Anchor Questions

[gjbentley]

Just got back from a week onboard our 35ft yacht Grand Slam, where my wife & I managed to get as far as Weymouth and back from Port Solent.

One incident did give us concern, and I would value the opinion of other skippers on it.

We were anchored in Studland Bay 1700 hrs using the 10 metres of chain and 30 meters of rope onboard in 4.5m of water. The wind was SW, F 3- F4. The number of yachts anchored in the bay built up quickly to over 40 as we conducted an anchor watch for the next few hours before going to bed at 2200 hrs. During this time Grand Slam seemed to behave as if she was on the end of a bungee cord, bouncing around as the wind and tidal streams varied. Having always used chain only on other yachts, this was a new experience.

The GPS showed we were keeping within a "triangular area", but the close nature of all the other yachts did give me concern. At 0100 hrs I got up to check our position which was still okay. The wind then changed to on-shore and increased to F4-F5. We were rocking around a bit making sleep difficult in the aft cabin, so we moved to the saloon area.

At 0330 hrs we heard a bang as Grand Slam hit up against another smaller yacht. Luckily there was no damage, but it caused a panic as I realised the anchor had dragged 150 ft. I put the engine on and got the anchor back in with just my thermals on !

I then made the decision that it was too crowded to attempt re-laying the anchor, so we made our way out to the Poole No 1 Bar buoy then plotted a course to Weymouth, our next port of call. Emotionally we were a little high and on edge, but it was a safe night passage and we arrived in Weymouth at 0930 hrs after an initial 2 hours of foul tide.

The question I have for you is over the experience of otghers of using the anchoring gear 6hat is chain & rope combined. I am not keen to use two anchors at the same time, and I am now uncertain about the existing length of chain and elastic nature of the rope. My thinking is to add 40 metres of 8 mm chain for shallow anchoring, with the option of using the additional 30 metres of rope for deeper water. I need to feel safe when anchoring as we plan to use mainly anchorages when we take our cruise to the Channel Islands in July.

What do you think ?

 

[Gunfleet]

A frightening experience. On the face of it 9 times the depth when you anchored in 4.5 metres looks okay, but you don't relate what was the depth when you dragged and what was the depth at high water. You sound experienced enough to know to make sure the anchor is well dug in. You don't say how much the anchor weighs.

 

[TigaWave]

Its going to one of those many differing opinions answers I feel, as soon as anchoring is mentioned.
First things for confident anchoring and getting a full nights sleep are practice, but there will still be sleepless nights especially when the wind changes and it gets crowded. However the majority of times anchoring is easy and peacefull, so persevere and maybe get some longer chain as it will reduce the bouncing swinging speed. A 180degree wind shift will always risk dragging the anchor and re setting it, some will stay held as they turn better than others but it will depend on the bottom. If you expect this change then its worth (if theres space) kedging the stern to keep the bow facing out of the bay, it also stops any rolling as small waves come into the bay.
But keep practicing...have a good trip to the CI.

 

[BlueChip]

Studland Bay is notorious for poor holding - so many people anchor there that the sand is ploughed constantly and is therefore quite loose. There are also extensive patches of grassy weed which prevent an anchor setting properly. Did you dig the anchor in well when you arrived? If not then you may well just have been laying to the weight of anchor and chain and when the wind came up you would drag.
In shallow water, too much chain/warp out can also give problems, the vessel shears around the anchor and at the end of each shear the pull on the anchor is almost at right angles and it comes free. This sounds to me like your problem. In deeper water the weight of the chain largely prevents that shearing.

My own belief, from experience is that in shallow water - less chain/warp is often better.

 

[Rich_F]

I just checked - the length of rode looks fine for 4.5 m, and 1700 last saturday was around high tide.

I'd be surprised if having a stretchy line would be an issue for holding; I'd have thought that the reduction in snatch loads would be an advantage.

It sounds, then, as if your anchor simply failed to re-set after the wind changed. It would be interesting to know the size and model of the anchor. And was there any weed, etc, fouling the anchor when you lifted it?

