[Rhodes22-list] Anchoring -- Phuzzy Physics

[Ronald Lipton]

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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