Anchors for big ships

[vyv_cox]

A man in the audience at one of my talks had been a RN deck officer on aircraft carriers. He told me that they always dragged, even in quite moderate conditions. A crewman was permanently stationed in the bow to judge when it was time to haul anchor and motor back to their original position.

 

[Kukri]

Source?

My source say otherwise, from 1.8 Examination of the anchor arrangement (http://www.aibn.no/Sjofart/Rapporter/2013-08-eng)

I stand corrected and have edited my posting. I was writing from memory and what I recalled was that at the time a defect was assumed.

 

[knuterikt]

I stand corrected and have edited my posting. I was writing from memory and what I recalled was that at the time a defect was assumed.

You should read the report. It's a thorough analysis of the whole event, shows how a chain of decisions lead to the grounding.

It is unfair to sum the report into one sentence - but..
Due to the weather forecast and the way the weather developed Full City should not have anchored in that position.

 

[Cruiser2B]

Well you're changing your tune to something much less contentious there, from "it (the anchor) isn't expected to dig itself in and provide any pull" to "the contribution of the chain to total mooring load is substantial"

You've taken what I said out of context. What was actually said:

once the chain is off the bottom, it's no longer anchored - if the anchor itself is dragging along the ground, it isn't expected to dig itself in and provide any "pull"

I think if you look at Classification Society documentation on anchors, you'll see it said that large ships' anchoring systems are designed for temporary holding in relatively benign conditions. The anchor system is not designed to stop a moving ship, as the shock loads would quickly overwhelm a system that is (ironically) small enough to fit aboard. Look what happened to Full City - broke the flukes off the anchor. So no change in my tune.

 

[Giblets]

In addition there are different problems ( for example we have limited crew, but our engines can be started quickly and provide thrust almost immediately)

I suspect one of the reasons why Southampton VTS (and probably others) requires all ships anchored in their area at times of strong winds to have their anchor windlasses and main engines at constant readiness for immediate action.

 

[RichardS]

I wish these people who think that both boats or ships "anchor on their chains", would throw their anchors away and just put chain out.

If they're correct they will have no problems. If not . . .

As you imply Tim, this has been discussed many times before. From messing about with 10mm chain on my 40ft cat, admittedly high windage, in crystal clear water (so I can easily see what's happening) and no tidal effect, I reckon that without an anchor I would drag in anything above 10 knots of wind even with 50m of chain out in 5m of water. With 20 knots of wind I would be planing backwards!

Would I risk even one apparently calm night without an anchor? Not on your life!

Richard