the air and water and it should have a damping effect. Since we know little about the details of the case, this is a cheap fix that has a chance of working.

...thank you for the explanation. It makes sense. Most likely their is a frequency ( if that is correct) produced by the cables that is causing these highly sensitive fish to stir excitedly.

It some how might be the answer to the first thread that I started about the electric trolling motor NOT scaring the fish.

In 30 years + of messing around in the net ( and design) business.....this is turning out to be a Sherlock Holmes mystery.......... thank you.

Hi Netmaker,
We all think and it is pretty sure that your problem is flow induced vibration, also read Von Karman vortices, relating to flow conditions and resonant frequencies and its harmonics.
It is the same phenomena where the wind can induce transmission lines to vibrate and fail if not dampened.
You can't modify your speed, so you are stuck with the Reynolds and Strouhal Nos. and you need the flexible member (cable) for on-board handling.
I can recall an experience in the past where we had an economiser tube bank located in the exit gas duct of a power boiler and the tube failures (fatigue) were caused by the gas flow conditions exciting flow induced vibrations of the leading row of tubes. The downstream tubes weren't affected as the vortices were disrupted and collapsed by the leading tubes. In this case, the solution was to divide the gas path by metal plates, creating narrower flow paths away from the natural frequency and harmonics conditions.

I think your solution to breaking the resonant frequency and the vortex shedding is going to be a trial and error exercise. If your cable was a tube or rigid section I would suggest you spiral wind some large diameter rope (like hawser rope in a barber's pole style), widely spaced pitch which is less likely to allow vortices to form. You may find large diameter rope, think ship's bollard line, with coarse spiral surface may be enough in place of the cable.
The ideal may be 9 x 1' segments of airfoil section, like mini-otter boards, pin-jointed connections to give you the flexibility to manoeuvre and stow.
We'd all like to know how the results go.

Thank you for the comments on this too.

Right now the crew is going to try the rubber hose thing. There is a new video out that I will post..You can plainly see the vibration.

Now, with the information given me, I see that frame HAS to be changed out for something better. The squared vertical supports HAVE to go. You can see the water pushing and coliding.

The whole thing comes down to accessablity and handling . There are limits that we have to stay within for use, speed, travel down the road, etc. A big Ol' shrimp boat is just tied to the dock and everybody drives home.

IN Shrimp fishing with these type rigs which are much larger, there is no issue. UNless you use plastic coated cable and the paltsic tears up....the shrimp are just NOT bothered by anything. Plus the shear weight of the shrimp rigs is 3x's that of these little frames so i am now figuring that what you all have said holds true....the weight changes the harmonics.

Commercial shrimp rig. frames are 16' wide x 10' deep. 4 cylinder GMC diesel.

Circa 1990's when we were heavily involved with commercial fishing.

All 2" Sched. 80 and heavily reinforced across the Top section.

5/16" cables for support.

* Ha Ha ...I snuck one past the Computer Gremilin... seems I get 1 ( one) shot at any posting and then the EDIT bar goes blank. The Image icon now seems to be staying active more ....go figure! *

Adding a weight in-line ( provided it did not create a lot of drag) may still be do-able as is the tear drop shaped frame work.

we have to stay above 2.75 knots with the nets to keep the fish inside for the required 30 seconds or so ( to wear out their BURST speed). With outboards and in a freshwater river environ, speed can be dangerous due to logs and sunken debris.

Thank you for the comments....more stuff later.

Using the rubber sleeving to dampen the steel rope's vibration works like any other damper. It dumps energy by heating up and if it can dump enough energy the vibration is damped.

You asked whether 2 springs would work. Only if the springs also ocillated and there was a damping mechanism to dump the energy eg friction, hydraulic, electro resistive...

Harvest the energy to run your boat lights. Done.

I just learned that springs were installed without my knowledge a few weeks ago. They tangled in the netting very badly when the frames were brought up along side the boat for travelling. The senior researcher canned the idea.

However, they are supposed to be re-rigging with the rubber hoses today.

thank you.

We have the same problem on our diveboat ladder, and Lyn and Passington and others have it right - add a weight (mass to be determined), not at the center of the cable length. To experiment, have them hang a small anchor or some other 10-20 pound weight on a shackle, with the shackle over the cable. Tie a tag line to the shackle so the weight can be pulled toward the boat. Start with the weight just out of the water so you don't have to deal with that added drag. Get up to speed you want, pull weight toward boat till vibration reduces or stops. Mark spot, put a crosby clip on the cable at the appropriate spot, and on the runs all the guys have to do is let the weight slide out to the crosby clip where it will hang till they haul it back in.

If the weight reduces the vibration a lot at a "sweet spot" but it's still not enough, get a bigger weight.

Don't be afraid to use two weights if you need to dampen more than one frequency.

