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  #1  
Old 12-26-2006, 01:03 PM
Jackknifed Juggernaut Jackknifed Juggernaut is offline
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How do ship anchors work?

I just got back from a cruise on a very large ship. While we were off of the coast of Cozumel, the ship was anchored and a smaller boat brought us to the island. While I understand the basic concept of an anchor, the one I saw didn't look big enough to stabilize the monstrocity of the boat I was on. I asked a few crew members about it, but no one could adequately answer my question. It seemed to me that the only possible way that it work would be if the anchor actually latched onto something at the bottom of the sea. But the general consensus was that the anchor doesn't actually latch to anything, and that the weight of the anchor itself did the work. So I toss this one over to my fellow dopers for some ignorance fightin'.
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  #2  
Old 12-26-2006, 01:13 PM
Johnny L.A. Johnny L.A. is offline
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A stereotypical anchor has 'flukes' that dig into the seabed. As a ship is drawn away from it, the anchor digs in. Think of using a hoe in the garden.

ISTR something about a heavy weight being used on some anchors, with the anchor proper being upcurrent on a chain.

There are many types of anchors: Fluked, plows, grapnels, mushrooms, and more.

I have a question: I was using a fluke anchor for my boat. When I tried to raise it, I couldn't. I can only guess that the flukes got under a boulder. No amount of maneuvering or pulling allowed me to pull the anchor up. I finally had to cut the line. Is there anything else I could have done?
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  #3  
Old 12-26-2006, 01:47 PM
Sapo Sapo is offline
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from my limited experience as a passenger in my friend's boats. You could have moved the boat to the other side of the anchor and pulled from there. And, of course, diving down could have shed some light on the problem or maybe even resolved it.
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  #4  
Old 12-26-2006, 01:52 PM
Johnny L.A. Johnny L.A. is offline
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Quote:
Originally Posted by Sapo
from my limited experience as a passenger in my friend's boats. You could have moved the boat to the other side of the anchor and pulled from there.
I tried that, to no avail. I didn't have my SCUBA gear with me, and there's no way I'd get in that water without a drysuit.
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  #5  
Old 12-26-2006, 03:46 PM
AskNott AskNott is offline
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Quote:
Originally Posted by Johnny L.A.
...
I have a question: I was using a fluke anchor for my boat. When I tried to raise it, I couldn't. I can only guess that the flukes got under a boulder. No amount of maneuvering or pulling allowed me to pull the anchor up. I finally had to cut the line. Is there anything else I could have done?
One other thing comes to my mind. When you cut the line (rode,) you could have attached a float, such as a puffy fender to the line. Later, when you had time and diving gear, you could go back and attach a line to the backout eye. Most anchors have a hole near the fluke end, and pulling on that will usually yank the anchor loose.

Stuff happens. There are big rocks, tree roots, and even power lines. That's one reason they can keep selling anchors.
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  #6  
Old 12-26-2006, 04:18 PM
wolfman wolfman is offline
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Quote:
I have a question: I was using a fluke anchor for my boat. When I tried to raise it, I couldn't. I can only guess that the flukes got under a boulder. No amount of maneuvering or pulling allowed me to pull the anchor up. I finally had to cut the line. Is there anything else I could have done?
My friends all carry a second anchor smaller anchor in their boat. You first make sure it's tied on(we learned that one from experience ) Then do a hammer throw to get it as far away on the same side as possible. Then you pull your boat over on top of itit. And hopefully the new angle will let you pull the first one free.
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  #7  
Old 12-26-2006, 04:48 PM
intention intention is offline
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As a commercial fisherman, I can tell you how we get them up. Generally, there's a bit of swell or waves. Pull up on the anchor line until you're directly above the anchor. Wait until you're at the bottom of the swell. Quickly wrap the anchor line around a strong cleat (being very careful not to leave your fingers entangled in the process). When the next swell comes in, it lifts the boat and either pulls the anchor out or breaks the line.

