[Rhodes22-list] Anchoring -- Phuzzy Physics

[Rik Sandberg]

Bill,

What you seem to be having trouble getting a handle on in this reply is 
that lifting the weight of all that chain requires energy. Much more 
energy than just lifting a nylon rode. That energy is taken from the 
motion of the boat as it tightens the rode, ie it slows. Therefore it is 
moving slower when it snubs up against the end of the rode and is less 
likely to jerk the anchor out of it's set.

Rik

Bill Effros wrote:

> Herb,
>
> It is true that a catenary curve develops between a tug and its tow, 
> but it only happens because both are moving.  As you noted, when the 
> tow is stationary, and the tug is moving, the line becomes completely 
> straight before the tow starts to move.  Once both are moving at the 
> same speed, the weight of the tow line creates a catenary curve.  This 
> can become dangerous if the tow starts to travel faster than the tug.
>
> If you look at Ron's example of wind force on a properly sized and set 
> anchor, a chain rode would be completely straight--just as it would be 
> if you attached one end of a piece of chain to your pickup truck, and 
> the other to an eye bolt embedded in a boulder.  Your truck would not 
> be able to budge the rock, but the chain, which initially was lying on 
> the ground, would form a straight line to your truck.  There wouldn't 
> be a catenary curve except for a brief moment as the truck took up the 
> slack.
>
> 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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