Anchoring A Large Ship - Observations


#1

Hey guys, after a long time away I am rediscovering this forum and must say I am impressed with the amount of really good and relevant posts I’ve seen lately.

Recently I was in the vicinity of a large ship anchorage area ad was fascinated by what I saw but a few questions popped up in my mind.

  1. On a boat, after laying out sufficient scope, we often back down on the engines to allow the anchor to “dig in”. One ship seemed to do this but most seemed to just lay out the chain. Is “digging in” the anchor common practice (or even a good idea) on large ships considering the amount of momentum involved?

  2. If noticed that the anchor chain can either be let down while engaged to the windlass or allowed to free fall. Obviously free-falling has its advantages when first laying out the anchor but what about paying out more scope? Let’s say you had 4 shots in the water, the wind kicked up, and you wanted to pay out another 2 shots… I imagine there is quite a force on the chain at this point so… do you let those 2 shots free-fall or do you pay it out with the windlass?

  3. How often do you paint the anchor chain on a large ship?


#2

[quote=yachtcapt;29281]Hey guys, after a long time away I am rediscovering this forum and must say I am impressed with the amount of really good and relevant posts I’ve seen lately.

Recently I was in the vicinity of a large ship anchorage area ad was fascinated by what I saw but a few questions popped up in my mind.

  1. On a boat, after laying out sufficient scope, we often back down on the engines to allow the anchor to “dig in”. One ship seemed to do this but most seemed to just lay out the chain. Is “digging in” the anchor common practice (or even a good idea) on large ships considering the amount of momentum involved?
    You do both with stern way. Seating the anchor and laying out the chain to the correct scope. Some even prefer to let the anchor hit bottom, then with little stern way, stop, just a ways from the anchor while letting out chain, to make a pile, then continue. Reasoning - is when everything does get tight, the chain will pull from the pile first which then transfers as a horizontal force to the anchor (ensuring no uplift). That method is used in deeper water.

  2. If noticed that the anchor chain can either be let down while engaged to the windlass or allowed to free fall. Obviously free-falling has its advantages when first laying out the anchor but what about paying out more scope? Let’s say you had 4 shots in the water, the wind kicked up, and you wanted to pay out another 2 shots… I imagine there is quite a force on the chain at this point so… do you let those 2 shots free-fall or do you pay it out with the windlass?
    Both again. Really doesn’t matter if you have a good brake and AB that can regulate the speed as needed. The biggest thing is not to pile chain on top of the anchor when first setting it.

  3. How often do you paint the anchor chain on a large ship?[/quote]
    Most companies, that invest in their vessels will have galvanized chain and there is no reason to paint - other than a few links or so to let you know when the anchor is all the way up in the bolster.


#3

Normally you swing head to the elements then back down… once the prop wash hits midships then you should have the perfect amount of sternest on to drop the anchor without it piling up on itself.

  1. You DO NOT need to “digg in” the anchor unless you are anchoring in deep water with a solid bottom. Otherwise (& even under these conditions) you run the risk of loosing your break. If you must back down (& again 99% of the time it’s foolish to do so) then make sure the break is set, riding pawl down, you have enough start/stop air and don’t let her get much momentum.

  2. I preffer to engage the chain but only in light conditions or if you have engines to assist. Otherwise you should have put out more chain before you got to this point.

My question is, with the rule of thumb being 5 to 7 times the water depth that most Captains choose 5. I understand wanting to limit your scope in a busy anchorage but, otherwise, why not just put out the full7x if you have enough in the locker??


#4
  1. anchorman is right, but conditions always vary. imagine heading a current? no sternway needed but that is given freely. in principal you want the chain stretched out one way or another.
  2. again many variables. but the salient fact is “you want the chain stretched out one way or another”.
  3. like anchorman: on my vessels we don’t repaint the black painted chain frequently other then the detachable links every shot: they are painted red, and the shot count is painted white onto the corresponding number of links. to wit: as the chain rushes out the hawsepipe at 0400 hours, Bosun sees a spot of white then red then white, a couple seconds later white white red white white. he’s seeing the number of shots going out. slick, eh?

#5
  1. I try to get what’s on deck painted but otherwise leave it till the next shipyard period. Never let the entire lenght of chain out unless the ship is secured by other means and you are in fairly shallow water. Your windlass was unlikely to be designed to lift the weight of the entire chain AND the anchor.

#6

cmjeff: I always instruct to put out the 7 if space allows. why not? I sleep better.


#7

Very interesting, thank you all. So what are the biggest reasons that ships sometimes drag anchor?

I know some offshore platforms have interesting anchor designs but Are there any new developments to traditional ground tackle.


#8

[quote=yachtcapt;29304]Very interesting, thank you all. So what are the biggest reasons that ships sometimes drag anchor?

