Anchoring Protocol revised

[rabbi]

Isn't scope more dependnig on your anchor setup and how much scope it needs?

Ours sets perfectly at 1:3 in good sand / mud, and under heavy load it just digs deeper into the substrate. If the substrate is bad holding, lots of chain can compensate somewhat but not really too much.


If we have the room we aim for 1:5, but with good substrate and 1:3 we feel the main danger is other boats drifting into us.

Of course we set our anchor to WOT reverse _every_ time and if there is even the slightest doubt I snorkel it. Never dragged so far in several hundret nights at anchor. Plus we use a smart anchor app ever time.

[skipmac]

Originally Posted by rabbi

Isn't scope more dependnig on your anchor setup and how much scope it needs?

Ours sets perfectly at 1:3 in good sand / mud, and under heavy load it just digs deeper into the substrate. If the substrate is bad holding, lots of chain can compensate somewhat but not really too much.


If we have the room we aim for 1:5, but with good substrate and 1:3 we feel the main danger is other boats drifting into us.

Of course we set our anchor to WOT reverse _every_ time and if there is even the slightest doubt I snorkel it. Never dragged so far in several hundret nights at anchor. Plus we use a smart anchor app ever time.

My favorite answer, it depends. In deep water say 15-20 meters and with all chain, I am much more comfortable with shorter scope than I am in 3-4 meters. And again, in moderate weather or a in a nasty blow I will give a different answer.

I've done the math and looked at the angles. With 3:1 scope the pull on the anchor (assuming strong winds and the rode straightened out) is almost 20 degrees which to me is pushing the limit. About 5:1 the angle drops to about 11 degrees which I like a lot better.

Another concern, if any significant chop or swell in the anchorage every bit of angle increases the pull up on the anchor as the bow bounces.

Of course anchors hold better as the angle of pull approaches horizontal and geometrically the point of diminishing returns as scope increases is about 7:1.

[skipmac]

Originally Posted by rabbi

Of course we set our anchor to WOT reverse _every_ time and if there is even the slightest doubt I snorkel it. Never dragged so far in several hundret nights at anchor. Plus we use a smart anchor app ever time.

Also always set the anchor with full reverse but often anchor where you couldn't see the anchor until you bump it with your nose.

Also, I'm a bit older and out of practice so don't think I could free dive to the anchor anymore in 20 meters or better.

[Simi 60]

Originally Posted by rabbi

I

Ours sets perfectly at 1:3 in good sand / mud, and under heavy load it just digs deeper into the substrate. If the substrate is bad holding, lots of chain can compensate somewhat but not really too much.


.

Ours does as well but I never make the assumption that other peoples anchors are as effective as I know they are not.
I also anchor for the 2am storm so regardless of the conditions when I anchor I know we are good if it turns to custard later. That usually has us at 5:1 minimum. Today we have about 10:1out.
I measure from deck to ocean floor as well, so for us, water depth +10ft
Chain is of no use sitting in a locker.

[Mike OReilly]

Originally Posted by rabbi

Isn't scope more dependnig on your anchor setup and how much scope it needs?

Ours sets perfectly at 1:3 in good sand / mud, and under heavy load it just digs deeper into the substrate. If the substrate is bad holding, lots of chain can compensate somewhat but not really too much.


If we have the room we aim for 1:5, but with good substrate and 1:3 we feel the main danger is other boats drifting into us.

Of course we set our anchor to WOT reverse _every_ time and if there is even the slightest doubt I snorkel it. Never dragged so far in several hundret nights at anchor. Plus we use a smart anchor app ever time.

What Skips says

At 3:1 the angle is too steep for comfort. A good anchor, that is dug in, in good substrate, may hold to a point, but anchors only work by maintaining the force vector such that they are digging down. Optimal rode angle is horizontal. At 3:1 I don’t know any anchor that is angled properly. So now you’re depending on whatever you’ve managed to get into the substrate. The more it blows, the less stable your hold will become.

[rabbi]

Originally Posted by Mike OReilly

What Skips says

At 3:1 the angle is too steep for comfort. A good anchor, that is dug in, in good substrate, may hold to a point, but anchors only work by maintaining the force vector such that they are digging down. Optimal rode angle is horizontal. At 3:1 I don’t know any anchor that is angled properly. So now you’re depending on whatever you’ve managed to get into the substrate. The more it blows, the less stable your hold will become.

Thats the point. One needs to know what angle (scope) is required for the anchor in question to bury deeper instead of skimming the surface of the substrate.

Once at 1:3 our anchor digs in and dives deeper and depper into the substrate as more force is applied. Its typical to find the last few meters of chain burried in sand, more so for mud.
At around 1:2,5 this doesn't work, as the anchor starts to plough at the surface, not penetrating deep enough.


We add more chain if conditions and the crowd allow, as more chain gives catenary, thus reducing shock loads on the anchor and bridle when a strong gust hits. But I feel that doesn't add much to the anchor's ability to dig deeper. That is, again, for our particular anchor and of couse only in adequate substrate.


