anchoring - straight question.

[Guest]

On Sunday night, I anchored in a quiet creek, protected on all sides against wash, swell and waves of any consequence. This question relates only to strength of the wind and tidal rate.

I had earlier in the day dropped my 45lb CQR bower anchor, and really regretted doing so when it came time to weigh. Something hurt in my back, and I planned to get an estimate for an electric thingummy later this week. (My windlass is a SL Anchorman and I think one of the lever operated jobbies would be better than a winch-type).

However, that night I came to anchor in the creek, and decided to use my kedge anchor. This is a 7.5kg Bruce with rope rode only. (I have a reasoned argument why it does not need chain, for another time and another day).

I set the anchor in nicely, and felt quite good about the prospect of getting it up early on Monday instead of winding in the bower beastie.

In the night, the wind got up, and so did I – many times. I increasingly worried that the Bruce would not hold my 36ft/8tons of boat, but each time I checked, I could see that I had not dragged. I used a 5:1 scope, in 6m, dropping during the night to about 3m, with no adjustments. By the time I left at 4.30am. the depth was up to about 7m. The wind had gone up from the F3/4 when I dropped to F5 during the middle of the night. The tidal rate was about 3kts at the worst of the ebb.

I was surprised I got away with it, and I am interested in others’ comments or accounts of similar/different experience. I love horror stories, so reply with these would be entertaining.

 

[Guest]

the key to your experience was probably the rope. because rope stretches much more for a given load, the peak force on the anchor is much lower than when a chain rode gets to its full stretch. thus, whilst the anchor was smaller, the max load on it was less, so no problem.
the grip offered by an anchor is not weight dependant once it has dug in. it depends more on area - hence the performance of the fortress.

you asked for horror stories. once had to wait off portishead in the bristiol channel to catch the tide up to the floating harbour. whilst i was there, along came a narrow boat(!) down from sharpness. first attempt at mooring involved running the narrow boat up on to the mud bank, and throwing out a grapple that would have been a disgrace in a dinghy, on the end of no more than 5m of blue polyprop. hung there for thirty seconds, and then slid back down the bank into the severn. he repeated that exercise at least 10 times before giving up and rafting alongside some helpful soul. the same 5 m rope had to do service when he finally got into the lock (not a marina lock but a ship one) which made him really polpular with the other boats he bounced off. no fenders of course.

 

[Guest]

Physics breakthrough

Where do I buy this miraculous rope that has the property of destroying energy? So Newton had it wrong all the time!

It doesn't matter whether your warp is made of a solid steel bar or elastic bands, the force transmitted to the anchor by a straight line pull from the boat is exactly the same. The rider to this is that a chain warp contributes its own mass to the equation, as a large proportion of it is also lying on the bottom, where it adds its friction to resisting drag. In addition it provides elasticity to the system by the action of its weight lifting off the bottom. Conversely, load is transferred directly to the anchor by a rope warp, as it has virtually no weight.

My comment on the original query - it's a brave man who lies overnight to a 7.5 kg anchor on an all-rope warp. I have a Fortress with 5 metres of chain plus Anchorplait but it is used only for daytime anchoring. Overnights are on the Delta with all chain, which has never failed, unlike the Fortress. Not a criticism of the anchor, but light arrangements cannot be relied upon to cope with wind and tide changes.

 

[ponapay]

Re: Physics breakthrough

I recently anchored in a very sheltered spot where the bottom was soft clay. I used my 25lb CQR with 10 metres of 3/8" chain and 20 metres of rope (plus tow headropes to trees ashore in a tideless anchorage with depths of 4.5 metres).

All went well until mid-afternoon, by which time I had been joined by others each side of me about 5 metres away. The wind (F5) came in from my beam and the anchor draggged, we lifted it and re-anchored using the same anchor as a bower anchor and stayed safely until the wind again increased.

What is the best anchor in soft clay? Please don't say use your main anchor (the main is a 55lbs CQR on 3/8" chain and the windlass was temporarily u/s awaiting spares from UK), I know tha would have been better.

 

[Guest]

It is not miraculous rope just physics.

