[Rhodes22-list] Anchoring -- Phuzzy Physics

[Ronald Lipton]

Bill,

   There is a nice site which has a detailed calculation of rode angle
as a function of chain lenght and weight:
http://www.johnsboatstuff.com/Articles/anchor.htm
he shows that 20' of chain increases the tension necessary to
lift the end of the rode off the bottom by a factor of 2.5

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 10:43 AM
Subject: Re: [Rhodes22-list] Anchoring -- Phuzzy Physics


> 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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