Anchoring in a blow?

[Uricanejack]

Sat through many a blow on a well set soddin great big CQR with 10m heafty chain and more than 5 x depth. never dragged.
Out with brother in Law. anchor soddin great CQR. dumped straight down 5 x depth chain plied on top. I asked if he was going go astern he told me oh thats just a myth you don't need to do that.
I always feel its rude to try and teach some how to sail or anchor their own boat. unless they ask.
I did ask if he ever dragged. He apparently found it quit a problem.
I sugested a simple transit might be worth while.
He showed me how to select one,
We dragged in a slight breez before dinner was ready.
Deciding the holding ground was obviously not good we picked up and went some where else.

 

[Neeves]

Impact loading is reckoned to be x10 the force applied in steady pull at the same load. In the case you describe I can only suppose that to be the cause.

Vyv,

For those of us with an inadequate education:

Can you elaborate on this: 'impact loading' is reckoned to be x10 the force in a steady pull at the same load.

I query because,

If you bend a steel beam beyond its yield stress it undoubtedly bends, but bending takes time. So if you load it (well) beyond its yield stress and immediately release the load then it does not bend as much as if you were to load it and provide a time element. I appreciate the query, of the snapped chain, was actually about absolute, or catastrophic, failure.

Jonathan

 

[vyv_cox]

£45 ! is it double time on a Sunday?

Not for us, we are giving our services free, although our travel expenses are paid. The fee includes refreshments, an excellent buffet lunch and printouts of every slide shown. Judging by my progress so far the printouts will approximate the size of Encyclopedia Brittanica.

 

[vyv_cox]

Vyv,

For those of us with an inadequate education:

Can you elaborate on this: 'impact loading' is reckoned to be x10 the force in a steady pull at the same load.

I query because,

If you bend a steel beam beyond its yield stress it undoubtedly bends, but bending takes time. So if you load it (well) beyond its yield stress and immediately release the load then it does not bend as much as if you were to load it and provide a time element. I appreciate the query, of the snapped chain, was actually about absolute, or catastrophic, failure.

Jonathan

You are correct, which is why ductile chains are better than brittle ones

The x10 statement is just a rule of thumb that I was taught in college.

 

[[2574]]

If you don't think that your chain is forming a useful catenary, your chain is too light.

Peter Smith makes a strong argument against the need for catenary effect and heavy chain here: http://www.petersmith.net.nz/boat-anchors/catenary.php

His recommendation of lighter high strength chains in place of G30/40 seems to make a lot of sense to me. Better 100m of 8mm than 60m of 10mm.

 

[vyv_cox]

A coal mining engineer told me you can tell when a piece of chain has been overloaded, it goes stiff. I imagine that would be very high quality chain.

That's a good indicator. When the links stretch the internal diameter at the contact with the next link reduces, causing the chain to become rigid. The samples we tensile tested can be held up vertically like a stick. They were Grades 30 and 40, both behaved the same.

 

[fisherman]

Peter Smith makes a strong argument against the need for catenary effect and heavy chain here: http://www.petersmith.net.nz/boat-anchors/catenary.php

His recommendation of lighter high strength chains in place of G30/40 seems to make a lot of sense to me. Better 100m of 8mm than 60m of 10mm.

The same chap says this in my link earlier

"It is a mistake to think to tandem anchor regularly. You must have a primary anchor of a good size and type for your boat. If not, get one."

 

[jimbaerselman]

Not for us, we are giving our services free, although our travel expenses are paid. The fee includes refreshments, an excellent buffet lunch and printouts of every slide shown. Judging by my progress so far the printouts will approximate the size of Encyclopedia Brittanica.

I reckon Agnieska's buffet lunches are worth a good £20. All you can eat, and a tremendous variety.

 

[jimbaerselman]

Peter Smith makes a strong argument against the need for catenary effect and heavy chain here: http://www.petersmith.net.nz/boat-anchors/catenary.php

His recommendation of lighter high strength chains in place of G30/40 seems to make a lot of sense to me. Better 100m of 8mm than 60m of 10mm.

