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Old 09 July 2019, 17:00   #1
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Overnight Anchoring

Hello everyone, Iím after some advice please, Iím planning on leaving my 22ft rib on anchor over night (sand seabed), Iím using 10 meters of 8mm chain with 12mm 36mtr rope what anchor/weight should I be using? Any recommendations welcomed. Thanks
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Old 09 July 2019, 18:21   #2
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Depends on the conditions you expects etc. The weight depends on the design you choose. Some people would suggest 1kg per m of length. If you were operating commercially then the size of anchor would depend how far from shore you plan to go - presumably however if you are planning to anchor overnight then it will be in benign conditions and it might be fair to compare to the least challenging coding requirement - 4kg.

There have been many threads on this so searching should bring you suggestions on designs to avoid etc.
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Old 10 July 2019, 01:11   #3
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10m of 8mm is 14kg - if you deploy it all.

How deep is the water?

In case it's not obvious a grapnel & fisherman are both the wrong design if you are making a choice for sand specifically.

This time it's sand. Will it always be sand?

You need to consider storage on board. Anchor locker size? Space left with chain and rope?

Are you staying aboard?

Do you have a plan if the wind / tide change overnight and she slips?

How are you getting ashore once anchored?
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Old 10 July 2019, 01:32   #4
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Maximum tide height will be about 9-10 meters.

I currently use a danforth whilst anchored in the day and watching the boat from the beach & yes it will always be sand.

I’m planning on attaching a marker buoy to the other end of the rope so basically be a temporary mooring and I have a small sib which I use to get back and forth from the rib.

Thanks.
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Old 10 July 2019, 02:11   #5
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Davos

Welcome to the forum.

For your size RIB I'd suggest a 7kg'ish anchor. Jimmy Green have a large range and anchor types have been discussed on here recently.

Your anchor will need to fit in your anchor locker unless you can tie it to the deck.

When you deploy the anchor you will need to ensure that the intersection between the rope and the chain and the rope and the boat can't chafe.
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Old 10 July 2019, 04:45   #6
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At 10m water depth but only 46m of rode I'd suggest that you need a longer line by the same again bar any tail. Your chain seems about right.


An anchor weight of about 7.5kg should be fine, but is there space to stow that shank?


Anchoring is as much art and compromise as science and there are plenty of rules of thumb about weights per metre of vessel and length per metre of water depth. All I would say - in the absence of any information about location and conditions - is to err on the side of caution. It might be overkill but that's better than waking up and finding out that your boat is either gone or ashore on rocks.


Hope it works out and have a great time!
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Old 10 July 2019, 12:24   #7
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At 10m water depth but only 46m of rode I'd suggest that you need a longer line by the same again bar any tail. Your chain seems about right.

The RYA recommend 4x for chain only, and 6x for a mix of chain and rope (with 5m+ of chain) so I donít think he needs double - but a bit more would be wise if it really is a 9-10m depth. Worth keeping in mind though that the boat will swing so simply chucking out more scope isnít always a good idea.
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Old 10 July 2019, 13:17   #8
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The RYA recommend 4x for chain only, and 6x for a mix of chain and rope (with 5m+ of chain) so I donít think he needs double - but a bit more would be wise if it really is a 9-10m depth. Worth keeping in mind though that the boat will swing so simply chucking out more scope isnít always a good idea.

As I said above ref. art and compromise. Anywhere between 5:1 and 12:1 depending where you are, what you're doing and who's rule of thumb you're following. So 8:1 is a happy medium and the diff between this and 6 ain't worth arguing about. I don't really care, it's not my boat and I certainly don't try to help to be snarled up in pedantics. After all I think we both agree his line is a bit short!!!!!



As I'm sure wiser members know, if you're worried about swing, there are more than half a dozen different ways to use your kedge............
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Old 10 July 2019, 13:32   #9
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Thank you everyone for all the advice!
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Old 10 July 2019, 13:37   #10
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I certainly don't try to help to be snarled up in pedantics.
You've given them too much scope.

Cheer up though - it's a long rode that has no turning!
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Old 10 July 2019, 13:48   #11
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that's dreadful.....
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Old 10 July 2019, 17:23   #12
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12:1? Really? Most of the yachty fraternity talk about 3:1 to 5 or 6:1

Dropping 14kg of chain plus 7kg of anchor is probably gonna hurt when you lift it if at high tide.

