KEY CONSIDERATIONS AND CALCULATIONS BEFORE ANCHORING IN DEEP WATER

Anchoring in deep water requires the Master and deck team to carefully assess the water depth, required anchor cable length, windlass capacity, and loads acting on the anchoring system. This article presents practical principles and calculations for deep water anchoring, together with key considerations during anchoring operations, anchoring large vessels, and anchor watch, with the aim of ensuring the safety of the vessel, equipment, and crew.

Deep water Anchoring

All vessels to walk back anchor under power to the desired scope in depths over 50meters. To avoid damaging the windlass, the anchor cable should be kept up/down and paid out continuously under power till the entire scope is paid out. When paying out the anchor chain, the speed over the ground should not be more than 0.3 knots. Engine and anchor brakes should be used as required to avoid over-speed and/or overload of the windlass motor.

Permissible water depth for anchoring is determined by the capacity of the windlass. Generally, the windlass has a lift capacity of 3 to 4 shackles with an anchor.

Sample calculation:

1. Cape size Bulker:             Dead Weight: 181,412 MT / Windlass Capacity 382.4 kN 

     382.4 X 0.102 = 39 MT

Anchor weight: 12.075 MT

Chain weight: 1 shackle of 97mm diameter of common link: 0.206 t/m

0.206 X 27.5 M = 5.665 MT

4 shackles’ weight = 5.665 X 4 =22.66 MT

Total weight of an anchor and 4 SS of anchor chain

= 12.075 + 22.66 = 34.735 MT 4 SS X 27.5 = 110 meter.

Considering safety margin, friction on chain compressor, hawse pipe, anchor bell-mouth and additional chain weight from the windlass to the sea surface, the company policy of maximum permissible water depth for anchoring is 110m.Considering safety margin, friction on chain compressor, hawse pipe, anchor bell-mouth and additional chain weight from the windlass to the sea surface, the company policy of maximum permissible water depth for anchoring is 110m.

Regardless of the calculation mentioned above, some vessels are capable of lifting more than 4 shackles with an anchor (110m) based on their windlass capacity, however, a uniform policy on all managed vessels is considered to apply.

If the Master needs to anchor or is requested to do so at the depth more than permissible water depth, contacting the company is essential before anchoring.

Anchoring large vessels

• In shallow waters (depths less than twice the draft) extreme care must be taken.
• It should be noted that when lowering anchor under power excessive load on the anchor cable could cause damage or wear of the windlass engine and gearing.
• Master’s should be aware, particularly on large ship’s that Pilots are not always well experienced in this aspect. Pilot’s intentions should be established well in advance of anchoring preferably at the Master-Pilot information exchange.
• On completion of anchoring, the cable stopper is to be locked in position across the cable, the cable should be adjusted to rest up against the stopper and then brake engaged and windlass gear disengaged.

               On Anchoring /Anchor watch

•  Immediately upon anchoring, a fix on the anchor drop position should be made and the Ship’s swinging circle ascertained based upon length of the cable.
• At sufficiently frequent intervals check this position by taking bearings of fixed navigation marks and/or marks monitored by automatic radar plotting aids or readily identifiable shore objects.
• Other than visual means positions should also be checked using all other available means onboard, including radars, GPS and ECDIS at regular intervals. The swing circle radius should be used and is calculated by adding the length of the chain paid out and the ship’s length.