[Rhodes22-list] Pneumatic damper

[Bill Effros]

Alain,

I don't use Polypropylene for the same reasons others don't use it.  In 
Long Island Sound, 3 Strand Nylon Line floats--if you don't attach too 
much chain to it.

I have attached a copy of your excellent anchoring illustration 1.3 from 
your "Rode -- Static Behavior" section because it contains information I 
have never been able to understand. 

Regarding the all chain rode, it seems to me that when the boat dropped 
anchor and 15 meters of chain, 10 meters of chain fell right on top of 
the anchor, potentially fouling it.  Now, I know that you are backing up 
slowly when you drop anchor, but I watch people around here, and that's 
not what they do.  They push a button--the bigger the resulting splash, 
the better--then they back up to within a few meters of the nearest 
boat, and stop.  That establishes their turning circle, but few have the 
vaguest idea of how to set an anchor, and my guess is that most are 
lying on chain.

So how does the anchor get set as shown in the illustration?

As I understand it, (and I have never dived on an anchor while it was 
being set, although I have set anchors in very shallow water so I could 
watch what they do from the boat) the anchors are designed to land 
right-side-up most of the time.  If they land as designed, they are 
sitting high on their flukes, while the shaft is pointed downward, 
providing a stable tripod configuration with the anchor at rest, ready 
to go to work.

Pulling on the end of the shank is designed to drive the points of the 
flukes into the seabed.  The angle of the flukes moves the shank into a 
horizontal position when force is applied at long scope from the surface 
of the water, causing the flukes to dig ever deeper into the seabed.  
The more force applied to the end of the shank, the deeper the flukes 
dig, until the flukes are driven all the way in, and the shank is lying, 
horizontally, on the seabed.

At least that is my understanding.

As you point out, by definition, there is never a catenary curve in any 
type of anchor line because the line is not free to fall below the level 
of the seabed, so all talk about catenaries is rubbish in this context.

It seems to me that the only horizontal force on the anchor is provided 
by the relationship between the flukes and the shaft.  No anchor line, 
regardless of type, pulled from the boat, high above the sea bed, can 
possibly exert a purely horizontal force on the attachment end of the 
anchor shaft.  And there is no material difference in the angle of pull 
dependent on the type of rode.  Heavy chain rode will simply fall to the 
bottom and pull the boat closer to the anchor, as shown in your diagram, 
without putting any horizontal force on the anchor shaft, at all.  Not 
until the boat  provides enough force, and sufficient scope, to lift all 
of the anchor chain off the bottom is any force applied to the end of 
the horizontal shaft--and that force cannot be a horizontal force at the 
point where the rode is attached to the anchor shaft.

What have I missed here?

Bill Effros

Alain POIRAUD wrote:
> "I use floating anchor rode.  I use little or no chain.  I can see when I 
> am driving the flukes into the bottom because all of the line disappears 
> from the surface.  I can use a sextant to determine scope--by applying 
> sufficient force at the proper angle I can drive any properly sized 
> anchor into the bottom where it will hold until I free it.  The harder 
> the wind blows, the more deeply my anchor sets.  My line never sits on 
> the bottom, so it is never damaged by an "aggressive" bottom.  I can 
> anchor better without chain than with it. "
>
> Interesting technique, few people are using floating rope to anchor, as
> Polypropylene, has a much lower strength than Nylon ( Polyamide) (i.e.
> Polyamide 100 daN :mm² - Polypropylene 35 daN/mm²) But when someone has to
> anchor in coral, a very common technique is to attach a buoy (usually a
> fender) at the junction chain/rope, then, like with your polypropylene rode,
> the rode will not rest on the bottom, and if the boat is pulling on the
> line, the rope will disappears from the surface.  
> Nevertheless, I will still suggest to use a few meters (5) of chain 
>
> --
> View this message in context: http://www.nabble.com/Anchors-t1344828.html#a3950628
> Sent from the Rhodes22 forum at Nabble.com.
>
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>
>   
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