Serious Anchoring Advice...... Honest!

pau1gray

pau1gray

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www.owlwise.co.uk
We use a 15m Cat for work and we are looking at anchoring in 50m of water.

My question has nothing to do with types of anchor - I'm just interested in the opinion of those more in the know.

If we use a three point mooring do we still need the same amount of scope per anchor or can this be reduced?

Honestly not a troll - just have limited knowledge of three point mooring as opposed to just sticking down an anchor and swinging off of it...
 
I've just checked my old Admiralty "Manual of Seamanship" which talks about using two anchors for naval vessels. It doesn't say anything about reducing the scope for two anchors, but it does say that for greater depths a relatively shorter scope can be used. (eg 4:1 rather than 5:1)

FWIW "Ark Royal" had 2 x 9.5 ton anchors with a cable of 517.5 fathoms!
 
Thanks for that - I suppose we'll just have to try it and see how it goes...

FWIW I can't help but remember the Foc'sle PO on the Newcastle couldn't find the third anchor that was on his PLR - How the **** do you manage to loose something that big!
 
If your 3 point mooring is what I imagine ... 3 anchors set at 120 degrees apart each joined to a centre point by chain with the boat attached to a riser from the middle....
Then yes the scope can be reduced dramatically.

The scope when anchoring is there to ensure the pull on the anchor through the mud on the bottom is horizontal. To get the anchor out you pull the anchor up vertically. The horizontal pull is achieved by increasing the horizontal distance of the boat to the anchor so reducing the angle of approach of the line to the anchor and by having a heavy chain which drops in a catenary to nearer horizontal. In fact it hopefully lays the chain along the bottom for some distance before rising toward the surface.

Now in a 3 point (or 2 point) mooring the chain between the boat and the anchor has another chain and anchor attached a small distance away from the anchor, the weight of which helps keep the main load chain close to the bottom. Hence the horizontal pull is achieved with a much shorter chain (scope).

Just how much shorter the scope can be depends on the distance between the anchors and weight of the chains. However from trigonometry you find that a 2 to 1 scope gives an angle of 45 degrees with the chain taught. This angle is reduced by weight of the chain in use and the additional chain. 3 to 1 scope gives more like 30 degrees of angle. More scope gives less angle but with less reduction so a scope of about 3 to 1 perhaps should be a limit depending on the load on the mooring.

Just draw a diagram of the situation and you can see pictorially how it works.

Rereading your post if the 3 point mooring is in fact 3 separate anchors on 3 separate chains or ropes either from one point on the bow or from sides and stern then each anchor and chain is functioning separately and probably only one at a time so it is just like having one anchor except you don't have the concern of resetting the anchor with change of tide or wind. The size of the scope can be reduced by attaching a weight to the anchor chain about 10% of the way up the chain. To give more catenary. (droop) olewill
 
"Just how much shorter the scope can be depends on the distance between the anchors and weight of the chains. However from trigonometry you find that a 2 to 1 scope gives an angle of 45 degrees with the chain taught. "

Not when I went to school
 
[ QUOTE ]
"45 degrees with the chain taught"

P'raps you wasn't taut proper?

[/ QUOTE ]

Might not have been taut proper, like - but he's correct. Its 30 degrees at a scope of 2 , 19.5 deg at 3 etc.

That aside, its an interesting question and much depends on how you lay the 3 point anchors system. If you have the ability to put 3 anchors on the sea bed and have chains joining them on the sea bed reasonably taut, then an upward pull on the chain to the boat would be converted largely to a horizontal pull on the three anchors and I would have thought you could use a shorter scope. If you have to join the chains at the surface level, I would have thought that what you would end up with would really be a series of three single anchors, most load being on one at any time, and the scope would need to be a normal one. Between those two scenarios, you could adjust.

Guess its suck it and see in real life. Much as I enjoy playing with the maths of this sort of question, I wouldnt dream of relying on it in real life. /forums/images/graemlins/grin.gif Apart from anything else, I am Olympic standard when it comes to silly mistakes.
 
I've anchored oin 40m and had a much reduced scope. I used about 70 feet of 10mm chain, and 200' of 18mm nylon, with a 15kg danforth on an 8 ton boat. Stayed for 3 days, squalls to 30+ knots and strong currents. Never budged. Sand bottom in a coral atoll. The hardest part was getting at the tackle back up again on a maual windlass.

My feeling is that 3:1 would have been preferable, but at those sort of depths, a reduced scope can still give a very good catenary.
 
When I responded earlier I had assumed that OP was wanting to anchor his boat to get maximum positional stability as he works for a sonar company.

If, however, he simply wants to lay a fixed mooring (in 50 metres?) then, forgetting the mathematics, WilliamH's main posting seems to be fair enough.

If it is a triple anchor arrangement then I also agree with Twisters comments about the lines getting knotted up. Difficult to sort out, but not impossible. The "Manual of Seamanship", which I mentioned earlier shows the Naval way of untwisting anchor cables on full size vessels. Not an easy job when you have several tons of chain to sort out, but the fact that they have a system for sorting it out shows that it happens to the big boys too.

