[Rhodes22-list] Anchoring -- Phuzzy Physics

[Bill Effros]

No,

Because the distance between the ends remains the same on the bridge, 
but it changes between the boat and its anchor.

Bill Effros

Ronald Lipton wrote:

> Bill,
>
>   As someone who "knows physics"  I am always hesitant to answer
> these questions because it embarssing to get it wrong. Catenary
> curves are long lost in classes ~30 years ago.  In any case the catenary
> (discovered by Liebnitz) function describes the curve formed by a rope
> under  tension held at both ends.  The cantenary equations describe
> the curve in all of these situations, modified by the buoyancy of the
> rope and complicated by a second section which has a different density
> (chain).  This complcates the solutions quite a bit - but the basic ideas
> of the catenary shape are unchanged.  But the same equations should
> describe a suspension bridge and a light anchor rode.
>
> Ron
> ----- Original Message ----- From: "Bill Effros" <bill at effros.com>
> To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> Sent: Thursday, January 12, 2006 11:50 AM
> Subject: Re: [Rhodes22-list] Anchoring -- Phuzzy Physics
>
>
>> Herb,
>>
>> Got to work, no time now, glad people are thinking about this, will 
>> get back, it's called a "Catenary Curve" and boaters got all involved 
>> with it a few years ago--in fact if you look in the West Adviser of a 
>> few years ago they drew it into the anchoring adviser--but it was 
>> gone the last time I looked.  Somebody who knows physics explained 
>> that when you are pulling the rode it flattens out.  It's not the 
>> same as the cables draped over the towers of a suspension bridge--on 
>> boats our size with chain that weighs just a few pounds, in order to 
>> pull the anchor you flatten out the curve.
>>
>> And when you aren't pulling the rode the chain all falls into the 
>> muck (the starting point of this discussion) it doesn't form a 
>> catenary curve then, either, because we don't have all chain rodes.  
>> And our rode floats. And...
>>
>> Gorgeous day here.  Got to fix the roof.  Where is Rummy when you 
>> need him?  (Don't bother, I already know the answer.)
>>
>> Bill Effros
>>
>> Herb Parsons wrote:
>>
>>> "When your boat pulls your anchor into the bottom, the anchor rode 
>>> becomes taut.  The angle of the rode is exactly the same whether or 
>>> not you have any chain between the anchor and the point on your boat 
>>> where the rode is tied off."
>>>
>>> That's not true Bill. I've never been underwater and watched a boat, 
>>> but I HAVE towed a few boats. We were taught in the class I took (I 
>>> forget the term though) about the thickness of the rope used to two. 
>>> If you use a thicker rope, it not only adds strength, but weight. 
>>> When you pull the other boat, it pulls taught, then goes to a 
>>> semi-slack mode. I forget the terminology, but they showed pictures. 
>>> A tug pulling a boat with a lighter line had less of a curve, and 
>>> the angle of the pull was steeper. The instructor even mentioned in 
>>> the class that this was similar to the effect of using a heavier 
>>> rode on an anchor.
>>>
>>>
>>> Herb Parsons
>>>
>>> S/V O'Jure
>>> 1976 O'Day 25
>>> Lake Grapevine, N TX
>>>
>>> S/V Reve de Papa
>>> 1971 Coronado 35
>>> Lake Pontchartrain, Louisiana Coast
>>>
>>>
>>>
>>>>>> bill at effros.com 1/12/2006 10:43:39 am >>>
>>>>>>
>>> Dave,
>>>
>>> I, too, have read the "horizontal pulling force" theory of anchor 
>>> rode chain.  As soon as you think about your mechanics and physics, 
>>> you will know that it's rubbish.
>>>
>>> When your boat pulls your anchor into the bottom, the anchor rode 
>>> becomes taut.  The angle of the rode is exactly the same whether or 
>>> not you have any chain between the anchor and the point on your boat 
>>> where the rode is tied off.
>>>
>>> The anchors we use dig in properly at an amazingly wide range of 
>>> angles. Horizontal is not one of them.  Vertical is also not one.  
>>> That's what the whole notion of "scope" is about--putting the rode 
>>> at the proper angle to cause the flukes to dig in.
>>> These anchors set properly despite the fact that people put chain in 
>>> their rode, not because of it.
>>>
>>> Bill Effros
>>>
>>>
>>> DCLewis1 at aol.com wrote:
>>>
>>>
>>>> Bill,
>>>>
>>>> There's no way I'm an expert at anchoring, but I do know a bit 
>>>> about mechanics and physics. You're certainly right about an 
>>>> anchor's generally sinking, but the chain rode issue may be about  
>>>> how the anchor engages the  bottom, and stays engaged with the 
>>>> bottom -  not about sinking the anchor.  Consider an anchor on the 
>>>> bottom with sufficient chain rode, if there is a pull from the 
>>>> anchor line with any vertical component, the weight of  the rode 
>>>> can counteract the effect of the vertical pull and ensure  that 
>>>> forces on the anchor are horizontal (i.e. notionally parallel with 
>>>> the bottom).  I believe anchors are really made to deal with  
>>>> horizontal forces; the anchoring effect is achieved primarily by 
>>>> flukes, plows, or other appurtenances optimally engaging the 
>>>> bottom, and that requires horizontal forces.  Without the rode, any 
>>>> substantial pull on  the anchor line that has a vertical component 
>>>> may cause the anchor to lift  vertically, in which case it may not 
>>>> engage properly, or at all, with the  bottom.
>>>>
>>>> Dave
>>>> __________________________________________________
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>>>>
>>>>
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