JumbleDuck
Well-Known Member
Perhaps because it gives a more horizontal pull on the anchor - or, to put it another way, because any vertical pull at the anchor reduces its effective weight?
Anchors for big ships
- Thread starter Captn D
- Start date
Perhaps because it gives a more horizontal pull on the anchor - or, to put it another way, because any vertical pull at the anchor reduces its effective weight?
RichardS
N/A
Johntheyachty
Member
If you think 10 mtrs of chain is enough..............!!!!
Cruiser2B
Well-Known Member
The point is that chain friction is not part of what holds the ship - if you drag a chain inline, there's not that much friction, so you need the anchor to keep it from sliding. You don't want the chain to move - it works as a static weight. As I said before, and this was confirmed in the Full City accident report, a dragging ship's anchor is not designed to stop a ship.
Let's put it another way - a common anchor used for moorings is a deadweight anchor - a big chunk of concrete; it has no flukes or other mechanisms to "grab" the bottom; it holds by being bloody heavy. It's not that easy for a ship to carry around a ginormous chunk of concrete; it's hard to store and would need a crane to launch it. However, a ship can carry an equivalent weight in the form of anchor plus chain; this is easily handled with a windlass, stores easily on board, gives a handy attachment for the anchor, provides shock absorption, and allows a variable deadweight. When you come to anchor, the anchor is let go or veered to the sea bottom, and if done properly the anchor chain is then slowly paid out while the ship moves, so that the chain is spread along the sea bottom, rather than piled up in a clump. The purpose of the anchor at this point is to keep the chain from sliding along with the ship, while a sufficient weight of chain is laid out.
Cruiser2B
Well-Known Member
So a ship already has somewhere between 6-10 the depth in length of chain out to the anchor. So if it's lying to 240m of chain in 40m of water, and the chain comes bar taut, what's the angle at the sea bed - something like 10º. If there's sufficient chain to let out 400m of chain, then that angle drops to about 6º. How much do you figure a 4º change in the angle of pull reduces the effective weight of the anchor? Not much I'd wager.
Cruiser2B
Well-Known Member
Well said ....... but I suspect that you're wasting your time.
Richard
Do you have anything intelligent to add to this discussion?
JumbleDuck
Well-Known Member
Do you think it would hold as well if it was on wheels? After all, it would still be just as bloody heavy.
If the purpose of the anchor is to stop the ship and chain moving (I agree) what do you think the point of the chain is?
JumbleDuck
Well-Known Member
Given a constant tension T in the chain, at 10º the horizontal force at the anchor is 98% of T and the vertical force is 17% T. At 6º it's 99% T horizontal, 10% T vertical.
So as the angle changes from 10º to 6º, the apparent weight of the anchor increases by 7% of the cable tension.
silver-fox
Well-Known Member
This guy has done the most comprehensive analysis of the forces at work on anchors and rode I have seen
http://alain.fraysse.free.fr/sail/rode/rode.htm
http://alain.fraysse.free.fr/sail/rode/rode.htm
Cruiser2B
Well-Known Member
If you wish to take it to the absurd, then yes it will hold just as well, until the horizontal force exceeds the rolling resistance. This would be akin to anchoring with chain alone; it would hold just fine until the horizontal force exceeds the friction of the chain.
You misunderstood me. As the ship is paying out cable, it is moving - if the chain isn't anchored it would have a tendency to be dragged by the ship while it is being paid out. The purpose of the anchor is to keep the chain from moving; the purpose of the chain is to be the deadweight that holds the ship. I thought I made that clear.
GHA
Well-Known Member
bikedaft
Well-Known Member
Try it on land
reckon its worth having heavier chain than spec'd for a boat, as long as you can lift it
bikedaft
Well-Known Member
Thanks for link
GHA
Well-Known Member
onesea
Well-Known Member
This really is a marmite subject it seems. Having played with the odd anchor system some thoughts for you...
The chain principle does work, as long as it is not taken to extremes. By this I mean as long as the anchor does not drag!