First step, though, is to go to the other side of the anchor and pull from there, often that works.

If there's no swell, back off a ways, then drive directly over the anchor from downstream. Have someone pull in on the anchor line as you approach it , then cleat it when you're right over the top of the anchor. The speed and weight of the boat will usually pull it out.

w.
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  #8  
Old 12-26-2006, 04:51 PM
Skipper Too Skipper Too is offline
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To answer the OP, Some anchoring systems work by shear weight. If it was a large cruise ship or Frieghter of Aircraft Carrier, they more than likely had a "Navy" style anchor. These have short stubby Flukes and a thick crown, and shank that pivots between the flukes. These anchors work on the principle of just use a shit ton of weight. The Anchors themseleves are a Cast Iron and weight a ton. they may not look very large in porportion to the ship but they are massive. On a aircraft carrier they look tiny up on the bow bu from the crown to the end of the shank they are as tall as two or three story building, and solid iron. The other part of any anchoring system is the Rode or what is attached to the anchor and to the vessel. These large ships with large "navy" anchors use all chain rodes. On the Abraham Lincoln CVN 72 it's all chain rode has link chain where each individual link weighs in at 350 lbs each it carries 1,082 feet of chain for a total weight of 308,000 lbs. The two anchors it carries are 30 tons each. By laying not only the anchor on the bottom but a good amount of chain, you get a very effeicent anchoring system with the chain adding a good amount of holding power on it's own.

Bottom types also affect anchoring systems, a navy anchor and all chain rode do real well in sandy and muddy bottoms due to the chain sinking in the sand and mud as well. This compounded when the system is given enough scope or length of rode relative to the depth of water. The more scope the better the holding power of the anchor due to a more lateral pull against it versus a vertical pull of a 1:1 scope. Common rule of thumb for stormy conditions where enough space is permited 7:1 is ideal that is seven feet of rode for every foot of depth.

To answer Johnny L.A.'s question. It all really depends, it could be rocks, it could be weeds it could be another anchor. I have seen a boat pull up a dozen anchors on Lake Washington after Seafair Weekend. I have seen Folks who run on the Columbia River use a bouy and a slide ring on the rode run full speed back towards the anchor that set and the bouyancy of the bouy combined with the speed set an anchor free. How were you retrieving your anchor? Best method with something like a Danforth Stock anchor is to slowly inch towards where the anchor is on the bottom so you are more or less over top of it taking in the rode as you go. Then try and pull it up. If you are using an electric windlass this is also the best case use the boat's engine to get you over top then use the windlass to wheel in the slack with no load, not use the windlass to pull you to the anchor under load. If you are not able to get it free with the widlass or by pulling by hand take in as much slack as posible, make the rode fast to a cleat then rock the boat fore and aft to use the boat's leverage to help break it free.

Skipper Too, who has not lost an anchor yet!
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  #9  
Old 12-26-2006, 05:54 PM
Improv Geek Improv Geek is offline
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Just a random doper saying thanks to the knowledgable folks, very interesting stuff!

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  #10  
Old 12-26-2006, 08:34 PM
Sapo Sapo is offline
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Didn't they have (way back when) what they called "deep sea anchors" which were just one big arse sail weighted to open underwater? How did those really work? Wouldn't currents pull them adrift just as easily as wind on the surface?
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  #11  
Old 12-26-2006, 09:18 PM
Rick Rick is offline
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You are thinking of a sea anchor which are used when the water is too deep, or you are trying not to blown all to hell in a heavy wind.
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  #12  
Old 12-26-2006, 09:58 PM
Sapo Sapo is offline
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I guess that when you are facing gale-strength winds, it makes sense. But for say, fishing, won't currents push you just as wind would?
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  #13  
Old 12-27-2006, 02:18 AM
GusNSpot GusNSpot is offline
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You don't anchor mid-ocean and fish. A sea anchor will slow you down but most will use engine power to hold position or just drift along. Open ocean fish boats also sometimes tie the helm down and motor in a circle at night with the 'unattended helm' flags (day) or lights indicating there is no one at the helm.

