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Paul
There is a difference between digging and holding. But they are related in
that a light weight anchor must dig to hold. Digging is a function of bottom
strength, density, material type, angle between the flukes and the bottom and
several other factors. It is a very complex operation involving soil
mechanics. Simplifying it offers some danger of misunderstanding. That said
here goes for simplification.
The ability to set a rigid the angle between the shank and the flukes of the
super max essentially sets up a tripod between the shank and the two flukes.
This tripod determines the initial attack angle of the flukes. This initial
angle cause the flukes to bite into the bottom and the soil moving up the
flukes cams the anchor into the ground. The angle of attack changes through
out the digging process. In many anchors this tripod is unstable and during
the setting process the anchor rotates.
>> As scope increases, actual angle of the anchor to the ground decreases.<<
This statement is only true if all the chain is off the bottom. The angle of
pull on the shank depends on the weight of your rode, the scope and the load
you apply with the engine. As long as some rode remains on the bottom the
angle remains the same regardless of scope. For any given chain weight and
maximum engine pull, the larger the scope the longer some chain stays on the
bottom. As long as some chain is on the bottom, the direction of pull on the
anchor shank is optimum. On the other hand, rode on the bottom produces
friction between the bottom and the rode and reduces the load reaching the
anchor, reducing digging capacity. Thus we have the Goldylock's situation.
Too much or too little is bad. Optimum setting occurs when the last link of
the chain just lifts off the bottom. Notice here I said setting not holding.
In order to determine when the last link is lifted off the bottom you need to
know the weight of your rode, the length of your rode, the depth of water,
the freeboard forward, and the load on the anchor line. If you can't get all
of those factors and few boaters can, you need to guess. In general the
heavier your rode and the weaker your power plant, the smaller the scope for
optimum setting. Light rode, and strong power plant large scope. In
general, upward pull degrades digging faster than increasing chain on the
bottom, therefore I recommend erring on the side of too much scope. If you
drag anchor at full power, you maybe pulling all the chain off the bottom.
Once the anchor flukes become fully buried, then the crown and shank must be
forced into the ground. This slows digging in sands and firm materials.
Tests have shown that in these types of materials anchors rarely dig deeper
that this position. In soft materials (muds) they can be overburied. But
not that once the shank gets buried the chain trends upward relative to the
bottom. It actually forms an inverse catenary. The greater upward trend of
the rode reduces the digging component, or horizontal force on the anchor
flukes. For example if the pull is vertical no digging force is applied and
the anchor is rotated and comes out of the bottom. The buried rode also
reduces the force applied by the engine to the anchor. So in soft material
there is a limit to how deep the anchor can be buried.
The mechanics of an overburied anchor are complex. But experimental soils
mechanics shows that once the anchor is completely buried the holding power
efficiency begins to taper off and remains constant at a depth of around 12
times the fluke height.
>>It seems that at two different shank settings, one could achieve the same
actual angle by varying the scope.<<
I hope the above discussion has shown you that there is a difference between
attack angle and angle of pull on the shank.
>>They say that at some point, as the scope increases, the actual angle of the
shank to the ground gets to small and the holding strength deteriorates. <<
I don't know who they are but they are wrong. Dead wrong. Optimum holding
comes with as much rode on the bottom as you can afford. The Navy and
commercial ships effectively use chain as additional anchors. Any chain on
the bottom reduces the load felt by the anchor. Given enough chain the
entire anchor load could be held by just the chain friction along the bottom
and no load would reach the anchor.
More scope is always better for holding, but not necessarily for setting as
discussed above.
>>the scope would have to be constantly adjusted.<<
Scope adjustment is a factor of life especially in areas with large tidal
differences, but increases in wind velocity or boat density may also require
rode adjustment.
Don Dodds
North Pacific Research
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