I would strongly recommend getting/using a GPS with an anchor alarm. It doesn't use much power to leave the GPS on overnight, and it certainly helps me sleep more soundly (as if I needed any help /forums/images/graemlins/smile.gif)

For what it's worth, we've anchored regularly on an overweight Spade anchor with 50m chain, and never dragged, or even had to re-set when anchoring. But I'm sure that you'll get lots of reommendations for other types as well.

 

[Evadne]

I can't speak for Studland but the amount of rode sounds more than enough, if the anchor will dig in.
I once anchored in Swanage bay, just off the end of the pier. It was flat calm but got up to a NE F6 shortly afterwards. This is not compatible with a good night's sleep and she bumped and snatched all night, but didn't drag an inch. I can confirm that the holding there is excellent and it is closer than Weymouth.

 

[Rich_F]

[ QUOTE ]
I can confirm that the holding there is excellent and it is closer than Weymouth.

[/ QUOTE ]
I think that it was last year (or maybe the year before) when we had a yacht drag onto the beach at Swanage - but that was at the north end of the beach. Though I've got a feeling that the wash from the Condor ferry might have been cited as contributory.

But, I agree, it's much closer than Weymouth. In fact, it's much closer than anywhere (given that I live here /forums/images/graemlins/grin.gif)

 

[Close hauled]

Bear with me on this! The only scientific analysis that I've read regarding anchoring with chain and rope combinations is in the book 'Oceanography and Seamanship'. There is a very detailed analysis of how anchoring using chain and nylon rode is effective.

The author arrives at the following formula

Length of rode + 0.8xLength of chain = 4.8 Depth of water

OK - I know that nobody is going to stand on the foredeck and try and do this in their head, but it is easy to work out a rule of thumb for your own particular combination.

In your case I get a result that would suggest that if you had 8m of chain out plus 14m of rode (22m total) that should be OK. (As near as damn it 5 x depth of water)

I'm interested because I have a cat with a rope/chain combination (20m chain + 60m rode). I find 5 times the depth works pretty well.

 

[Evadne]

Well I've no intention of repeating the experience on purpose. Swanage is very pretty and a sheltered anchorage mostly, but in an onshore F6 it was 'orrible. I anchored at the place recommended in the Shell pilot, just north and west of the pier if memory serves. We have a 20lb Sowester CQR copy and about 20m of chain. In conditions worse than "nice" I tend to put the lot out, following the adage about how much good it does you in the locker compared to on the sea bed. I'd behave a bit differently if it were more crowded.
I have to say also that I almost never sleep when anchored, I'm an eternal worrier and I've dragged too often (once is too much in my book), but this restored my faith in the anchor and chain that I currently use. I probably still wouldn't sleep unless the keel was buried in the mud as well as it would be in Pyefleet creek though!

 

[sttpt01]

I spend most week-ends anchored overnight in Studland Bay and so far I have never had any problems. I have a 9.4m yacht , delta anchor with 20m chain/30m warp. I usually anchor in 2-4m of water more towards Old Harry and usually find the bottom to be clay/mud.
I find that letting out too much rode can mean another boat over your anchor when it is busy. This can make life interesting retrieving the anchor when making an early start in the morning (4-5am sometimes!!).
Reading some of the posted replies has now put a little doubt in my mind !!

 

[jrt]

There is an excellent French book on anchoring, going into all aspects of the subject. A whole book on the subject shows how complex the subject can be. On the subject of rode length the book gave a simple rule of thumb: The rode length is given by the maximum expected wind speed (Beaufort scale) multiplied by the maximum depth (high tide). It assumed that the rode was all chain. And it advised a minimum of 5x the depth. A rope rode was longer.

 

[starboard]

I use a similar combination of heavy chain (10metre) 14mm rope and Delta anchor for my Albin Cirrus 26ft. I find this a great combination and will anchor nearly every trip.With only one case in the last 4 years of dragging. However what you will find with a chain rope combination is that in certain conditions the boat will range about a fair amount compared with a vessel anchored by chain alone. With this in mind you must take extra care and leave more swinging room when in a busy anchorage.

Paul.

