[Rhodes22-list] Anchoring -- Phuzzy Physics

[Bill Effros]

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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