Mobo Anchor Sizing

[RogerRat]

Sorry for the possibly stupid question but could you explain this further please.

Ta

Questions like this are not stupid and can save you a lot of trouble and scares some day. Most instructors' will go over this in a day skipper course. Most factory fitted anchors I've come across have the anchor chain shackled directly on to an eye bolt molded into the glass matting deep down inside the anchor locker.

In changeable weather conditions, especially if you're anchored further out, fishing over rocks or you've been forced to anchor in an unfamiliar area over rocks. You may also inadvertently, snag your anchor an old anchor chain, mooring tackle or even a cable.

For several reasons, it may be safer to cut your anchor free, mark it with a fender or buoy and return to collect it another day.

The problem will be obvious when your trying to cut through a shackle with a junior hacksaw cos that's all that will fit in the locker with you. You could even be upside down trying to reach said coupling. You might be asking now, 'Why not undo the shackle?' Well, this sits in salty sea water most of the time because most people forget to wash it down regularly. They're rarely stainless or big enough, so the galvanising rusts and the shackle screw seizes. All this is happening while you are being thrown around with the possibility of waves spraying over the top. In worse cases the bow can be dragged under if the anchor is properly fouled!

The remedy or get out is to splice a rope end, often called a 'bitter end' minimum 2 metres, preferably 5-10 metres to the boat end of the chain and tie this onto a new larger stainless shackle in the locker. This piece of rope is effectively a sacrificial piece. Should you get into trouble and totally snared, one swipe with a sharp pocket knife and you'll be free as it's much easier to cut through rope than chain. If you were to extend this extra rope to say 30 metres you can also increase your anchoring scope/depth.

Important: This rope should be the correct diameter to match the chain and chain gypsy so that the rope will grip in the notches around the gypsy like a jamming cleat. By using a 'chain splice' this will allow the rope to move continuously around the windlass making the transition automatic from rope to chain.

Hope that's clearer. :encouragement:

RR

 

[TheOrs]

Questions like this...//...Hope that's clearer. :encouragement:

Much clearer; thanks. My set-up (on my fairly recently new-to-me boat) consists of anchor <-> ?? metres of chain <-> ?? metres of rope <-> boat. The chain/rope and rope/boat connections are achieved using carabiners (aluminium I think). I've bought a couple of stainless shackles to replace these. Next step is to measure it all and do a bit of research to see if it's fit for purpose.

 

[RogerRat]

Much clearer; thanks. My set-up (on my fairly recently new-to-me boat) consists of anchor <-> ?? metres of chain <-> ?? metres of rope <-> boat. The chain/rope and rope/boat connections are achieved using carabiners (aluminium I think). I've bought a couple of stainless shackles to replace these. Next step is to measure it all and do a bit of research to see if it's fit for purpose.

Sorry to hear that, sounds a mess and carabiners crumple, even if they're cross strutted, keep them for hanging up a towel or hanks of mooring line, even your dinghy.

What's the type of boat, anchor and chain size???
Where do you boat?
How do you intend to anchor, beach, lunch/picnic, fishing, overnight???

These factors are important in determining whether it is fit for purpose. You may find that all or at least some of the setup is fine, but just put together badly.

RR

 

[Nick_H]

I haven't used Navionics but is it not possible to tap the screen as you drop the anchor to mark it as a waypoint? This will give the anchors exact position.

I normally forget to do that, but if you use the "track" function on the plotter/ipad, then by the time you come to set the anchor watch in the evening there is usually a nice arc drawn on the screen, and this makes it fairly obvious where the anchor position is.

 

[BartW]

squals can blow up out of nowhere, particularly in the med.

I've posted this picture before, of A&K's boat in Sardinia. We went to bed in flat calm, with no strong wind forecast, and we woke up to this. As you say, once the chain is taught, you're completely dependent on the anchor, and it arguably doesn't then matter whether you have chain or rope. The test shows that in some conditions at least, the anchor is 4 x as effective at 5:1 scope as it is at 3:1.

we had two occasions like this last season,
in one occasion, red dot bay, anchored in 5m depth, and 40m chain out,
the forcast from the nearby airport showed a thunderstorm the following morning at 8am,
my clock alarm was set at 7am, but the squal came 2 hours earlyer then prediction from the evening before, and surprised us when we were sleeping.

when we woke up from the movement and noice from the strong wind, we found the boat 180* turned, and moving between saling boats anchored around us,
(we touched one, but it was not severe)

even if you know such a squal is expected, even then its difficult to prepare yourself for such a event,
ones the wind is at full force, and you're anchored, its very difficult to manouvre the boat

 

[MapisM]

once the chain is taught, you're completely dependent on the anchor, and it arguably doesn't then matter whether you have chain or rope.
The test shows that in some conditions at least, the anchor is 4 x as effective at 5:1 scope as it is at 3:1.

