Sorry. I replied to the #1 post without proceding to see that others have already covered it.
In that case, the boat isn’t being dragged by current, so it doesn’t matter that the anchor isn’t doing anything. When the boat begins to drift, then the anchor will be dragged close to parallel and it should engage the seafloor.
Think about it… if you are carrying an anchor chain massive enough to have any significant effect on the inertia of a 100,000 ton aircraft carrier, it would be so heavy that you wouldn’t have any room for anything else.
I remember back in high school physics class using math and some formulas to prove that there is always a catenary curve on a rope, chain, string, etc hung between two points, no matter how much tension there is pulling the ends apart. It would take infinite tension to overcome the weight of the string/chain/rope itself in order to make the line perfectly straight w/ absolutely no catenary. Unfortunately I couldn’t remember the math to save my life, but I specifically remember doing it because I thought it was incredibly interesting. You can never pull a line like a tightrope, or a tugboat line, or a telephone cable, etc, perfectly straight without some sag (the sag always being a catenary curve). Physically impossible (at least on earth w/ earth’s gravity).
Incidentally, an upside-down catenary curve is the most efficient shape of arch for construction purposes. I also find that extremely interesting.
But IMHO, the most interesting thing about catenary curves is that they are NOT parabolas. You’d think they would be, but it’s a totally different formula for a parabola. Catenary curves are not parabolas. I remember doing the mathematical proof of that too back in high school too… but alas, that knowledge has long since vanished from my brain. Before then I’d always assumed a hanging string, etc, was a parabola. But a parabola is not the most efficient form. A catenary is.
Yes that is true. And a reverse scenario. Lets say you want to cross a gorge or canyon or between 2 trees on a rope. So you run a rope from point A to point B. You pull it nice and snug so that its (almost) straight. And lets say the rope doesnt have much stretch to it. Now you throw a pulley on it and decide to roll from point A to point B. And lets say your rope has a 2000 lb breaking strength and you only weigh 200 lbs. Should be safe right? Nope, depending on the exact details that rope could well break.
Back to the anchor. The more chain you have and the heavier it is in relation to the anchor the better the anchor will work. Lots of chain but no anchor is better than nothing. Anchor but lightweight rope is better than nothing. Anchor with good bit of heavy chain is best of all.
Agree about the catenary curve. I also agree the more chain you have on your anchor the better it will work, as it will allow for a smaller, more acute angle between the boat and the anchor - as close to parallel as possible without the boat being on the seafloor with the anchor.
But I disagree that the weight of the chain has anything to do with it other than being proportionally strong enough to hold and hoist the anchor and maintain the tension between the anchor and the vessel without snapping. The bigger the vessel, the bigger the anchor it needs (some like the supercarriers even need 2 huge anchors), and the bigger the anchor the heavier it will be… and the heavier the anchor the stronger connection you need between the vessel and the anchor. As such, with super-huge mega-anchors like those on carriers and huge cargo ships, extremely heavy chain is the only thing that will work, only thing capable of holding the huge, heavy anchor between the huge, massive ship. Especially when it’s being used and there’s a lot of tension on the line, up to 10 times the weight of the anchor itself. Huge, heavy anchor chain is a limitation on currently available materials, not part of the anchor design/function.
If we ever develop carbon fiber filament strong enough (think space elevator), it would work just fine on a ship to hold the anchor even if 1000 feet of it only weighs 10 pounds and is less than half an inch thick. In fact, I’d think it would be preferable in every way to huge, heavy chains with 350+ pound links… it would save huge amounts of space and weight on the ship, and it would be rustproof. In addition, it would allow much longer lengths (due to the saved space and weight) so as to make the anchor work better (more acute angle with a mile of line than with 1000’)… instead of 1000’ could have 10,000’ or more. Of course I’m assuming the carbon fiber filament would be strong enough and possible to produce in sufficient lengths at a reasonable cost. One of these days…
Based on the Wikipedia entry on anchors and everything I’ve read about how they work, I’m confident this would be true. If they could hoist a huge 200-ton anchor with this, light carbon fiber filament half an inch thick with a negligible weight, assuming it is as strong (or stronger) than the huge chains used today, I’m certain they would do so. The anchors would not lose any efficiency, and would probably gain some for the reasons stated above. A heavy chain isn’t necessary except insofar as it’s needed to maintain the connection between the anchor and the ship. The size of the chain is proportional to the mass of the anchor, which is proportional to the size mass of the ship.
And I am going to say your are wrong there. The heavier the chain the more the pull on the anchor will be horizontal rather than vertical.
Assume an infinitely light chain. Then assume an infinitinly heavy one. Think about how that will affect the shape of the curve between the anchor and the boat. When you do that its obvious the weight has something to do with it.
