Use of anchors on huge ships?

Factual Questions
41

I was going to say, if the anchor must have flukes and the flukes must be set in the seabed, then how can it possibly be correct that it’s always and only the chain holding the ship in place? That can’t be right.

I mean surely the weight of the chain must provide SOME anchoring. But I have to think that the practice of paying out 5x more chain than depth has more to do with allowing for a safe amount of positional variability.

As mentioned before, a ship anchored at shore will oscillate in arcs around the anchor point due to tidal fluctuations. You really want enough length in the chain to let this happen in an orderly manner rather than being whipped around like a balloon on the end of a string. Or in the case of a large tsunami… you want enough slack so that the boat can safely rise the full amplitude of the tsunami, not get swamped like a buoy vertically anchored to the bottom.

42

OK, just ignore my previous post. According to Wikipedia, the exact reason to pay out so much extra chain is because the anchor can only remain lodged if the direction of strain is mostly parallel to the seabed. When the direction of strain is vertical to the seabed, then the anchor pops out.

[QUOTE=Anchor - Wikipedia]
The ratio of the length of rode to the water depth is known as the scope. Anchoring with sufficient scope and/or heavy chain rode brings the direction of strain close to parallel with the seabed. This is particularly important for light modern anchors designed to bury in the bottom, where ratios of 5-7 to 1 are common, whereas heavy anchors and moorings can use 3 to 1 or less.
Since all anchors that embed themselves in the bottom require the strain to be along the seabed, anchors can be broken out of the bottom by shortening the rode until the vessel is directly above the anchor (at this point the anchor chain is “up and down” in naval parlance).
[/quote]

I always wondered how, if you had achieved an anchoring sufficient to hold a 100,000 ton boat, you could dig it out from the surface without thousands of tons of TNT. Apparently all you need to do is manipulate your good friends cosine and friction.

43

My 27 foot sailboat has a fluke anchor, which is really just a much smaller version of the gigantic anchors used on ships. I have a length of heavy stainless steel chain attached to the anchor, and then an appropriate anchor strength line (rope) tied to that. I can’t off hand remember how many feet of chain I have, but I seem to remember I have about 20 feet. The chain has multiple purposes, first off it weighs down the anchor so the anchor lies flat. If the anchor lands on it’s end and “sticks” in the sand that way, it can’t do it’s job. Secondly, the chain is far more abrasion resistant than rope, so it won’t chafe or cut on any underwater object. Thirdly, just the weight of the chain itself aids in holding the boat in place. If I anchor in calm water with no tides or winds, I can tell when I pull up my anchor that the flukes never even engaged because the chain itself was heavy enough to hold me in place.

Determining how long your anchor rode (chain + rope) should be is as simple as multiplying the deepest water you expect to anchor in by 8. As for rope size, the rule of thumb is 1/8" of rope diameter for every 9’ of boat length. So if you expect to anchor your 26’ boat in 30’ of water, you need 240’ of 3/8" nylon rope. Unlike oversizing the anchor, oversizing the line is not recommended because that reduces its beneficial elasticity. As a practical matter, however, rope with a diameter smaller than 3/8" is difficult to grip.

44

IANA big ship sailor. But I did live at the mouth of the Panama Canal for several years and both drive & fly over it daily. I also fly over NY, LA, & Long Beach harbors fairly frequently & have for decades.

There were/are almost always (90+%) several big ships anchored at both ends of the Canal. Ditto on the outskirts of NY & LA / LGB harbor. Seeing a dozen anchored ships would not be extraordinary. And these represent several of the biggest & busiest shipping points on earth.

Gatun Lake. And yep, they anchor their on occasion. At least when I was there, the scheduling process tried to avoid ships sitting in Gatun Lake; the dredged portion is not *that *spacious. But if somebody got hung up or had a malfunction or they had a lock problem, then a couple ships would get stuck in Gatun waiting for the problem to be cleared.

As a yachtsman what I was always taught / have read is that it takes both the anchor & the chain working together to have holding power. Having a length of chain on the bottom (or a much longer legth of rope for yacht-sized vessels) is necessary to ensure the pulling force on the anchor is always parellel to the bottom. Which keeps the anchor’s flukes dug in.

Paying out too little length either because the water is too deep for the length of anchor rode you have aboard or you’re concerned abut sea room vs. your swing radius is self-defeating. An anchor simply won’t hold unless there’s enough rode out to make the pulling force horizontal.

So in that sense, it’s the chain laying on the bottom which makes the anchor work. Which could be restated for simplicity as it’s the chain laying on the bottom which holds the ship. Or in a reverse formulation, *not *having any chain laying on the bottom *won’t *hold the ship.

45

They’re not as visible as the anchors on surface vessels, but U.S. Navy submarines also have anchors. (When retracted, the anchors are flush with the hull.)

We used them a few times during my time on board. One time was just for practice, and the other two times were in foreign ports.

To add to what others have stated, U.S. Navy anchors are most certainly not for show. They are important, necessary, functional pieces of equipment.