If OP comes back on then maybe he could explain exactly what he is trying to achieve?
 
However from trigonometry you find that a 2 to 1 scope gives an angle of 45 degrees with the chain taught. "

*************************************************

Even if this was correct you do not want a taught 45 degree scope as this would lift the anchors.

The general idea of anchoring is that you have sufficient scope so that a proper catenary forms from the boat to a point some way from the anchor so that the anchor is only subjected to horizontal forces. This is why if you drag anchor in certain conditions then the easiest way to stop dragging is to throw out more chain

If the original poster intends to bring the three anchor lines to deck for his 3 oint system then he will need the same scope as he would have used on a single anchor system.

In the event he doesn t have help to lay the moorings then he needs to devise a proceedure for doing this. It is difficult but not impossible. I have seen an oil rig lay an eight point mooring to an acceptable standard when no anchor handlers were available but it took some planning. They did have 5000 ft of 3" chain on each anchor to play with.
 
Worries me to think of having three chains to the boat - if the boat turns on the anchors you are twisting the chains and they might become a real bugger's muddle to recover.

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I think you missed the point.....This mooring is used to stop the boat swinging..........the third anchor is to the stern .


/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
Ok I'll put my hand up - maybe I didn't give you all enough information to start off with - sorry about that....

As Login name suggested we are looking to get a stable position on the surface (around 10m) in 50m of water - 3 hours either side of low tide.

I maybe misled people by speaking about a 3 point mooring which I now realise was the wrong terminology - what we want to do is lay out 3 anchors 120 degrees apart, connected to the two bollards on the stern quarters and one on the bow.

The anchors will be laid out using our rib, and the anchors will be buoyed off to the surface (clearly marked off course!) so that we can break out the anchors from the rib and return them back on the boat.

We have successfully used this method in Cawsand Bay a number of times, with 5 times depth as the scope for the chain and rope that we used - however going deeeper to 50m will require an awful lot of rope!

Thanks for the answers so far guys - they will give us some things to mull over - to be honest - until the weather improves a bit I can't seeing us doing an awful lot with this yet.....
 
The only suggestion I can make is that you probably don't need a 5:1 scope because of the depth. I can only guess at what is need but would suggest that 4:1 would be OK.

Presumably you are not near land so, if it does drag, it won't be a disaster even though it may be inconvenient.

The problem with doing this either side of low water is, of course, that you will also be holding the boat against its tendency to swing in the tide which will put an extra strain on the anchors. Forgive me if I am teaching granny to suck eggs, but it would be sensible to check the tidal flows at the location you are planning to do this. It would certainly be beneficial to do this at neap tides as the boat will be less likely to be pushed away from its "static" position.
 
Firstly my appologies for bad Trigonometry quite right.
Separate anchos could be used to one fixed point on the bow if they were joined together then to a swivel shackle to one chain this all lowered to a point well below the boats keel.

However separate anchors and especially in Paul's case will each be functional one at a time depending on wind so as suggested the anchors need to have the usual rode length. The rode length requirement can be alleviated alittle by adding weights to each chain about 7 metres up the chain from the anchor. A weight comparable with the anchor weight would be good.

At least at that depth the rise and fall of tide becomes a small percentage of the total depth so may not be so much concern.

So just add more rope and see how you go. good luck olewill
 
Paul,

Probably because it's not run of the mill this one intrigues me /forums/images/graemlins/smile.gif

Just out of interest why is it necessary to be limited to such a small amount of movement? I know you can't give away commercial confidentialities but I'm just interested to know what a sonar company does.
 
10 metre target in 50 metres.......that should be absolutly no problem. You are basically trying to do what oil rigs do though they use an 8 point system. However as you have catenary system then the boat will move position due to wind and tide requiring that the position is monitored.

We did various tests on this many moons ago and now it is universally accepted that GPS can give you this information so long as you have DGPS correction.

There are domestic gps which will achieve 1 metre with no problem and if you want to be pernikity then you can rent a system from various folk in Aberdeen to give centimetre accuracy.

This is how oil rigs are positioned though who the hell is bothered if they are a foot or so off defeats me as the drill string is easily steered.


Waas gives this accuracy in the USA however the sytem isn t operating yet in the UK.

try this

and check DGPS is available in your area
 
Hi BilgeDiver,
yup done more rig moves than I'd care to think about.
We've got our own RTK system to monitor the amount of movement we are experiencing but what we are trying to do is limit the amount of movement on the surface to 10m.

The current accuracies hunted down by Survey companies do indeed boggle a few especially with the advent of directional drilling /forums/images/graemlins/ooo.gif but hey - that's how we earn our money /forums/images/graemlins/laugh.gif
 
Have you considered using a sand screw instead, i.e. put down a temporary mooring. Safer and you move far less, and need to carry no rode. We were discussing it recently in Liveaboard. My Google search finds the thread but YBW doesn't serve that thread so you might need to dig for it. Google sand screw mooring and add ybw to find the thread.
 
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