And better still, his recommendation to use a mixed rode, plenty of nylon snubber, so that the snatch forces are spread over time.

 

[binch]

Load

My figure for load on anchor cable comes from the standard table in my naval architecture text book.
Everything depends on how you calculate the block coefficient below the waterline and the effective wind resistance of masts and rigging as well as the hull block above the waterline. Above waterline, the envelope is used.
The following table is based on tests (I cannot say exactly what the tests consisted of) and is known as Martin's formula.
Wind force Beauf Pounds force per sq ft.
2 0.25
3 0.5
4 1.0
5. 2.0
6. 3.25
7. 5.0
8. 7.4
9. 10.4
10. 14-4
11. 19.6
12 25.6
The equivalent tractive effort to achieve a speed of seven knots in still water would have been just a trifle under the force 11 wind effect on an 80 ton three masted schooner.
The formula is standard for sailing ships.

 

[binch]

confusion

I laid out the table above, but when pressing the send button, the figures all hustled over to the left. It should be possible to sort them out.
I'm not very good with blasted computers.

 

[PhilipH]

The same chap says this in my link earlier

"It is a mistake to think to tandem anchor regularly. You must have a primary anchor of a good size and type for your boat. If not, get one."

We have used tandem anchoring on three occasions and it has worked - all in very soft mud bottoms. Our boat is 41ft LOA and our primary anchor is a 20kg Delta and we have a Fortress FX37. We attach the FX37 as recommended by Peter Smith on about 7 metres of chain from just aft of the shank on the Delta i.e. both on one rode. However, it has got a bit messy on recovery as, well ... all three places were tidal rivers: Luis Correa in NE Brazil, Kourou in French Guyana and Charleston in South Carolina. The big BUT is that while other boats around us were dragging we were not, even with winds of 30+kts and even with reversing tide. It may well be that the Fortress would have worked without the Delta but I'll have to rethink ...

 

[NormanS]

We have used tandem anchoring on three occasions and it has worked - all in very soft mud bottoms. Our boat is 41ft LOA and our primary anchor is a 20kg Delta and we have a Fortress FX37. We attach the FX37 as recommended by Peter Smith on about 7 metres of chain from just aft of the shank on the Delta i.e. both on one rode. However, it has got a bit messy on recovery as, well ... all three places were tidal rivers: Luis Correa in NE Brazil, Kourou in French Guyana and Charleston in South Carolina. The big BUT is that while other boats around us were dragging we were not, even with winds of 30+kts and even with reversing tide. It may well be that the Fortress would have worked without the Delta but I'll have to rethink ...

Is your "winds of 30+kts" possibly a miss-print?

 

[PhilipH]

No. Is there a problem? This was very very soft mud

 

[NormanS]

I readily accept that very soft mud can be a problem, but I still wouldn't expect to drag in 30 knots of wind. What strength of current was there?
I do, on occasion lie to two anchors, normally in a vee. I've never tried tandem anchoring, but I know it has been used successfully in Patagonia, for example, where holding is notoriously poor due to kelp. The thing which has always put me off is the hassle of retrieving the outer anchor. Do you have the length of chain between the two anchors, greater or less than the depth of water? I think it would be easier to retrieve if the chain length was greater than the depth.

 

[PhilipH]

I readily accept that very soft mud can be a problem, but I still wouldn't expect to drag in 30 knots of wind. What strength of current was there?
I do, on occasion lie to two anchors, normally in a vee. I've never tried tandem anchoring, but I know it has been used successfully in Patagonia, for example, where holding is notoriously poor due to kelp. The thing which has always put me off is the hassle of retrieving the outer anchor. Do you have the length of chain between the two anchors, greater or less than the depth of water? I think it would be easier to retrieve if the chain length was greater than the depth.

As I said, very very soft mud and tidal: current about 4kts. Doesn't matter what you expect, it's what was there and happening. I use a fixed length of chain because other wise there's an extra dangly bit and if things can go wrong they will. KISS.