Well, it's easy to see why. It's a simple matter of geometry: the higher the ratio, the more horizontal is the pull on the anchor.
I would rather have chain than rope in any sort of wind, though.
I've yet to see a 5:1 anchorage where the chain stays taught above the seabed for all its length.
In most occasions, before overnighting, I have the habit to check the anchor, and even in the strongest winds the chain was always lying on the seabed for at least the last 5 meters or so.
Sometimes, the pull was enough to make also that last part of the chain swing around a bit (which might be a problem on rocky bottom), but never enough to lift it completely.
And if seen from the surface, the chain was as straight as in your pic. Which I agree that leads to think that only the anchor is working, but it isn't.

PS: in my experience, a properly anchored boat can withstand any sort of wind alone (well, almost). It's the wind+wave combination that can make any anchor drag.
In the very few occasions where I had to pull the hook and move elsewhere, it was always because the wind turned unexpectedly from the unsheltered side of the anchorage, building up waves and making the boat pitching.
And I can confirm that in those conditions, you don't really need any anchor alarm, because if the boat pitching badly doesn't wake you up, neither any alarm will...!

 

[MapisM]

we had two occasions like this last season

Well, rain aside, you can say that BA held her own well in THREE occasions like that, the 3rd being that night on the E side of Cap d'Antibes in a westerly F8...

 

[SwedeSkipper]

Good reply, I have to agree with you on all principals.

For a light weight Merry Fisher tho. The real question is where you are trying to anchor and in what conditions. The Brittany coast with major tidal range is a far cry from Osbourne bay.

Basic point to remember is that the chain / scope holds the boat and the anchor holds the chain. 5Kg Bruce for your boat is fine in normal conditions and I would expect you to have a 6mm chain in place.

Over the years... I have found that everyone has their own 'favourite' anchor. I remember the blurb when the 'Bruce' was introduced and the claim was, that you could use half the size and weight of the ever popular 'CQR'. I doubted it of course, but they do work!


RR

I think you need to consider the difference between authentic Bruce anchors and copies. I have used both, S/S authentic is way much better than the galvanized copy. Maybe it's S/S vs galvanized that makes the difference rather than the authentic/copy - I don't know.

 

[MapisM]

LOL, that surely makes one difference: in how nice the boat looks in photos!
I've yet to see any sort of commercial vessel with a s/s anchor...

 

[Nick_H]

And if seen from the surface, the chain was as straight as in your pic. Which I agree that leads to think that only the anchor is working, but it isn't.

OK, I was there, but you know best

 

[MapisM]

OK, I was there, but you know best

Well, of course that's not what I pretended.
I simply reported that I've been more than once (never in heavy rain TBH, but I can't see how that could matter) in the water with a mask to actually see the chain/anchor behaviour while all boats, in a proper blow, had the chain as pulled as in your pic.
And in none of those I looked at (mine, and also other boats anchored nearby) I ever saw the WHOLE chain pulled above the seabed.
If you have a different experience, I'm all ears.
Otoh, regardless of who was there in a situation like the one above, if as I guess you didn't jump in to check, how could you understand from the surface if only the anchor is holding and the chain is above the seabed or not?
Not a sarcastic question, mind. I'm genuinely curious.

 

[Deleted User YDKXO]

Sometimes, the pull was enough to make also that last part of the chain swing around a bit (which might be a problem on rocky bottom), but never enough to lift it completely.
And if seen from the surface, the chain was as straight as in your pic. Which I agree that leads to think that only the anchor is working, but it isn't.

Yes I agree. I have also made a point of snorkeling to check how the chain and anchor behaves in windy conditions and I have never seen the whole chain lift off the seabed even though the chain looks bar taut at the surface and even with only a 3:1 scope. Of course having a heavy chain helps. As you say, the final part of the chain may swing a little from side to side but I've never seen it lift. To my mind that is the situation you always want to have where there is always a horizontal pull on the anchor. If the chain starts to lift completely off the seabed then there is a vertical pull component on the anchor and the risk of it breaking out is much higher

 

[MapisM]

If the chain starts to lift completely off the seabed then there is a vertical pull component on the anchor and the risk of it breaking out is much higher

Precisely. Which as I said might happen with strong wind+waves, where the pitching "snatches" the chain.
Other than that, I can imagine that also wind alone could make a properly deployed 5:1 all chain anchor drag/break out eventually, but at a guess it must take hurricane conditions....

