Example 3.1;  Forces calculations for a real anchor

 

In this example the earlier discussed theory will be used, for a real anchor at a certain depth, to check the different forces on this anchor.

 

 

First of all the vertical and horizontal forces ‘created’ by the fluke will be determined, the results of these different forces are shown in the figures below

 

Vertical fluke force 3D          
                                           

 

 Horizontal  fluke force 3D

For this situation the α = 0,15 (for example) β = 0,3.

So:

Fv =      5570N

Fh =      3470N

 

 

Now the forces perpendicular to the shank will be calculated for the case that:

B=        0,1m

y=        3,0 m

γ=        1900 kg/m3

            Φ=       0,611 rad

 

          = 1280N

 

After this, the friction force of the shank will be determined to be:

 

 

 with:

            =     4000Pa

            =       0.6m

 
















Forces on the anchor


When looking at the point where the anchor becomes stable a force and moment balance can be made out of all the forces on the anchor and mooring line. In fact this is the moment were the anchor reaches his maximum holding capacity.

 

Vertical equilibrium of forces:

 

 

Now one can calculate the maximum vertical component of the pull force Tv :

 

 

The maximum horizontal component of the pull force (Th) can be calculated in the same way:

 

 

Moment balance to point A:

 

                                                           

















Maximum pull force vs. depth


In practice:


When positioning an anchor, at each anchor position the maximum pull force can be determined.

For the next point this process will be repeated and the maximum pull force at that specific position can be calculated as well. As a result of these positions, a curve can be made of the holding capacity of the anchor at a certain depth.