Should work, provided it’s long enough, and the anchor is sufficient for the task.
Might be a problem when it drags the ship down to the center of the earth with it.
Funny, but there is an actual point if you bother to think about it (and BTW letting things go to infinity or zero is almost a physics 101 method of thinking about things to understand how they work).
One point about anchor rope. The synthetic rope is pretty close to neutrally buoyant. The preferred material - nylon - has a density of about 1.125, and polypropylene is less then unity, and actually floats. Thus for all useful intents the rope portion of rode makes no contribution to the jobs of keeping the anchor lying flat, and thus set, nor any contribution to absorbing shock loads to the system. (There will be little no catenary shape, the rope will pull essentially straight.) The upshot is that you will need to use a significantly larger anchor than if the rode includes a length of suitably heavy chain. The chain keeps the anchor set, and absorbs the shock loads as the boat rides waves. In a well sized system you want the anchor to be responsible for the lateral loads, and the chain to be responsible for keeping the loads lateral, and for absorbing shock loads that might unset the anchor.
Good point Francis. But I would say the rope part helps a good bit as well. If it has some stretch to it (and most rope certainly does) it will “absorb” some of the dynamic loads that the chain or anchor don’t.
Here is a site purporting to explain why the anchor is not what holds the ship in place:
Your reasoning is way off. With enough chain out the chain (being very heavy) can be and should be laying on the seafloor, pulling parallel to the seabed. The chain can be pulling on the anchor while resting on the seafloor in the same way you can grab someone’s legs and pull them across the floor even while their upper body and shoulders remain on the floor. You don’t have to pull them so hard that only their head is touching the floor for force to be exerted on their head.
This factor is crucial in proper anchoring.
I know you have conceded on the bulk carrier point but just to rub it in, you really need to check this photograph. Large ships wait at anchor for berths all the time. Sights like this are commonplace of major coal ports for example.
And by the way, a large proportion of them are far heavier than tiddlers like mere aircraft carriers.
I agree that anchors are a safety measure. All large commercial vessels transit channels with (usually) the first officer at the bow anchors standing by to let go if there is a steering or engine failure. And anchors are used to try to hold a vessel off a coast in the event of adverse weather.
However, the rest of what you say here is wrong. In bad enough weather and in the wrong location, anchors simply will not hold. If your first statement above were applied, no boat (or ship) would ever go to sea. The *Pasha Bulker *to which you refer went aground because the master failed to depart the Newcastle anchorage in the face of prospective bad weather. He had the anchors out. They would not hold, so he picked them up and attempted to get out to sea, but couldn’t make headway in a ballast condition with the prop not fully submerged.
The full ATSB report is on line if you feel like reading up. The wikipedia entry is cursory to the point of being misleading and if I wasn’t in a hurry I’d correct it.
If you assume an essentially weightless chain, then it essentially has no catenary curve, which means you would need a considerably longer line to maintain the necessary angle.
An excessively long chain could be problematic if you’re trying to anchor close to shore or port, because the tidal variability in your position would require you to anchor further away. Or if you were trying to anchor a fleet of ships in the same general area, they’d have to be farther apart so as not to tangle their lines.
But I don’t doubt they probably would lighten the existing chains somewhat if they could. If you had an ultralight chain except with the last 20% (WAG) heavy, that ought to maintain the correct angle to keep the anchor engaged in the seabed.
It’s just an elaborate semantic game and the author knows it. He’s saying they do no work in the strict physical definition of ‘work’, which is not the same as effectiveness. Then he goes on to say that the chain keeps the boat in place, and the anchor keeps the chain in place. But of course without the anchor keeping the chain in place, the boat wouldn’t remain in place either, so naturally it follows that the anchor keeps it in place.
He also mentions that given a long enough chain, an anchor wouldn’t be necessary, but he doesn’t really linger on the fact that every boat in the damned world uses an anchor instead of piles and piles of chain.
So, in short, the statement “anchors don’t work” doesn’t mean what it seems.
Minor nit picK. The chain, cable or what ever that attaches the anchor to a ship is called rope. In fact it is the only rope on the ship.
Anchoring with enormous scope implies the freedom to swing in an enormous circle in response to changes in wind and current. As Cosmic Relief notes, this could be a serious problem in a restricted anchorage (which many are).
Never seen the 1st mate or anyone near the bow when coming into or out of port. If anything he was on the bridge with the captian.
Your experience is naval, isn’t it?
It may be persons other than the 1st, but as to standing by anchors during navigation in channels…
From a Pilotage Plan checklist:
From a document apparently concerning crew duties:
US Marchant Marine. Commercial ships.