Q. What are ordinary stockless anchors?
Ordinary ‘stockless’ anchors are very commonly used design on ships. They comply with the rules or practice of the Society.
The mass of the heads of stockless anchors including pins and fittings are not to be less than 60% of the total mass of the anchor.
Q. What is High Holding Power (HHP) anchor?
A ‘high holding power’ anchor has a holding power of at least twice that of an ordinary stockless anchor of the same mass. When special type of anchors designated ‘high holding power anchor’ of proven superior holding ability are used as bower anchors, the mass of each anchor may be 75% of the prescribed mass required for ordinary stockless bower anchors. Up to 25% reduction in anchor weight is allowed, when high holding power anchors (HHP), such as the AC14 type, are provided.
Q. Do you also have anchors of Higher Holding Power?
Yes, this type of anchor is called ‘super high holding power’ anchor. It is an anchor with a holding power of at least four times that of an ordinary stockless anchor of the same mass. Such an anchor is suitable for restricted service ships’ and the mass is generally not over 1.5t.
Q. How will you prepare anchor for letting go?
The engineers, electrician, the forward crew, etc are duly given notice. The anchor station crew and the responsible officer must reach forward in time, with flash lights, walkie talkie, etc. The power is switched on. In case of hydraulic machinery, the pumps are started after due precautions. One of the first things that must be done is to re tighten the brakes before doing anything on the forecastle. The sea passage is done with brakes, cross bar / devil’s claw, wire lashings, etc on. Hawse pipe cover is secured and the spurling pipe cover with cement is applied to prevent water from entering, during a sea passage.
The gypsy is not kept in gear in the sea passage. The drums are run to ensure that the windlass is operational. While removing the lashings, the gypsy should be engaged in gear prior opening the lashings. The lashings and cross bar are removed. The hawse pipe cover and spurling pipe covers are removed. Now, the anchor is only on the brakes and the gears. The brakes are eased and the anchor is lowered under power i.e. walked back.
The anchor is walked back and hove through a few metres to take care of the possible problems due static friction. Depending on the depth at anchorage, the anchor is held on brakes or on gear. The anchor is kept ready for letting go, generally, just outside the hawse pipe.
Q. What will you do upon getting an order to let go?
The bosun must be instructed not to let go anchor until the go ahead is given by the officer in charge on forecastle. He must not open the brakes simply on haring the order on walkie talkie. This is because there may be a boat, etc under the bows. Any kind of obstruction must be ruled out. Chief officer must look over side and upon ensuring that all is clear and it is safe to let go, the bosun is instructed.
Q. How will you know that the vessel is brought up?
With the slight sternway and the impact which is made by anchor on the seabed, the swivel helps fluke embed the ground. A good holding ground and slow sternway ensures the cable lays nicely on the sea bed. The cable gently grows to long stay. The exact behaviour of cable should be indicated to bridge. The cable is seen stretched and tension seems steady in cable and then easing slowly. It appears as if the anchor is holding position and as if the vessel is being pulled forward. Instantaneous speed indicated by doppler log or GPS would show up the similar movement of the vessel.
Q. What is the effect of the cable length being insufficient in moderate weather?
Anchor provides maximum holding power when its flukes are embedded in the sea bed. This occurs when the anchor shank lies on the sea bed and the anchor cable pulls horizontally at the anchor shackle.
When the pull increases, the cable lying on the sea bed is lifted off, creating a larger angle above the horizontal. As the angle increases, the holding power reduces. It is said that when the shank makes an angle of 5o to horizontal, there is a 20% reduction of holding power. At 15o, there is a reduction by 60%.
Q. How much cable should generally be used?
Assuming that the factors such as weather, holding ground, stay, etc. are average then very commonly used thumb rule states that number of shackles of cable should be equal to 1.5√D (D= depth in metres). Another formula being the length of cables in metres should be about 6 to 10 times depth of water in metres. A very old conventional formula being number of shackle lengths equal to 2 x .