46

Subs have anchors? Innarestin’ …Never knew or even considered that. I assume they’re on the very bottom of the sub?
“Having a length of chain on the bottom (or a much longer legth of rope for yacht-sized vessels) is necessary to ensure the pulling force on the anchor is always parellel to the bottom.”

That’s geometrically impossible… as long as the boat is above the water, then any pulling force strong enough to take out all the slack in the chain and pull on the anchor would mean there was no chain left laying on the bottom. So there could be no pulling force parallel to the bottom. The longer the chain, the closer to parallel it will be (more acute angle between the floor and the chain going up to the ship from the anchor).

47

For the record, I am 100% satisfied that there are valid uses for anchors on huge ships. Consider me 100% convinced. It wasn’t so much that I doubted they had a use (but I wouldn’t put it past them to stick them there just for aesthetics), I just couldn’t think of any that made realistic sense.

48

Odd “full circle” event for me…

I reported aboard the Nimitz just after they changed ports from East coast to West coast, so I just missed out on their visit to Rio de Janeiro. The guys in my department all told of the wild time they had, and they talked about how the nuke protesters wouldn’t allow the ship to dock, so they had to anchor in the bay and use liberty boats to come in.

…Fast forward four years…

I was a civilian, with my Brazilian wife, visiting her parents’ house in Rio de Janeiro. Her cousin needed to give a friend a lift to work and so he was in the car with us.

As we drove across the Rio-to-Niteroi bridge, her cousin’s friend mentioned the visit of the Nimitz, and we talked about it for awhile. He told me about the protests and the general mayhem of the sailors, from the Brazilian point of view.

I thought that was a really cool moment: though I hadn’t been there at the time, I was told about the same port visit from the sailors and from the locals, years apart.

49

I think you are confused. One of the reasons that a vessel puts out so much chain is so that all of the slack is NOT taken out of the chain. That’s kind of the point.

50

They’re not on the very bottom, but they are well below the waterline (and outside the pressure hull), IIRC.

No shipbuilder would place multi-ton steel objects on a vessel just for aesthetics.

51

You can’t pull a chain dead straight between two points; it will always sag and form a catenary curve due to the chain’s non-zero weight per unit length, just like you see in the wires between two utility poles.

In the case of an extremely heavy, extremely long chain - as one would find attached to a properly deployed anchor for a large ship - the chain sags enough so that a significant length of it is laying on the bottom of the ocean, even when severe wind+waves are pushing the ship backward. Quoting spifflog from upthread:

That length (measured relative to water depth) is important, otherwise, yes, you can pick up all of the slack in the chain. With enough length, your catenary curve intersects the bottom of the ocean, and all is well.

52

Chains: OMG, I have a headache.

You generally need an anchor, chain and rope to have the most reliable anchorage.

The anchor digs it and holds. An appropriate amount of chain lays on the ground, pulls the anchor horizontally (essential) and starts the curve upward where the rope comes in – essentially offers slack.

Boats/ships are constantly moving and pulling up/down. THINK: With huge seas and lots of chain, the ship transfers its movement to the chain first. The chain is buffering the ‘noise’. The chain goes up/down, around, and the ship/boat isn’t moving enough to disturb the anchor… the chain is filtering the noise.

Now, take the chain out of the equation and the rope is tugging on the anchor relentlessly – it would require every nickle and dime movement of the ship/boat to transfer to the anchor and the anchor would be fighting to hold the vessel constantly.

Anchoring is not locking the ship in place. The ship is allowed to move, for if this were not permitted (via a gracious am’t of chain… and chain is the only effective slack), the anchor would be challenged constantly.

The chain also drags the anchor flat, and fluke type anchors work best pulled along horizontal lines, and fail when pull upward.

You need chain to properly drag the anchor horizontally; the chain is – indeed – THE SLACK, and chain relieves the anchor from non-stop tugging and vertical tugging that risk freeing it. Anchoring is drifting within a certain acceptable range.

WHEW. The chain is NOT holding the boat. The chain is allowing the system to work properly.

.

53

As the chain angle approaches being parallel to the ocean bottom, the anchor has a stronger purchase. As the chain angle diverges from the ocean bottom, the anchor is more likely to pop out. It can’t be exactly parallel for reasons you mentioned, but don’t let the sloppy terminology confuse you.

54

Even a shipbuilder (really more the designer than the builder… the builder is just following directions to spec and not making design decisions) paid by the US government? I’m clearly more cynical than you about gov’t waste, especially when it comes to military spending… But I do concede that anchors are not examples of such pointless, wasteful displays of military aesthetics.

55

Yea I understand - the more acute the angle between the ship and the anchor (requiring a longer chain relative to the depth of the water) the more secure the anchor is. If the ship is right on top of the anchor (i.e. a 90 degree angle/perpendicular to the sea floor) then the anchor won’t work well, if at all - in fact hat’s how you’d go about un-hooking the anchor from the bottom.

But say the ship is in 300 feet of water and lets out 1000 feet of chain. If the chain is stretched out the full 1000 feet then the anchor will be dug into the ground and keep the ship secure. But if there is a big pile of 700 or so feet of chain in a lump on the ocean floor, the anchor won’t be doing anything. The chain will provide some friction/drag before the anchor does, because the slack in the chain will have to be gone before the anchor can dig in and start working. Basically, you want the ship to be as far away from the anchor (on the ocean floor) as possible, thus causing the most acute angle between the ship and the anchor. If it’s parallel to the anchor, it means your ship has sunk or the anchor is somehow floating.

