Using a ball weight on the anchor chain

[Uncle Bob]

Originally Posted by mrcarson

Yea sorry Old fart mixing irish words again. I married 2 of them.

Wot, you married a kellick, wow.

[MathiasW]

Originally Posted by mrcarson

Mathias.
Have you ever seen the size of SHIP Chain links. they can weigh up to 1/2 ton and more each. I've been anchoring for a while. Mostly Yachts etc NEED the pick to stick a point into the bed. but heavier chain can hold. I've seen Admiralty and navy film of pick and chain trials. back in the 50/70's.
Mainly Bulk and Tankers. The picks just laying there. Lots chain out and just rising and falling with the sets, but never all down to pick.
as long as you lay plenty
I was a crane operator in prev life Back in '60/70'sas well as a diesel Tech. Often went into local oil refinery to lift and change the mooring chains every 5ish yrs. Tanker moorings where they lay just off and discharged.
MONSTER mussels on there, one was a full meal Yummm. Talking to crews and skippers enlightened you a lot to the anchoring and running off rather than heading for harbour.
NOTHING will hold any vessel in REAL bad weather. The further offshore the safer.
We used to oxy cut the chains into 3 or 5 link lengths and use them as the tug and work boat mooring anchors. after tidal movement had buried them in the sand along the coastline (divers checked)

Yep, I am well aware of that. These beasts are heavy!

Some time ago I got myself a report of professional folks who analyse ship accidents at anchorage, and one example I recall was a vessel with about 2000 square metres windage area, a chain of 102 mm, which is about 200 kg / metre in water, and an anchor of about 10 metric tonnes.

With that windage area, at 21 kn of wind, your wind load is 14 metric tonnes, rising to 52.6 tonnes at 40 kn.

Let's see what your theory of "it is the chain only, by way of friction" would mean in this case. If I assume a friction coefficient of 0.5, then 0.5*10 = 5 tonnes of that wind load are taken care of by the sheer weight of the anchor itself (assuming it has no extra holding power at all, only its weight on the ground matters). That leaves 14 - 5 = 9 tonnes (at 21 kn) which the chain's friction should then take care of. As the chain weighs 200 kg / metre, and with a friction coefficient of 0.5, this means a total of 9/0.1 = 90 metres of chain need to be resting on the seabed, minimally, to provide the required friction.

OK, perhaps... this is half a cable length.

But then look at 40 kn of wind... Now the chain needs to absorb 52,6 - 5 = 47,6 tonnes, which means 47,6/0.1 = 476 metres or ~2.5 cables of chain need to be lying on the seabed. At that sort of wind, in 17 metres anchor depth, another 90 metres or 0.5 cables are needed for the catenary to form, and thus the chain will need to be 3 cables long, at least.

That is clearly not realistic anymore and some load will instead need to be taken care of by the anchor having dug into the soil.

BTW - do not assume that just because some part of the chain is still lying on the seabed, there is no load transferred to the anchor. It is perfectly possible for a chain to lie on the seabed, seemingly at peace with itself, and still transfer a substantial load horizontally.

And finally, have a look at the guidance professionals give. Just one example, there are many more:

https://www.standard-club.com/filead..._oct_09_v1.pdf

They use formulas for the minimally required chain length as provided by the British Admiralty (BA) - essentially a square root of anchor depth, which is a good approximation to the correct formula for strong winds / shallow water. But why would they do that, if all that matters is the length of chain lying on the seabed? In the latter case, the BA formula for the needed chain length would have to be very different and more along the lines outlined by me above. Yet, they use the square-root formula, which is derived from catenary only.

And why would they talk about "high holding power (HHP) anchors" to begin with? If the holding power of the anchor does not matter?

So, bottom line, the guidance the British Admiralty is giving is very different from yours! You may dismiss a yachty like myself, but the British Admiralty????

Cheers, Mathias

Anchor Chain Calculator

[RaymondR]

It's quiet common for mobile oil rigs to test their anchors to 100-150 tons and run a second anchor in series if the main anchor does not hold.