 

[Neeves]

I readily accept that very soft mud can be a problem, but I still wouldn't expect to drag in 30 knots of wind. What strength of current was there?
I do, on occasion lie to two anchors, normally in a vee. I've never tried tandem anchoring, but I know it has been used successfully in Patagonia, for example, where holding is notoriously poor due to kelp. The thing which has always put me off is the hassle of retrieving the outer anchor. Do you have the length of chain between the two anchors, greater or less than the depth of water? I think it would be easier to retrieve if the chain length was greater than the depth.

I find it difficult to understand why an anchor that will not hold in kelp is able to hold when their are 2 of them. Our experiance with kelp, this is the real stuff that can be 20' long, like NW Scotland, is that nothing holds and you are much better planning to anchor somewhere else, maybe a gap in the kelp bed. The only anchors we have read of that, might, hold in kelp are monster fishermens styles, the pre 1900 types, but to work they need to be huge and we do not carry one. But to find that one's anchor does not work in weed and then feel the need to deploy a second looks to me like an immediate demand for a 24 hour personal, ie not electronic, anchor watch. Spade (and in Oz, the Excel) work well in weed (better than a same weight Supreme) because they are heavily weighted and sharply toed - but certainly heavy weed (and kelp is worse) defeats our Excel - and we move on, or find a sand patch.

But we have never tried tandem anchoring but often use 2 set at an angle, as they stop the yacht swinging and a steadier yacht has less snatching.

 

[Neeves]

PhilipH,

Have you ever worked out your 'area' and if so what and what sort of yacht are you sailing. I've seen a similar table, do not recall where, that equated simply to length, on the basis that windage was related to beam and that beam related to length. Designs, and proportions, have changed and your area might be more useful - but I assume this is frontal area?

We sat and watched a row of 13 yachts, all using Delta/CQR's drag in 25 knots, excellent holding - but there were biggish seas. The forecast was optimistically max of 15 knots, it was a lea shore and the nearest shelter 6nm away. As you say, its what happened, not what one expects.

Jonathan

PS Its really refreshing to see someone who has the same problems with layout. I do it so often I knew exactly how to sort it out.

 

[NormanS]

I find it difficult to understand why an anchor that will not hold in kelp is able to hold when their are 2 of them. Our experiance with kelp, this is the real stuff that can be 20' long, like NW Scotland, is that nothing holds and you are much better planning to anchor somewhere else, maybe a gap in the kelp bed. The only anchors we have read of that, might, hold in kelp are monster fishermens styles, the pre 1900 types, but to work they need to be huge and we do not carry one. But to find that one's anchor does not work in weed and then feel the need to deploy a second looks to me like an immediate demand for a 24 hour personal, ie not electronic, anchor watch. Spade (and in Oz, the Excel) work well in weed (better than a same weight Supreme) because they are heavily weighted and sharply toed - but certainly heavy weed (and kelp is worse) defeats our Excel - and we move on, or find a sand patch.

But we have never tried tandem anchoring but often use 2 set at an angle, as they stop the yacht swinging and a steadier yacht has less snatching.

I don't anchor in weed either. One of the best bits of my anchoring equipment is a colour fish-finder. It gives you a really good idea of the bottom. There are some anchorages on the West Coast of Scotland, which are notorious for kelp. Canna and Craighouse immediately come to mind, but with the FF, clear parts can be found. I do carry a 20 kg fisherman anchor, in addition to a Bruce and Danforth, of the same weight, but very seldom use the fisherman. I find the (genuine) Bruce to be excellent.

 

[KellysEye]

It depends what you mean by a blow. If wind looked likely to reach thirty knots plus we would put out two anchors. For hurricane conditions we carried a third monster anchor.
One general thought it is always worth buying an anchor spec'd for at least one size bigger than for your boat, that's what we did.

We always put up a riding sail to reduce the degree of swinging and thereby cut the snatch load. I find it very odd how few boats use one.