[Rhodes22-list] Dragging Chain

Bill Effros bill at effros.com
Fri Jan 13 01:51:51 EST 2006


Ron,

Sorry about the previous reply.  It was a bit short.

Please help with the physics.  It's not my long suit, and I would love 
to understand it better.

My problem is that in sailing publications we learn physics as 
interpreted by graphic artists--and one look at their diagrams tells me 
that physics isn't their long suit either.

Let's look at the Fortress site again. 
(http://www.fortressanchors.com/safe_anchoring.html)  The first 
illustration is fairly typical.  A boat in perfectly calm seas anchored 
in 20 feet of water with an anchor 70 feet in front of the bow on 100 
feet of rode--5:1 scope, beautiful catenary curve from the bow cleat to 
the bottom.  Anchor set deep with horizontal shank.

Only that's not what happens.  I've watched these clowns try to anchor 
this way.  Here's what really happens:  They push the button on their 
windless.  The anchor drops into the water, followed by 100 feet of 
chain.  The chain falls directly on top of the anchor, fouling the 
flukes.  Then they back up until they get too near another boat, at 
which point they go forward.

I watch the angle of their rode.  It is always 90 degrees, straight 
downward, just like the drawing.  It never flattens as it would have to 
if the boat were moving relative to a stationary anchor in order to bury 
the flukes into the bottom as shown in the drawing.

Then these boats start "dragging their anchors" until they motor off 
somewhere else.  Only they're not dragging their anchors, they're 
dragging their chains.  Their anchors are never set--they are just 
bouncing around the bottom with their flukes tangled in a ball of 
chain.  Not much different than Slim's cinder block.  Except that the 
ball of chain creates the illusion of a set anchor "dragging" from one 
place to the next.

A properly set Fortress anchor won't drag as long as the wind and 
current don't shift more than 60 degrees plus or minus--and even then it 
probably won't drag.

It's getting late.  More physics tomorrow.

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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