[Rhodes22-list] Dragging Chain

[Ronald Lipton]

Bill,

   I will try to do a proper calculation.  All the ingredients are
available in some form - force vs windspeed, holding force
vs angle, and I can calculate angle for a given force and rode/chain
combination.  What I can then calculate is the windspeed where an
anchor begins to drag as a function of various assumptions.
I don't have much time these days and it may have to wait
a few weeks.

Ron
----- Original Message ----- 
From: "Bill Effros" <bill at effros.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Friday, January 13, 2006 12:51 AM
Subject: [Rhodes22-list] Dragging Chain


> 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
>>>>> __________________________________________________
>>>>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>>
>>>>>
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