 

[ashanta]

The length of scope seems considerable for a tidal range of around 3-4 mtrs and HW being around 4.5 - 5 mtrs. Allowing for the rope and chain combo, IMHO, i would have let out about 20mtrs max. Your scope was double that and I'm wondering if the excessive scope has not allowed the the scope to have the correct draw on the anchor and also created a very large area for your boat swing around when the wind got up. I feel it's important that the right pressures and leverages are being applied to the anchor for it do it's job as it was designed. This means therefore that too much scope can be detrimental. I like you prefer all chain but last boat has a combo and it was OK. It was octoplait spliced to te anchor and it did a good job but it means using 4 x the length rather than the 3 x the length with chain.
This purely my opinion and doesn't mean it right but It's the method I employ. I'm glad that there was no harm done and everyone was safe.

Regards.

peter.

Regards.

Peter.

 

[Guest]

There is no substitute for chain ....

The number of threads that have been posted on these forums and number of times old-hands have said .... CHAIN and the number of others have said ROPE ....

Sorry - but your tale is not a surprise, for years the value of weight in the anchor rode has been accepted and valued, the need for easier managed anchoring has given rise to this hybrid rope to chain affair ... and I would only ever use rope to chain similar to this for lunch hook or kedge. To anchor and go to sleep feeling secure ............... all chain for me.

The only bit of rope in that situation I would possibly use is to quieten the rode as it comes over the bow .... BUT still I would have the chain secured as well.

QED .....

 

[William_H]

Re: There is no substitute for chain ....

I am hardly experienced at anchoring however. It seems to me that you need to have enough chain or chain /rope out so that the angle of pull of the anchor is near to horozontal. After you get the rode beyond 5 times the depth of water the angle does not improve much. This model of course is based on maximum straight pull. (In practice the boat will shear sideways forward and back.)
However if it is all heavy chain the drag of the chain on the bottom sideways as the boat shears will tend to stop the shear. Rope on the other hand will not drag on the bottom so will not dampen the shear. In this case provided the chain is heavy enough / long enough to remain on the bottom most of the time then additional rope warp will not help the shear and will in fact encourage it by giving the boat more swing room. So it becomes a balance of less rope to prevent shear while enough to enable the anchor to be pulled horozontal and so stay set.
There is no doubt heavy chain is better but then not necessarily the best arrangement for you considering cost weight and handling difficulties.
I would suggest for your trip you consider a bigger anchor and more chain and be carefull not to let out too much rope rode. certainly total rode no more than 5 times depth. this may help you to sleep better. regards olewill

 

[gjbentley]

Thank you all very much for your comments and suggestions which I will take into account next time I anchor.

For the record, I was using a 25lb CQR. It was the first time I had used it, being the first season we have owned our yacht Grand Slam.

 

[Guest]

With all respect ....

25lb CQR with mainly rope rode ..... I consider a) your anchor small or minimum for a boat of your size, b) a rope rode on a minimum anchor to be unsuitable.

IMHO

 

[Anonymous]

Re: With all respect ....

[ QUOTE ]
25lb CQR with mainly rope rode ..... I consider a) your anchor small or minimum for a boat of your size, b) a rope rode on a minimum anchor to be unsuitable.

[/ QUOTE ]Maybe, but have you calculated the windage and taken into account the displacement? It doesn't say much for the holding in that anchorage if his CQR won't hold in a 20kt wind. Maybe there is a lot of weed down there? Or a smooth rock bottom at that particular place? Others have commented on the bad holding there so maybe it's not unexpected.

 

[hylass]

Hi JRT,

Many thanks for your positive comments about (I suppose!) my book.. This book has already been published in German and we are now in discussion with an Editor for the English version.. I hope it will be printed soon..

May I also suggest a very technical but also interesting Web site talking about all aspects of the anchoring line:
http://alain.fraysse.free.fr

 

[Ships_Cat]

Re: With all respect ....

I agree with you Nigel. The anchoring system described is in my view the minimum and as such I would not consider it reliable for overnight anchoring - I would have an anchor watch stood by a crew member at night and would never leave such a vessel unattended at anchor.

John