I know some offshore platforms have interesting anchor designs but Are there any new developments to traditional ground tackle.[/quote]

As far as a ship - 9 out of 10 times, you need more scope for the prevailing conditions. The coast guard tests just give you a rule-of-thumb, but there are always exceptions 5 to 7 times water depth, which can even be excessive. Some bottoms, you’ll be lucky to get your anchor back.

As far as anchors on large platforms, I think I’ve participated in 10 world records over the last 14 years. Now days, most rigs in ultra deep water use taut leg systems - meaning no scope at all. The moorings are made of synthetics, typically 7", 900 ton breaking strength, and weigh 4lbs/foot in the water - very nice stuff. This was a very important evolution because in 10,000’ water depth, wire would break under its own weight, never mind holding a rig in place. Of course, you can break the wire in sections and add submersible buoys in the design, but the added rigging, time, and trips to the dock to get all of this - just did not prove to be cost effective.
I could go on, there are so many options and technologies out there, but that gives you an idea of some of the limitations.


#9

Not sure if I agree totally. You need to make sure that the flukes are set and holding - not sure if that what you meant by “digg in”. You need to know that at a minimum to help determine scope, or even if it’s wise to anchor in that particular spot. If you know the anchor is set and holding good, extra scope is just a safety factor to what you already know.

If the anchor comes out - good. That something that you want to know while putting it out, not during the mid-watch.


#10

Anyone ever hear “the anchor holds the chain, the chain holds the ship”?


#11

The chain holds the ship is pretty true. The west coast method of anchoring barges uses that philosophy. a tug will slow way down, shorten up, then pay out 3 or four layers of tow wire, in a BIG loop like a question mark, then return to the barge, or more frequently just stay out at the end of the wire. the catenary (or what would be the catenary if stretched out) holds both tug and barge quite well. BUT, the west coast uses two shots of chain as a bridle pennant, in addition to the one shot bridle legs. So there’s a whole lot of chain down there. The only part I never liked about this method is: You are intentionally using your main tow wire in bottom contact. This is sort of like asking for chafe and snagging. Towboat skippers try their whole career to keep the wire OFF the bottom. But as usual the west coast does things a little differently!

The original question about digging in, and how to do it is answered by individual preference. When on a large Unit I have both fair tide and head in dropped anchor. The deciding factor is often how hard the current is running, and or the wind. sometimes I don’t have the sea room to round up head into the wind/current! Either way when the anchor sets well you can both see it from the bow answering to the anchor, and the bow watch can see the chain get taut and not skip. How much scope you use depends on location, depth, bottom characteristics, and the ‘reputation’ of the anchor. Some barge companies use a short (15’ or 20’) piece of chain, and the rest wire. That stinks! There is NOTHING quite as satisfying as having a chain rode. Chain holds better, takes less scope per depth, takes abuse better, and make me sleep better.

Your question about increasing scope is pretty important. Sometimes by ‘veering’ out is enough of a ‘jerk’ to pull the anchor out of the mud by free wheeling then stopping suddenly. The term is ‘Walking the anchor out’. By using the windlass and slowly, evenly letting out more scope is the preferable way to get more scope out when needed in adverse conditions. The earlier comment about paying attention and letting out more scope is tantamount. An ounce of cure is often worth way more than several more pounds of cure (or anchor, rode, chain, etc etc etc.) I don’t see why some people ‘chince’ on a little scope when just a shot more would make it all better?


#12

cappy208,

“west coast” of where??spent my formative years w/ the “bayou boys”…use to do it in the GoM…been “anchoring” on my tow wire all along all over the world…has always worked out well for the most part but have managed to foul it a few times…“luck of the draw”…the only “west coast thing” that blew me away was when I was a mate and the AB asked me to get him the peavey…been trying to have one or get one aboard ever since!!


#13

[QUOTE=yachtcapt;29304]
I know some offshore platforms have interesting anchor designs but Are there any new developments to traditional ground tackle.[/QUOTE]

I am C/M on a brand new vessel which I anchored last week. Not much has change as far as technology except I was happy to see a gauge on the side of the windlass that told me how many meters of chain I had paid out. I simply converted the number of shots needed to meters then placed a nice thick grease pencil mark on the correct number.

The gauge wasn’t much use to me last week but I imagine, after the paint marking each shot has rusted off in a few years, it may be of great use in the future.


#14

The CFRs specifically refer to the method of using towwire, chain pennants as an alternative to anchoring on the west coast because of deep water right up to shore. The connection to the west coast is because it is specifically mentioned in the regs concerning having a working anchor, or acceptable insurance wire. (unless you have a double hull, then it appears an anchor is a frivolous luxury) Sounds goofy, but hey, when in Rome…