I've enjoyed reading your Asian Carp stories. These videos are really amazing and educational. Please send a note to the admins to see if they can help you with your posting problem. Hate to lose you over something silly like a browser issue.

*Can not post ANY type of jpeg, png, gif, dwg or anything thing else....CR4 screen freezes*

This was me in Internet Explorer. Did you try Firefox yet?

cnc jim

I tried Google Chrome and messed up my setting so badly I had issues with Auto CAD and FOXIT PDF which turns my DWG into readable format for my customers.

I am really hesitant to touch anything.....I can take a ripped trawl net with as much as 75% of the netting gone....and make a forensic pattern ie, rebuild the dang thing to within 95% of what it was originally....no design , pattern or anything else required.

Its the only gift God and the angels gave me. With this computer, I am just a key banger....ignorant in so many aspects.

The admin. told me that a lot of folks had success with IE. I have Windows 7 and Office 2010.

I keep hoping someone will send me a note about how to fix or adjust. I am sure you are correct about suggesting Fire fox. I am seriously afraid it will mess up like Chrome did and I might NOT be able to get back to where I need to be. My fear of the unknown repercussions and ignorance in this for sure.

Glad you liked the carp stories.....if CR4 had an Environmental section I'd feel comfortable posting even more strange stuff that gets run through this little family business of ours.... but that's another story... ha ha ha !!

I once tried google chrome too. I was in a mess for days and ditched it, so I understand your reluctance to try something else. However, maybe when this vibration problem is sorted, give Firefox a try, it installs and works sweet and smooth.

cnc jim

Did you try firefox yet?

cnc jim

Take a look at "Strouhol Vortex Shedding." (If I spelled it right!) I worked with things in air, not water, where it occurred at very low wind velocities. It could make poles vibrate violently if the shedding matched the pole frequency. You will also notice that high-tension electric transmission cables usually have some sort of damper (Stockbridge is one brand) to keep the wire from galloping. I don't know what happens in water, but increasing damping; changing tension, wire size, mass per foot, etc would change the resonant frequency. Most of these were mentioned above.

Thank you too for the comments.

I think we have an idea of what we are faced with and which of all these really great options we can actually get away within our limited parameters.

One of the fishery crew mentioned that she would like to try just idling into these fish and see what happens. I received their nets back via FEDEX and they have asked me to install some additional lift points and such.

Next week they will get a chance to test this. For sure, the lead researcher stated that his review of their video cameras shows the largest of the carp ( 20-30 lb. fish) moving away well in advance of the nets ....while the rest seemed not to budge until the frames were right upon them.

Thank you very much for locating this study.....it sounds just like what we are facing.

Have you heard of/used this type of tech?

http://www.hydrowave.com/freshwater/faq.htm

No sir.... I've never heard of that. i wonder if its like that belching noise that drum fish and some grouper make. I once heard a story of how Back in the day, a sub tracked a 'sound" for miles and miles only to find out it was giant grouper.

The concept of imitating a prey noise like that is very good idea for sport fishing.

Ol' Skool hook and line ( slaughter pole it was called) Tuna fishermen used to use high pressure nozzles to spray water right at the sea to imitate bait fish being gobbled up. An occasional bucket of anchovies was thrown in. It worked very well for years. With the ban on Ring nets ( purse seines) in some countries territorial waters, we may see its return.

This gadget here would certainly do it.

Once again its pointing to vibration and the harmonics that was so well explained here, as a huge element in catching the Asian carp.

* FYI They just eat plankton zoo and phyto....no meat..... but if they could imitate the sound they make at the surface when they are "gulping water and air" while they are muching on plankton....then we'd have something. Hmmmmm it might be worth investigating.

Thank you for the site.

If the problem is oscillation from a length then break up the length, use opposite sided pressure like a wire straitening roller system along the length of the cable for tension. Every 6 inches put a guide up, and then one down, this will prevent natural harmonics in the cable (oustside of the gap distance)

If oscillation is an issue then it's simple NEVER EVER allow length of structural material to reach a natural harmonic, and if one does then simply obfuscate the path so the harmonics self dissipate.

Springs are one of the WORST harmonic dampeners in existence, they simply add their own characteristics to the system, and can often make it worse, you need to critically damp. Friction from sending the cable through a set of rollers/rods increases and the fact that any two points of contact from the cable itself to the rigid frame decreases dramatically low frequency harmonics.

Your in my head mate.

Practicality of solution implementation is a big deal though...normally.

I'm looking forward to see how the broad spectrum damping of the rubber hose sleeving works out.

Next week is when they run the new set up.

I've sent them two sets of nets:

12x5 x 24' long

8x5x 23' long

the 8x5 frames are much sturdier and beefed up...still just round pipe for trials.

the 12x5 frames are being dismantled and rebuilt also.

They have 1/4" and 3/8" pnuematic air hose for dampeners.

we'll see and I'll report the results.........

thank you