 

[Nick_H]

Really? In a 3m anchorage with 15m chain out you don't think all the chain (a whole 30 kg of it) would come off the seabed if a squall blowing F8 came through? As well as the time above, I've had two other occasions, in Rondinara, Corsica, and a worse case in Garoupe, Antibes last year. Both times the same, light offshore winds during the day, then suddenly in less than a minute the boat swung through 180 degs and was in a howling onshore. I've no doubt the chain was off the seabed and the anchor was doing the work, I could feel it in my spine each time the boat snatched. Fortunately these two occasions were during the day, and with approx 5:1 scope, so the boat didn't drag, or at least not enough to cause a problem, and of course we quickly let out more chain or headed for port.

If they had been at night, with 3:1 scope, as was the discussion, then I may have joined the two sailing yachts on the rocks.

If you were swimming around with a mask and snorkel, then you're talking about different conditions.

 

[Deleted User YDKXO]

Nick_H, I'm only calling it as I've seen it. I could cite another example of when we were anchored in Cala St Vicente in Ibiza where a gentle offshore westerly breeze turned into a howling onshore easterly wind in the middle of the night which was pitching the boat so badly that one of my crew took it on herself to distribute lifejackets. I sat on the flybridge all night on anchor watch ready to start the engines and go but amazingly the anchor held. At first light, the wind hadn't abated and I swam out to check the anchor because I honestly believed the only way it could have held was because it was stuck under a rock or something and I had to assess how to clear it. However, there was no rock and it had just buried itself deep in the sand but, as I said before, to my huge surprise, the final few metres of chain was not lifting off the seabed

 

[MapisM]

If you were swimming around with a mask and snorkel, then you're talking about different conditions.

LOL, "I was there, but you know best?" springs to mind...
The most popular anchorage near my home harbour is very well sheltered from W to N, which is great because the mistral is the prevailing wind.
But the coastline is low and smooth, therefore the wind blows at the sea level as strong as it gets.
In spite of that, it's possible to anchor at 100m or even less from the coast, being able to swim in flat water, no matter how strong the wind blows, due to the lack of fetch.
And as I'm sure you know, the Mistral in that part of the Med DOES blow properly, at times.
I swam in that situation (or other bays with similar conditions) quite a few times, and as I said, if seen from the surface the chain always reminds a violin string...
Otoh, if you say that in some occasions you've been able to "feel in your spine" that the chain was off the seabed, I can only take your word for it of course.

Re. shallow anchorages, I already said in other occasions that I have a habit of using a somewhat higher ratio in them.
In fact, the first marks I have on my chains are at 20m, 'cause I never let down less than that.
Not even in situations like the one of a video I once posted, with the keel at 2/3 feet from the bottom.
n fact, coming to think of it, in a few occasions after a whole day in very shallow anchorages, with sustained strong wind, I did see that the anchor dragged, maybe a couple of meters, in a soft sandy bottom.
Not sure this implies that the chain went off the seabed, though (I never spent the whole day looking at it! ).
I always felt it was just a matter of poor holding properties of the bottom, though I can't be 100% positive on that.

 

[NGM]

.......
Re. shallow anchorages, I already said in other occasions that I have a habit of using a somewhat higher ratio in them.
In fact, the first marks I have on my chains are at 20m, 'cause I never let down less than that.....
.

I'm always curious as to the best practice/etiquette in busy, shallow anchorages. Obviously I would prefer to put out more chain rather than less but it's impossible to tell what others have done. If I was there first and swung the most, potentially into another boat, should they have considered this first or is it my issue for being overly cautious ?

 

[Deleted User YDKXO]

I'm always curious as to the best practice/etiquette in busy, shallow anchorages. Obviously I would prefer to put out more chain rather than less but it's impossible to tell what others have done. If I was there first and swung the most, potentially into another boat, should they have considered this first or is it my issue for being overly cautious ?

For me its clear. You were there first and everyone else who wants to anchor after you has to assess how far your type of boat is going to swing in the prevailing and forecast weather conditions and anchor far enough away to avoid you. If another boat anchors after me and I consider he is too close, there's no way I'm moving; I just put a couple of fenders out. They soon get the message after you swing a bit close to them. I have to say that proper yotties tend to know how far away they should drop their hook and it tends to be inexperienced charterers who try to anchor in your cockpit!

 

[MapisM]

is it my issue for being overly cautious ?

Afaik, it's generally accepted that whoever arrives last has to maneuver, if and when necessary.
I understand that your question is more specific/subtle, but frankly, even assuming that it would be fair to discriminate somehow the "over cautious" helmsmen, how could that be done?
I mean, who's saying if I let 15 or 20 meters of chain out? And what my neighbours did...?
Fwiw, I just tend to avoid too crowded anchorages!

 

[NGM]

Fwiw, I just tend to avoid too crowded anchorages!

...ideally yes !!