[Rhodes22-list] Dragging Chain

Bill Effros bill at effros.com
Thu Jan 19 09:58:24 EST 2006


Ron,

I think this question was answered by the guy who developed the Spade 
anchor, even though he did the calculations empirically.  It seems to me 
that he really understands the issue, and everything he said makes sense 
to me (although I worried about his spelling and syntax until Michael 
mentioned he is French, and the site is clearly a poorly translated 
version of his ideas.)

His conclusion is that you need chain if the bottom is chewing up your 
line, and only for this reason.  You need enough chain so that it will 
form a catenary curve at rest--a short length of chain just pulls your 
line to the bottom where it is more exposed than it would be if there 
were no chain at all.  (The shaft of the anchor holds floating rode off 
the bottom.)

For all of the other benefits commonly ascribed to chain, he says that 
chain actually makes the situation worse:  it weakens the line; becomes 
bar tight under load; and defeats the catenary curve effect when you 
most need it.

He says everyone, regardless of boat length, should carry and deploy 
roughly 75 feet of chain if they encounter aggressive bottoms.  This is 
more weight than I want to deal with, so I will not carry this amount of 
chain unless I see wear at the bottom of my rope rode--which I have 
never seen.

I continue to believe the problem with "dragging anchor" is really a 
problem with "dragging chain", but it doesn't take a whole lot of 
physics to demonstrate the point.  Everyone tries to anchor far enough 
away from their neighbor.  (Except for some people who anchor much too 
close to me.)  The wind and current aren't constant as different people 
anchor, and everyone doesn't have the same ability with regard to 
spacial relations, and the swing of other boats.

Most people anchor by dropping the anchor in the center of their 
perceived circle, and backing away at 7:1 scope to set the anchor.  At 
this point, the rode, whether rope or chain, must be fully extended in 
order to properly set the anchor.  The weight of the chain, and the 
release of the tension on the rode then pulls them back toward the 
anchor.  This reduces the scope, but few people then pull any of the 
rode back on board because they think that will pull them even closer to 
their anchors.

When the wind starts blowing, the rode extends again, and it appears 
that they are "dragging anchor".  They frantically pull up chain and 
shorten scope until they actually do rip their anchors out of the 
bottom, and then they try to reset the anchor under impossible 
conditions.  That's what I think.

Bill Effros

Ronald Lipton wrote:

> 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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