Small boat sailors have a trick of running a small line tied to the crown of their anchor and a float attached at the surface if they are in unknown water or places where there is sometimes a problem with raising the anchor. You can pull the anchor out backwards this way.

If you are not rich, and have a good set of anchors, $100 - $200 each and say 200' of BB chain, ( will mold into a almost solid bar before breaking ) it costs many $$$ and are in remote areas, you can't afford the $$ and the danger of losing your ground tackle.

Only in the worst conditions do you want the anchor itself to do any work. The chain should do all the holding most of the time. You can also slide down an additional weight on a loop fed buy another line to help hold the chain down

If it is not feasible to go in after the anchor and you are near stuff, (coral heads - wrecks - stuff that you can wrap the anchor rode around and can't get off of ) a double set ( two anchors with the boat between them so you do not move around a lot ) should be used.

If you have a $10 cast mushroom on old ski rope on your bass boat and it gets stuck, just cut it and go on. Not worth the possible damage to the boat or risking falling overboard in the winter.

There are whole books written about anchors and the techniques for using them.

On the water, being prepared is a life saving thing. Pay heed.

YMMV
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  #14  
Old 12-27-2006, 09:52 AM
Jackknifed Juggernaut Jackknifed Juggernaut is offline
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Quote:
Originally Posted by Skipper Too
To answer the OP...
Thanks for the explanation. In the example you gave, the rode weighed 308,000 pounds and the anchors weighed 120,000 pounds. I'm just curious about the total weight of the ship, not including these.
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  #15  
Old 12-27-2006, 10:40 AM
butler1850 butler1850 is offline
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Quote:
Originally Posted by Jackknifed Juggernaut
Thanks for the explanation. In the example you gave, the rode weighed 308,000 pounds and the anchors weighed 120,000 pounds. I'm just curious about the total weight of the ship, not including these.
From the CVN 72 Abraham Lincoln official page it displaces 97,500 tons. IIRC, the amount of water displaced equals the total weight of the ship.

The rode weighs 154 tons. So, I'd say the ship, minus the rode = 97,346 tons... or somewhere thereabouts.

I have no idea if this is fully loaded with men and material, ready for a deployment, as it sails out of Everett, but it's close enough.
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  #16  
Old 12-27-2006, 01:26 PM
mlees mlees is offline
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It doesn't take that much weight to keep the ship from drifting sideways on casual swells and tide movements. You (obviously) don't need weight equivalent to the boat itself.

When stronger than usual conditions arrive (say, hurricanes...), then you will need extra precautions.
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  #17  
Old 12-27-2006, 02:00 PM
AskNott AskNott is offline
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Quote:
Originally Posted by Sapo
I guess that when you are facing gale-strength winds, it makes sense. But for say, fishing, won't currents push you just as wind would?
The idea for using a sea anchor or drogue is not to maintain location, but to keep the same orientation to the wind or current. They are often used in storms to prevent getting sideways to the waves. A couple of guys fishing on a windy day might trail a little sea anchor or a bucket to keep from fouling each other's lines as they tight-line fish the bottom while the wind pushes them across the lake.
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  #18  
Old 12-27-2006, 02:20 PM
Billdo Billdo is offline
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As Skipper Too pointed out there are two elements, the anchor and the rode (the anchor line or chain). Although the anchor itself is important, too few people recognize that the rode is doing a huge portion of the work.

As was mentioned before, most anchors have smallish flukes which will dig into the sand, gravel or other soft material of the bottom. The design of these anchors is such that when dug in, they strongly resist a lateral (parallel to the bottom) force.

Most anchor rodes in small boats consist of a several foot length of chain attached to the anchor and then a braided nylon anchor line. In larger vessels, the anchor rode is typically all chain.