The anchor is set in the bottom and can withstand a certain number of pounds pull before it breaks loose or drags. So long as you don't load to this break-loose point, it will hold.

By using a nylon (stretchy) rope, you avoid a shock load when the boat moves away from the anchor and pulls it up tight. Stopping 8 tons in 1/10 second when the chain comes up tight can take 25,000 pounds - more than enough to crack the anchor loose. BUT if you spread that load out over several seconds, as happens while the nylon rope is stretching and slowing the boat mass, the peak load can be kept to a level that doesn't break it out.

While it is true that the catenery of a chain does spread the load out over a period of time, similar to a rope stretching, when the chain finally does come taught, it has absolutely no give and is much less reliable than the stretchy nylon, especially in shallow water where the catenary is minimal.

If you want the strength and abrasion resistance of chain, then after setting the anchor and letting out the scope, you should then attach a nylon snubber to the chain, let some more out and tie off to a cleat. You then let out extra chain that lies under the snubber as a back-up in case it breaks.

On our boat (75 tons) we use about 30 feet of 3/4" 3 strand nylon for a snubber on 1/2" chain. A smaller boat should size the nylon so it is quite springy under expected loads.

 

[Guest]

Re: Physics breakthrough

there's no simple answer to your question. if you want to read some of the technicaldetails, there is a book written by a frenchman called (i believe) alain gree. this goes into questions suchas the loading generated by the wind on the boat etc, and then looks at comparative trials of various anchor designs. available in translation atyour local library - or at least it is at ours.

 

[vyv_cox]

Re: It is not miraculous rope just physics.

The original post described anchoring in a quiet, fully-protected creek. I based my response on this information - there are no snatch loads. I stand by my posting: a nylon warp, nor any other light type, does not reduce load on the anchor. A 7.5 kg anchor is not adequate for a 38 ft boat overnight.

 

[incognito]

Re: Physics breakthrough

I started this thread because I was surprised at how little held so much. VC has referred to a quiet little anchorage, but the wind buffetting was quite significant and the rode was springing quite a bit.

However, the book to read is "The Complete Book of Anchoring and Mooring"
Earl R. Hinz. This is a complete treatment of everything you could ever want to know, except what I asked - "what can you get away with?".

 

[Mirelle]

Tiny suggestion

I think you will find the 35lbs CQR much easier to handle than the 45 and still more than adequate for ordinary anchoring. The difference is remarkable when it comes to weighing the brute.

37ft gaff cutter weighing 9 tons. Have never dragged it yet, in almost 20 years, but have the 45lbs one in the bilge as insurance. Much cheaper than fancy windlass!

 

[Bergman]

I don't know the answer so I will ask another question.

Where I sail we have lots of ships anchored, 8 of them yesterday. Many of these are very large tankers both full and empty. The anchor cables, chain of course hang almost vertically down from the bows. When I anchor the cable, runs almost horizontally away from the bows.

Anybody hazard an opinion to explain the difference?

As an afterthough I remember once fishing on a large coble out of Bridlington. The skipper found his mark off Flamborough Head, literally dropped a large (about 3ft long) fisherman anchor over the bows and started fishing. No setting no chain just rope and the coble never moved all afternoon despite the strong tide around Flamborough, a 8 ft swell and f5 wind. It was a big heavy boat too licensed for about 20 people.

Do we worry too much about anchors?

 

[Guest]

straight question ??

