The ground tackle system of a vessel is an important safety feature and should
work reliable under all circumstances.
It is therefore necessary first to understand the requirements put on the
system as a whole and also on the specific implementation.
Especially due to the strong forces imposed on the ground tackle system under
extreme weather condition but also due to the aggressive environment in which
it is used, the choice of material with respect to corrosion resistivity
and maximum strength is an important consideration.
Normally mooring coves will be sheltered against the prevailing winds and the open sea.
In unfavourable weather conditions however, e.g. when the wind turns from
shore to offshore and without shelter from the open sea, the mooring system of
anchored vessel may become exposed to strong forces due to wind and swell.
The force (Fwind) the wind imposes on a fixed object
is proportional to the square of the wind speed (v_wind):
Fwind = 0.5 * ro_air * A * v_wind2
with ro_air=1.3kg/m3 and A the effective area of the body in the direction of
the wind (which for a sailing yacht is about 0.7 * the geometrical area)
To obtain an idea of the order of magnitude of these forces, the following table
shows some experimental values for a 40 ft. sailing yacht of average beam and windage:
wind speed wind force
30 Kts 550 daN
60 Kts 2200 daN
The forces imposed by the swell depend on the state of the sea, the shape of the
underwater line of the vessel and whether the waves are breaking or not (which is
related to the depth of the water at the anchorage)
Dynamics of the Yacht:
Besides the rather static approach of wind and swell load, in gusty winds
the vessel may also "sail at anchor".
This happens because - especially for modern fin-keel yachts -
the bow drifts faster than the rest of the yacht, resulting in a half
circle swinging of the yacht in a gusty wind.
The load imposed by this movement of the vessel
(backward acceleration in the gusts and abrupt slowing down
at the moment the anchor rode is stretched to its maximum lenght)
is considerably larger than the forces obtained through a simple static
load analysis due to wind and swell.
Some kind of damping construction may reduce these shock loads.
For example a strong but elastic line might be used to shunt a part of the chain.
This line can also be used to fix the chain to the bow cleats, which
are constructed and dimensioned for mooring the vessel.
This setup will effectively take away the load from the anchor windlass,
which normally is not constructed to hold the vessel at the mooring.
Since so many factors are involved in the forces acting on the anchor system,
a detailed analytical analysis yielding some useful results does not seem to be feasible.
And while poring over the different anchor tests in nautical magazines it becomes clear
that also gathering meaningful and comparable experimental data is hardly feasible.
From the analytical and experimental data that is available, only a very rough estimation
for the extreme worst-case loads on the anchor system can be given.
For an average 40-ft sailing yacht these maximum loads may have to be taken into account:
wind speed max. anchor load
30 Kts 2500 daN
60 Kts 10000 daN
The maximum breaking force of the ground tackle system must be dimensioned accordingly.
When choosing a suitable anchor, remember that the anchor system also includes shackles, rope, chain,
and deck cleats and that every single item must be dimensioned to deliver the required strength.