The key to successful anchoring is to allow sufficient "scope", that is the ratio of the length of the rode to the water depth. With a primarily rope rode, a scope of 5:1 (for example 50 feet of rode in a 10 foot depth) usually acceptable and a scope of 7:1 is preferred. With chain 3:1 can be acceptable, but 5:1 is preferred.

When a vessel is anchored, the rode does not pull in a straight line from the anchor to the bow of the vessel. Instead, it hangs more or less in a catenary, the curve that you get when you suspend a bit of line or chain without any weight on it.

When the anchor rode is put under stress because of the efforts of wind or current, there is the catenary tightens somewhat, but most of the stress transmitted to the anchor in a lateral pull, the direction in which the anchor has the best holding. All in all, the amount of effort required to keep a vessel in place against typical wind or current is not particularly large compared to the size of the vessel, so even a relatively modest anchor can keep a large vessel in place in normal conditions.

If there is a wind gust or wave action, there will be a momentary excess stress, which will raise the catenary, but when the stress abates, the weight of the rode will cause it to drop back into place, giving the system a great deal of "springyness". In addition, a nylon rode is itself quite elastic, so the line will extend in response to stress, and spring back when the stress reduces.

To pull up an anchor, you pull in the rode until it is a 1:1 scope, i.e. entirely vertical. Most anchors are designed so that a vertical pull will break them free in most instances, so it is usually fairly easy to get them up. The trick intention mentioned of tying the rode off and letting the swells do the work can help.
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Old 12-27-2006, 03:19 PM
mnemosyne mnemosyne is offline
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I took a cruise last year (RC Mariner of the Seas) and they never dropped anchor - for the most part, the ship's position was controlled entirely by their GPS/vessel navigation system (I forget the name they said). Basically the propellers kept moving, making sure that the ship was, essentially, not moving with respect to the land and the tender boats. We only had tenders in Labadee (Haiti) and Georgetown (Grand Cayman) but the anchors always stayed up (they also, perhaps obviously, never dropped them for the two ports where they were at a dock). http://i65.photobucket.com/albums/h2...s/DSCN0946.jpg

Looking at a picture of the ship in Labadee, they tied the ship to a couple of buoys, one at the front and one at the back, but the anchor is clearly up. Another of the pictures I took shows two Carnival ships and the Rhapsody of the Seas in Georgetown, and neither has their anchors down, and I don't see any buoys either. http://i65.photobucket.com/albums/h2...s/DSCN1015.jpg

And just for fun, this is a shot of the Mariner's anchor in Cozumel (docked): http://i65.photobucket.com/albums/h2...s/DSCN1050.jpg
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Old 12-27-2006, 03:31 PM
mlees mlees is offline
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I dont have access to photobucket while surfing at work. Sorry.

But anyway, with the ship tied to bouys (presumedly not pierside), those bouys are the anchors. The bouys are "anchored" to the (harbor/bay) bottom, right? Those types will use the same principle.
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Old 12-27-2006, 05:00 PM
Billdo Billdo is offline
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Quote:
Originally Posted by mlees
But anyway, with the ship tied to bouys (presumedly not pierside), those bouys are the anchors. The bouys are "anchored" to the (harbor/bay) bottom, right? Those types will use the same principle.
It looks like those bouys are "mooring bouys", which are floats attached by chain or rope to an anchor or other type of ground tackle, making essentially a fixed point for anchorage. Vessels will tie up to a mooring in lieu of using their own anchors.

Anchors for moorings use similar principals to working vessel anchors, but there are some differences. More commonly the anchor will be one or more mushroom anchors, which can be set deep into the seabed given some time, but are very difficult to pull out after they are set. Sometimes an anchor of simply very heavy weight is used, which can be quite effective if sunk into sand or soft ground. A vessel's anchor has to balance holding strength, weight, and ease of storage and use. A fixed mooring anchor has holding strength as its primary criterion, and is usually more able to handle vertical and diagonal loads than a vessel's working anchor.