Anchoring is one of those sciences that have so many variables it becomes an art. In simple terms nylon will stretch and then relieve the shock loadings from the anchor but the boat will tend to yaw around and try and twist the anchor out of the bottom. Chain on the other hand relieves the shock by its weight and the boat having to lift the chain off the bottom. So in general terms the Warp for shallow water and chain for deep water. There are so many anchors these days and on average one is as good as another but where one is excellent in one type of what am another will be adequate and vice-versa. The answer is before you anchor to test the bottom either by arming a lead or getting the scuba gear on, and then choose a your anchor assuming you carry six or eight different types!
My anchoring story goes something like this
First time in St Mary's, Scilly Isles arrived to find the only anchoring space at the back of the bay anchored at 6 o'clock (important point) tidied up had a meal went to bed to be woken at about 1 o'clock the next morning by a parachute flare going off Overhead. Realised it was a touch windy, so I let out all I anchor cable 28 Fathoms in 8 ft of water at low-tide. The remaining night and following day was spent anchor watch. When the wind really got up and shifted slightly to the north giving us a good lead off Newfoundland. By which time I had started the engine and ran it slow ahead. Firstly to relieve some of the a shock loadings, secondly if the anchor or chain parted I would just be able to openly engine and a motor away!! The following day the wind dropped to a gentle force 5 and we decided to move up the Anchorage, the only way we could get the anchor out was to let out six to 10 ft of chain out from the up and down position and motor flat out over it to pluck it out. I've still got the anchor shackle very twisted and bent. It's known as our Fastnet shackle.
Incidentally all the boats that dragged and went ashore were trying to anchor on more than one anchor! Usually they had twisted the Cables round each other some way and plucked them out.

Peter

 

[Guest]

Re: Physics breakthrough

I think, if you dug your little anchor well down into the sand on the beach wouldn't it take an awful lot to shift it? You could imagine needing to drive your car quite hard to pull it out. Maybe a ton or more of load?

Why should it be any different for your boat?

 

[Twister_Ken]

Weight has got (almost) nothing to do with it

Logically, the weight of the hook has got nothing to with the holding power of the anchor ONCE it has dug in.

What a dug in anchor has to do is resist the more-or-less horizontal pull generated by tide and wind on the boat. This is a function of a) the resistance of the bottom material, and b) the cross section the anchor presents to it. So a bent teaspoon buried in concrete would provide good holding, as would a six foot by four foot sheet of plywood buried in soft mud (provided it was perpendicualr to the direction of pull.)

So, if you get lucky with your bottom, a small hook well buried in something sufficiently sticky would hold the QEII, while a big hook buried in raspberry jelly wouldn't hold an Avon.

Where weight (and shape) really count is getting the thing buried in the first place. A heavy weight, a sharp point(s) and a horizontal pull increase your odds of getting dug in. That's why my favourite anchor last season was a 40lb danforth-type, with two really sharp points.

I used to be a sea angler in my youth and I remember walking across the sands, bait digging. Even a relatively short stretch of beach could present very differnet hardnesses of sand. If you were to anchor in the right bit you'd stand a gale. The wrong bit and you'd drag. And the right and wrong bits could be only a few feet apart.

 

[incognito]

How to revivify an old thread...

Just seeing what aged threads do to loading. I suspect from performance so far today, that it goes down with this new technique of bubbling everything new to the top of the heap.

Please excuse me if I am right.

 

[jimi]

Re: How to revivify an old thread...

I was going to do that ... but managed to restrain myself!

 

[incognito]

Re: Physics breakthrough

Spotted this after all this time, and realised you were wrong!!
A spring is analagous to an inductance in an electric circuit, and does indeed absorb the energy - there is a formula for the potential energy stored in a spring and I can't remember what it is... So the anchor warp absorbs the shock of the snatch load, by extending the rope and flattening the catenary. It then SLOWLY (comparatively) loses the energy by holding the pull longer. The effect on both ends of the rope, boat and anchor, are that shock loads are smoothed out into flatter, longer lasting loads.

If you want to argue the toss, I'll look up the formula in my mechanics book tonight.

 

[Twister_Ken]

Re: Physics breakthrough

As Vyv has headed Balticwards, the toss is unlikely to be argued for a while, unless an Internet caff intercedes.

 

[graham]

Re:Bruce Anchors

I once anchored in Bridgwater bay in hard sand waiting for a bit more tide before going into Burnham.

After only an hour or so the Bruce had buried itself so deep we had to use the rising tide to lift it.The boat went well down by the head before the sand let go.

Anchoring is something entirely dependant on the bottom quality Which can never be accurately determined in the real world of small boating.

 

[Grehan]

Re: Weight has got (almost) nothing to do with it

. . get lucky with my bottom? . . huh!

 

[Twister_Ken]

Re: Weight has got (almost) nothing to do with it

Nice to know someone is paying attention.