Moorings using anchors usually lay two with a chain between them and an upriser. Less swing, but also not lifting and resetting the anchor in a change of tide / wind.

If you are effectively creating a mooring for a week... Would it be better as a mooring?
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Old 10 July 2019, 18:48   #13
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12:1 yes in extremis which is why it was at the upper end of my figures. Extremis might mean coarse homogenous sands in one of Scotland's grander embayments sitting out a F10, or a rough day west of the Lizard after September. Squalls in Scotland can hurtle out of the hills with no warning. Little Loch Broom is known as the loch of a hundred winds for a reason - and it's not alone. However, if it's a muddy little pill up a sheltered creek in Devon then who needs to bother. It could also be a muddly little pill on the east coast - a NE'ly 'starvation wind' set's in and you're there for days riding it out. Anyway this is all in extremis. Let's let the OP decide - they have, after all, hinted that this is not a brief lunch stop.

However less is definitely not more when it comes to anchoring. Why argue over 20m of rope when the boat is worth thousands.

You could drop chain size or length but will it then do either of it's two jobs? Is the boat heavy or light for it's length? As for anchor weight this is more an expression of strength and plate area so would you want a smaller spade when out of sight of your boat overnight?

If weight is a problem then perhaps a shallower and more sheltered anchorage (notwithstanding tidal range) needs to be found. Or, horror of horrors, split your weight over two anchor systems or even run a traveller down one to give the catenary something to think about.


The one thing about anchoring is that you spend your entire time wondering if it will hold!


Seamanship, it's all about judgement calls.
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Old 11 July 2019, 04:13   #14
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Shiny, I think you are needing to get to the gym, or recall that anchor/chain in water weighs less than in air. Personally if this was a temporary mooring for more than a week Iíd be considering making the whole think chain.

Dr Hooks points all go back to the point I made in the second post. Is this an anchor regardless of conditions, in an exposed place and leave it situation; or is it a holiday situation where facing a bad forecast you can put it on the trailer.
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Old 11 July 2019, 07:25   #15
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This anchoring location, is it subject to any tides which will cause the boat to do a 180' degree turn every 6 hours. The reason I ask is that Danforth anchors don't have a great reputation for resetting after a turn in the tide.

There are better choices, naturally there is cost involved. Finally are there any limits in your insurance policy? certainly worth a check.

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Old 11 July 2019, 16:49   #16
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that anchor/chain in water weighs less than in air.
by about 15% if peeps are wondering.
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Old 12 July 2019, 01:38   #17
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or even run a traveller down one to give the catenary something to think about.
Sadly running a weight down an anchor rode to increase the catenary and therefore anchor holding doesn't work once the wind pipes up. It would be better to put the weight into a larger anchor in marginal conditions.

Scope vs catenary (Rocna Knowledge Base)

Quote:
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The one thing about anchoring is that you spend your entire time wondering if it will hold!
So true.

Pete
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Old 12 July 2019, 02:03   #18
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However less is definitely not more when it comes to anchoring. Why argue over 20m of rope when the boat is worth thousands.
Because 20m of rode increases the swing diameter 40m. That reduces your choice of anchoring spot without risking swinging into others.

And if he's in a depth of 10m as suggested, does that not mean for 12:1 you are looking at 120m rather than 50m.

So you are now claiming 2 football pitch lengths of water. But how does the yacht that comes into anchor after you went ashore know you have 12:1 out. He assumes you have 6:1 to be on the safe side and drops in what he thinks is a gap. He has a keel and so experiences the tidal effect more than you so swings more quickly / less quickly depending on wind and tide.

If it needs 12:1 to be sure it will hold... I wouldn't be leaving my boat at anchor.
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Old 12 July 2019, 05:53   #19
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Hi Iíve been researching into a new type of anchor following comments on this thread..

A Rocna seems to be my best bet with everyoneís experience and knowledge on here would you recommend I go for a Rocna 6kg or a 10kg??
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Old 13 July 2019, 04:49   #20
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Rocna recommend 6kg for your size of rib. We have the 10kg version on board a 31ft 5000kg yacht and it glues us to the sea bed.

Just make sure you set the anchor by powering backwards, after its had a little soak to settle, to drive the anchor in and ensure it has set.

Steve, has done a huge amount of work to document anchor setting for no reward, so worth watching:


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