[Rhodes22-list] Anchoring -- Phuzzy Physics

[Ronald Lipton]

Bill,

   I would think that a catenary would be used to describe both.
The catenary equation describes a flexible rope supported at
it's ends and under uniform graviational load.  This best describes
the suspension bridge (or telephone wires ...) but the same equations,
with modified end constraints can be used to describe an anchor
rode.  As I said the solution in a rode/chain situation is more complex.

By the way, thanks for bringing this up, I haven't though about this
stuff in years.  Maybe I will actually do the calculation.


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 12:47 PM
Subject: Re: [Rhodes22-list] Anchoring -- Phuzzy Physics


> 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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