Moorings typically use much less scope than a vessel's anchor, both because the anchor is proportionately stronger and because mooring areas usually have limited room for vessels to swing. The cruise ship in the picture was moored fore and aft (moorings attached to both the front and back of the ship), which means that it is quite tightly fixed into place.
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Old 12-27-2006, 09:15 PM
Sharky Sharky is offline
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Quote:
Originally Posted by mnemosyne
I took a cruise last year (RC Mariner of the Seas) and they never dropped anchor - for the most part, the ship's position was controlled entirely by their GPS/vessel navigation system (I forget the name they said). Basically the propellers kept moving, making sure that the ship was, essentially, not moving with respect to the land and the tender boats. We only had tenders in Labadee (Haiti) and Georgetown (Grand Cayman) but the anchors always stayed up (they also, perhaps obviously, never dropped them for the two ports where they were at a dock). http://i65.photobucket.com/albums/h2...s/DSCN0946.jpg

Looking at a picture of the ship in Labadee, they tied the ship to a couple of buoys, one at the front and one at the back, but the anchor is clearly up. Another of the pictures I took shows two Carnival ships and the Rhapsody of the Seas in Georgetown, and neither has their anchors down, and I don't see any buoys either. http://i65.photobucket.com/albums/h2...s/DSCN1015.jpg

And just for fun, this is a shot of the Mariner's anchor in Cozumel (docked): http://i65.photobucket.com/albums/h2...s/DSCN1050.jpg
That is called Dynamic Positioning. Back when I was an oilfield supply vessel captain, DP systems were becoming common on OSVs and standby vessels. It is a blessing for platform supply captains, where you are required to hold the vessel in position for hours while the rig crane offloads and backloads cargo. You get in position next to the platform and press a button, and it holds the boat remarkably still even in rough seas. Beats the hell out of working the throttles manually while staring at the legs of the rig for hours.
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  #23  
Old 12-27-2006, 11:50 PM
mnemosyne mnemosyne is offline
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I assume the mooring buoys might have been more necessary in Labadee because it is a fairly small bay, as opposed to in Georgetown where the ship was basically out in "open" water? As I said, I don't think the buoys are used at all out in Georgetown (there was even a fifth ship out there, Norwegian cruise lines, I think, behind the Mariner).

Google Map of Labadee with a Voyager Class ship (the Mariner is one of them) - you can see how small the bay is with respect to one of these monsters!:
http://www.google.ca/maps?f=q&hl=en&...5,0.017488&t=k

Only slightly off-topic: my husband and I leave for another cruise on the Mariner in January... so excited!
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  #24  
Old 12-28-2006, 12:47 AM
Sharky Sharky is offline
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Quote:
Originally Posted by mnemosyne
I assume the mooring buoys might have been more necessary in Labadee because it is a fairly small bay, as opposed to in Georgetown where the ship was basically out in "open" water? As I said, I don't think the buoys are used at all out in Georgetown (there was even a fifth ship out there, Norwegian cruise lines, I think, behind the Mariner).

Google Map of Labadee with a Voyager Class ship (the Mariner is one of them) - you can see how small the bay is with respect to one of these monsters!:
http://www.google.ca/maps?f=q&hl=en&...5,0.017488&t=k

Only slightly off-topic: my husband and I leave for another cruise on the Mariner in January... so excited!
The ship would not need moorings in addition to DP, the DP system would be capable of holding the ship's position within a couple of feet regardless of the size of the bay. My guess is that they did not use DP in Labadee due to shallow water (which would also make it difficult to anchor). The ship's thrusters would probably stir up the bottom in a shallow bay, damaging seagrass beds and clouding the water.

Those are beautiful ships. Enjoy your cruise, that sounds like fun!
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