# Vibratory System

Discussion in 'Hydrodynamics and Aerodynamics' started by gonzo, Jul 25, 2018.

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### MikeJohnsSenior Member

Yes by fundamental definition it's acceleration due to gravity. The context though is the storage of energy in a system against "standard gravity". Then the force component of the work done and the energy stored is F=mg where by the SI definition "the gravity of the earth" is denoted g . Practical Engineers or theoretical physicists ? Gonzo keeps a foot in both camps

If every post is covered to miniscule levels of definition we'd have a paper presentation rather than a discussion typed off the cuff in my case.

The fundamental issues that i was getting so frustrated about that I'd like you to address now:

1: An anchored vessel is a dynamic system not a vibratory system.
2: With chain and Nylon rodes, Surge energy can be stored gravitationally and as strain energy in the two cases.
3: A chain mooring with an offest horizontal pull has a force defined by a series of functions and is not sensibly modeled by f=kx.

I have problems with Gonzo's attempts to kill a discussion with abrupt one liners that are misleading at best and in my view completely incorrect. mainly "There's no such thing as gravitational energy" and " There's no such thing as strain energy"

As engineers we abbreviate terminology where meanings are crystal clear, in a system with multiple energy sources. We should be able to have a discussion using terms such as wind energy, wave energy, gravitational energy, strain energy when the type of energy ( potential or kinetic) is implicit in the very definition, and it's necessary to discriminate between them when there are multiple sources.

Last edited: Aug 4, 2018
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But now you say:

Sorry Gonzo, you don't get a free pass on this one. It is 100% clear what you said, as you made a point to say strain is not any kind of energy. That's not a typo.

I wouldn't say they are unprofessional. He was asking a very clear technical question and requesting clarification which was, even you have noticed too, clearly incorrect. Pointing out the constant obfuscation was required for clarification to ensure readers to this thread do not get confused. It was a reasoned technical debate which just may be an unpalatable truth to others. Just like i did with his 'strain' statement...clearly incorrect.

This forum is about advice/information and technical facts, not throw away one liners in order to give the appearance of knowledge.

Aaahh...now you're taking me back to my days of A-level physics, the days of endless questions and confusion

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### MikeJohnsSenior Member

Firstly just me being being pedantic about terminology: we term it damping. Dampening occurs from putting it in water!

As I posted already, the vessel can be excited by wind tide and wave energy. A combination of 3 excitation functions. You should be able to come up with a few very basic scenarios for overshoots for a start without trying too hard ? Craft with keels can even sail ( extracting the energy in the different velocities of the two fluids) around a fixed mooring point and will sail to windward. But even neglecting that consider:

A nylon rode stores a large amount of strain energy that is returned by accelerating the floating body at f=kx at up to 20% extension of the rode. Eddy making is minimal in the fwd surge, we design boats specifically for this. A vessel has a different damping function depending on the orientation of the principal axis to the direction of travel.
The small craft often gets to the equilibrium limit with it's principal axis at an angle to the rode. The rode can yaw the crafts mass into alignment, and then pulls the craft fwd now with a reduced frontal area and aligned with the direction of travel. Now at this point for example, stop the violent wind gust ...... . Damping coefficients are never going to close to critically damped for that type of system. A rode dragged through water does not produce enough damping on it's own. Chain does a much better job than nylon for reducing motion, it has a significant damping factor when dragged on the seabed.

Also synthetic ships mooring lines are quite different to 3 strand nylon usually used in anchoring systems for small craft.

Last edited: Aug 3, 2018
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### Mr EfficiencySenior Member

Right, so what's the difference between a shudder and a judder ? Or a shiver and a jitter ?

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### MikeJohnsSenior Member

shudder or should'a done something about the judder .

It does help to get the terminology on the same plane or words have quite different meanings, especially if arguments are devolving into theoretical definitions. When you dampen nylon it stretches more. If you apply damping you take energy out.

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### gonzoSenior Member

I didn't ask for a free pass. It was a mistake from not paying attention and writing in a rush.

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### MikeJohnsSenior Member

Now you are suggesting that a balloon lifting a bit of string is a sensible model for a catenary mooring ???

Sketch your basic free body diagram in some simple balanced condition with a constant force on the vessel and think about it.

The chain in the mooring system is being lifted through the action of a Horizontal Force. That force is related to the Tension Vector as a cosine function, while both the angle and magnitude of that vector change as a function of horizontal displacement.

And that’s without even starting to consider the real world dynamics.

It’s not linear. It's not a balloon.

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Indeed.

A simple cursory glance at the forces involved from any text book will tell you that.

Still...maybe he didn't mean a balloon, ..perhaps writing in a rush.

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### Remmlingerengineer

I do not understand the purpose of this thread.
It does not add any information, that could not be found in a textbook on the subject.
It is not even entertaining.
For me it is only a contest between alpha-males: "I know better than you", "I am the greatest".

My understanding was, this forum should be fun for everybody. I quote the moderator:
"Thanks for your understanding, and for helping to keep Boat Design Net a site where everyone can enjoy visiting to share the common interest in Boat Design, Boatbuilding, Boats and Boating no matter what their diverse backgrounds."

Uli

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Hi Uli

I don't understand the purpose of many threads actually, especially when they deviate into armchair schematics and/or conspiracy theories!

Agreed. But one can easily use that premise for about 90% of the threads on this website. Posters come on here as they are unable or unaware that such exists. Often wanting their one liner reply condensed from endless years of study and practice - not an easy task!

Which goes to, when you see posters using incorrect statements ignoring what is commonly accepted and taught and shown in textbooks and redefining definitions to suit their own narrative, do you just ignore it, let it go by so those coming on most likely end up going down the road path, or, do you, question it?

That's what i see/read here, not an alpha-male issue. (Although there is plenty of that on here...)
Definitions, ones which, as you point out, are found in many textbooks, when incorrect when denied, must be challenged. Do you as an engineer, not agree?

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### gonzoSenior Member

Actually, it is impossible to lift something with a horizontal force. Only a vertical force can lift an object.

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### gonzoSenior Member

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### MikeJohnsSenior Member

To recap: Gonzo is maintaining that the response of a vessel on a single point mooring on either elastic or chain rode can be modeled as a simple vibratory system.

I actually thought we might get there sooner rather than later by addressing a few of the fundamentals. Especially since Gonzo himself has asked for that approach rather than just being told that he’s wrong.

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### gonzoSenior Member

Not at all. I answered to a post where the member was concerned about the waves increasing the vibration on the system through excitation if their frequency was equal to the natural frequency of the mooring system.
I think that we should first address the fundamentals. For example, the definition of a spring, and whether a horizontal force can lift a body or not.

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### MikeJohnsSenior Member

What about the "action of a horizontal force" ?

Did you sketch the really basic free body diagram I suggested ?

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