Having shifted chain around my experience is once it has started moving chain is not hard to keep moving. Its the initial force required against the chain soaked into the sea bed that does helps keep the ship at anchor.
Once any anchor starts dragging at any rate not allot will make it get a hold again, until you can get it stopped and chance to soak in.
The main reason ships will have the anchor systems they do will be commercial. I would hazard a guess ships like yachts spend 95% of there time in calm anchorages trying to squeeze into over full anchorages. Heavy anchor and chain would seem sensible.
Commercially the ship yards know how to install reliable anchoring equipment, the crews know how to use it. Inspection/ maintenance chain will be a known quantity it does not rot quickly it does rust. Rope on the other hand will be checked thoroughly allot more frequently this is all time and money for the ship owner.
Those that talk about all rope rodes also talk about snubbers, imagine the snubber that would be needed for even a small 10,000 ton ship? Once that starts getting any momentum its going to take a hell of allot of stopping.
The chain principle does work, as long as it is not taken to extremes. By this I mean as long as the anchor does not drag!
Having shifted chain around my experience is once it has started moving chain is not hard to keep moving. Its the initial force required against the chain soaked into the sea bed that does helps keep the ship at anchor.
Once any anchor starts dragging at any rate not allot will make it get a hold again, until you can get it stopped and chance to soak in.
The main reason ships will have the anchor systems they do will be commercial. I would hazard a guess ships like yachts spend 95% of there time in calm anchorages trying to squeeze into over full anchorages. Heavy anchor and chain would seem sensible.
Commercially the ship yards know how to install reliable anchoring equipment, the crews know how to use it. Inspection/ maintenance chain will be a known quantity it does not rot quickly it does rust. Rope on the other hand will be checked thoroughly allot more frequently this is all time and money for the ship owner.
Those that talk about all rope rodes also talk about snubbers, imagine the snubber that would be needed for even a small 10,000 ton ship? Once that starts getting any momentum its going to take a hell of allot of stopping.
JumbleDuck
Well-Known Member
So what proportion of the horizontal load put on the chain by the ship do you think is balanced by chain friction on the bottom?
Are you saying that once the chain is laid out, the anchor could be disconnected from the far end with no effect on overall holding power?
NormanS
Well-Known Member
It seems highly unlikely that the ship would drag in a southeasterly direction, under the impact of strong southeasterly winds.
Are you sure that you are giving "factual information"?
vyv_cox
Well-Known Member
Last year, as a consequence of yet another of these 'you don't need an anchor as the chain does all the work' arguments, i pulled my 60 metre, 8 mm chain along the yard in a straight line. I had to pull pretty hard but I got the whole lot moving. I am far from muscular, in my late 60s and weigh less than 13 stone. I would guess that it would be considerably easier if the chain was under water.
As I have written many times before, I have seen all of my chain lifting off the bottom in winds of about force 6 at a scope of more than 10:1.
NormanS
Well-Known Member
I had my chain re-galvanised a few weeks ago, so was stretching it out to measure and mark it. Because you have mentioned before about dragging your chain by hand on the yard, I tried mine. OK, mine is 66m of 10mm, and I am delighted to say that there was no way that I could move anything like the full length of chain on the chuckies (small round stones) of our yard.
onesea
Well-Known Member
In moderate conditions yes, when the force becomes greater the dragging effect of the chain on the bottom still assists the anchor. If the ships thinks it is dragging its anchor as others have said it will start its engine to help the anchor. It also helps dampening the ships yawing as it offers more Resistance through the water than a wire would. Certainly it offers allot more friction along the seabed than wire or rope would.
If chain had no effect rigs would not use it when anchoring, this could save allot of cost and time during rig moves. The dynamics of a 2" or 3" weighting in the region of 100kg's+ per meter. I am sure the same can be said of anchors, measured in tons rater than pounds. Kind of like model yachts v real yachts.
I doubt ships use shock absorbers or snubbers?
I would rather be like a ship and have all chain than composite but life is a compromise, I have a fortress and 10m of chain plus rope as my anchor for a 35' boat. Although given extreme conditions I will be using my CQR as an angel or in tandem depending on how you feel it will work....