Q. How much cable length would you use for high tensile steel?
High tensile steel cable means the reduced scantling for given strength. This would also mean that the weight of cable would be less. The contribution of the weight of the cable and the caternary which is formed because of this is very important in ensuring that the shank lies horizontal. It is for this reason that an extra length compared to the usual cable should be used.
Q. What is scope? Why a good scope is beneficial?
The ‘scope’ of the cable is the ratio of the length of cable paid out to the depth of water. If the scope is not sufficient and if there is significant yawing or heavy pitching, there is a risk that the anchor may get dislodged. It will then drag. A scope of six or more is generally used. Big ships are generally, fitted with about 12 shackles of cable on each anchor. A good scope also ensures that the cable near the shank remains horizontal.
Q. What precautions must be taken while anchoring in deep waters?
Deep waters to a very small extent may be said relevant to the size of a vessel. Generally, a depth more than 45 meters may be considered as deep. The ideal scope may not be possible in deep waters. The tidal stream pattern and the underwater topography are the factors affecting the holding power. The anchoring must be done by walking back the full length of cable. While heaving the anchor, the engine movements and the rate of current must be watched. The heaving up usually is done on the first gear and the time taken to heave the cable must be accounted for.
Q. How do you generally use the transverse thrust to lay the cable on sea bed?
A traditional style of anchoring a general cargo ship with mid ship accommodation used to take the port anchor for letting go with right hand single screw propeller. The anchoring site was approached with vessel on dead slow ahead over the ground with the tide/current or wind being streamed. A slow astern movement was then given with anchor ready for letting go, usually outside the hawse pipe. The propeller wash amidship, generally meant that the vessel had stopped over the water and the ship would fall stern due wind and current. Astern movement is also associated with a transverse thrust that would change the head to starboard. A very slow dropping astern with stem opening to starboard laid the cable nicely on the sea bed.
Q. What precautions are taken while heaving in the anchor chain?
If the situation allows and time permits:
- the cable must be properly inspected for missing studs, elongated links or any kind of deformity; and
- painting of shackle and securing of seizing wire can be done.
The deck water is kept running to wash the mud, etc. The direction in which the cable is leading, with the weight on it is monitored continuously and conveyed to bridge. The cable is hove on first or second gear, depending on the weight that comes on the chain. The moment the the anchor is aweigh or leaves the sea bed, bridge is informed. The bridge is also informed about small boats etc maneuvering close to ship ahead. The bridge must be informed in advance if the cable is likely to get excessively stretched. Flukes sighted, cable at long stay, cable up and down, etc are duly reported. Sometimes, the flukes pick up a foul wire and at some occasions the flukes may not come up in the correct way. The bridge is duly informed. The lowering of anchors a couple of times may rectify this problem, As the anchor gets aweigh, the anchor ball is lowered.
Q. As a Master, how will you go about anchoring after a sea passage?
Master must pick up the required information from publications like Guide to Port Entry, relevant Sailing Direction, etc. Many useful tips regarding approaches, dangers, etc can be found from these publications. Master also must study the navigational, approach as well as large scale charts thoroughly. Depending upon the size of ship, weather, depth, visibility, traffic, etc the speed is adjusted. It is assumed the engines have already been reversed and duly tried out as per rules.
The anchor lashings may be removed well in advance or about say half an hour before scheduled anchor stations, depending on the depths, approaches, etc. Master must call the chief officer well in advance and brief him about: the method of anchoring; location of anchoring; the anchor that would be used; etc. The anchorage is approached with caution. The wind / tidal stream is stemmed. The tidal streams are calculated in advance. The headings of the ships at anchorage, particularly, those which are close to the anchoring position and of the similar size and draft are the good indicators of tidal streams. Duty officer can plot own position and at the same time the positions of ships at prospective anchoring position. This is to get a better idea about the space available for anchoring. The duty officer must plot positions regularly and note the soundings at the same time. The port control is duly reported at various stages.
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