56

It’s a complicated issue. Military procurement has a certain built in inefficiency. You have to jump through so many hoops to sell anything to the military, or to win the contract to build aircraft carriers, or to get on that contract as a subcontractor, that the military procurement process effectively prices you out of market price. That’s one of the big reasons that a hypothetical company outfitting a U.S. nuclear carrier with an anchor is going to have to spend more money than it might for the same job on a supertanker.

When people then look back over the military’s spending, they’ll note “um wow, we spent like $50,000 for something that normally costs $25,000, WTF!!!” But that’s just the nature of the beast, and to a degree the incredibly complex procurement process is because the military shouldn’t and isn’t interested in acquiring the cheapest product at the best price. The military is interested in buying a product or agreeing to contracts which mandate a high degree of redundancy, testing, provings, etc.

In addition, with any big organization there is obviously other types of more insidious inefficiencies. No one is going to deny that the procurement process can be monkeyed with to benefit certain parties and introduce further inefficiencies.

But actually adding huge things to ships that serve no purpose at all? That’s not the kind of inefficiency ship designers or the military normally engage in at all. Anyone who has ever been involved in procurement probably won’t argue a lot of money gets spent inefficiently, but it’s not being spent on things that don’t exist for a reason, it’s being spent through a process which has built in inefficiencies.

57

Another use for an anchor is to serve in emergencies to hold the ship off a lee shore (rocks or land toward which the ship is blowing) if the engines are insufficient or break down, or if steering is lost. Even the largest of ships can be, and have been, outright sunk or lost in such disasters, if the decision to anchor was made too late or the anchor failed to hold in the storm.

Case in point: the famous Amoco Cadiz oil spill. The Amoco Cadiz, a supertanker, was roughly comparable in size to a Nimitz class carrier (for example, the tanker was 1095 feet long and the carrier 1092 feet long).

[QUOTE=Wikipedia]
…the ship encountered stormy weather with gale conditions and high seas while in the English Channel. At around 09:45, a heavy wave hit the ship’s rudder and it was found that she was no longer responding to the helm. This was due to the shearing of Whitworth thread studs in the Hastie four ram steering gear, built under licence in Spain, causing a loss of hydraulic fluid. Attempts to repair the damage were made but proved unsuccessful.
[/QUOTE]

Bolding mine:

[QUOTE=Wikipedia]
Several attempts were made to establish another tow line and Amoco Cadiz dropped its anchor trying to halt its drift. A successful tow line was in place at 20:55, but this measure proved incapable of preventing the supertanker from drifting towards the coast because of its huge mass and Force 10 storm winds.
[/QUOTE]

It’s worth noting that a book I read made the additional point that the real problem was that the anchor and the tow line came too late, moreso than the force of the storm per se. The Wikipedia article doesn’t mention the detail that the ship struck the rocks literally 7 minutes after the tow line was finally established.

[QUOTE=Wikipedia]
The Amoco Cadiz ran aground on Portsall Rocks, 5 km (3.1 mi) from the coast of Brittany, France, on 16 March 1978, and ultimately split in three and sank, all together resulting in the largest oil spill of its kind in history to that date.
[/QUOTE]

The anchor did not save her in this case but it was certainly tried and had it been used earlier (farther from the rocks) it might have done the trick; riding out a storm at anchor off a lee shore is a traditional safety measure used for hundreds of years.

58

That whole cantenary curve thing is pretty interesting. When a tug boat is pulling a barge the tow line has a sag in it that can’t be pulled out but, at the same time, is pulling a very heavy object. Seemingly, with slack in the line. I never noticed this in my long-ago days as a harbor patrol type towing small boats with my small boat. We used a simple rope so I guess the curve was too small to notice.

59

That’s a negative, Ghostrider.

I served four years on the USS Ranger (CV-61), and two 6-7 month “West Pacs” (1986-1989). During that time, the ship anchored off the coast, or out in a bay, near:

Diego Garcia (Indian Ocean… atoll?)
Perth, Australia
Hong Kong
Singapore
Pattaya Beach, Thailand
Vancouver, British Columbia

The places we tied up pierside:

San Francisco (Fleet Week)
Pearl Harbor
Subic Bay, Phillippines
Sasebo, Japan
Pusan [Busan, now?], South Korea
and, of course, it’s home port: San Diego

It went into dry dock in Long Beach in '89.

The super carrier needs a deep dredged channel/harbor. Wiki shows a draft of 37-41 feet. That’s pretty deep. In those places that don’t have that kind of clearance near the piers, you’re going to have to anchor out. (Dunno why we anchored out for Hong Kong or Singapore. Maybe so we [the USN] didn’t have to pay expensive docking fees.)

They run “liberty barges” for the crew to visit these places when they achored out.

60

Nitpicky clarification: 7 minutes after the tugboat began towing (about 9 minutes after the line was established).