Of course in nasty substrate nothing works but tons of chain help to hide the problem that the anchor can't set reliably.

[john61ct]

Originally Posted by rabbi

Once at 1:3 our anchor digs in and dives deeper and depper into the substrate as more force is applied. Its typical to find the last few meters of chain burried in sand, more so for mud.
At around 1:2,5 this doesn't work, as the anchor starts to plough at the surface, not penetrating deep enough.

So which type of anchor(s) have that capability?

[foggysail]

Originally Posted by skipmac

OK, found my little, more or less official, sketch chart. The no go zone in the middle is bounded on the north by a line at roughly 41 degrees, 27.6 minutes, on the south a line at 41 degrees, 27.13 minutes. South of there is OK. North of there is OK except for a circular area of eel grass about 300 yds diameter from the west bank out to the main channel.

Thanks Skip-

This year is over for harbor hunting and on top of that, we didn’t even make it to Tashmo this season. I can think...next year for sure and for sure, there are limited number of years left that things and trips can be put off. Tashmo is a beautiful harbor so enjoy!

[Mike OReilly]

Originally Posted by rabbi

Thats the point. One needs to know what angle (scope) is required for the anchor in question to bury deeper instead of skimming the surface of the substrate.

Once at 1:3 our anchor digs in and dives deeper and depper into the substrate as more force is applied. Its typical to find the last few meters of chain burried in sand, more so for mud.
At around 1:2,5 this doesn't work, as the anchor starts to plough at the surface, not penetrating deep enough.

I’m sure you know your gear, but that is too fine a line for my sleeping comfort. I’m never happy unless we’re on at least 5:1. More is better. What anchor are you using? I didn’t realize any anchor would do this at modest depths — unless it is way over-sized perhaps? Or perhaps really good substrate?

[rabbi]

Originally Posted by john61ct

So which type of anchor(s) have that capability?

Nothing special. Its a relatively cheap but oversized Bügel anchor from some German internet shop. Supposedly made in china or eastern Europe.
But since these are all just knock-offs I guess one needs some luck to get one with the right angles and dimensions.

On our first cat we also switched to the same type after some bad surprises with a more traditional design.


Our FP Mahe came with a Kobra 2, which required more like 1:3.5 or 1:4 to start diving deeper into the substrate.

[rabbi]

Originally Posted by Mike OReilly

I’m sure you know your gear, but that is too fine a line for my sleeping comfort. I’m never happy unless we’re on at least 5:1. More is better. What anchor are you using? I didn’t realize any anchor would do this at modest depths — unless it is way over-sized perhaps? Or perhaps really good substrate?

I certainly agree that it's a fine line between 1:3 (works) and 1:2,5 (does not work). If there is any slope in the bottom this can lead to serious problems.


This summer we sat in a nice sandy spot at Isla Colom / Menorca in maybe 3m of water with 10m chain. No shelter from the wind, but safe behind a shallow reef. After a nasty 40kn thunderstorm passed the anchor and shaft were fully burried, as was at least half a meter of chain.

We were only a few meters from the rocks so I sat there in the rain for an hour with both engines in idle.
Would I have stayed dry inside had we anchored with 1:5 or 1:7 scope? I don't think so, as this was more due to the rocks being close.

PS: For true storm conditions I am happy when the wind blows exactly from shore, the anchor sits in 5m of water and I have 1:15 out. Nothing to lee and no neighbours, especially no charter cat upwind.

[rabbi]

Originally Posted by Simi 60

I measure from deck to ocean floor as well, so for us, water depth +10ft

I measure chain length between anchor and the point where the bridle is attached to the chain. Under heavy load this point is around or just under the surface.

So 1:3 in 3 meters the bridle is attached at the 10m mark.

[Don C L]

Boy I am not sure any anchor manufacturer would say 3:1 is ok for their anchor. There are certainly times I have a hard time pulling my anchor out of the sand even when I am almost on top of it, but I'd never sleep well with 3:1 and any kind of a serious wind. If someone had 3:1 upwind of me, I'd be concerned. It is not just the wind(age) but if any swell is involved and the rode starts to yank at the anchor, the scope or lack thereof will become evident.
BTW, who are you super divers who routinely dive to 20m to check anchors?

[StuM]

Originally Posted by rabbi

I measure chain length between anchor and the point where the bridle is attached to the chain. Under heavy load this point is around or just under the surface.

So 1:3 in 3 meters the bridle is attached at the 10m mark.

Which is NOT the standard understanding of scope and hence has confused other people here. It helps if you use the commonly understood meaning of scope.




Scope is a ratio of the length of an anchor rode from the bit to the anchor shackle and the depth of the water under the bow of the boat measured from deck height



Your "1:3" is considerable more rode out than everyone else would consider 1:3.

[rabbi]

Originally Posted by Don C L

Boy I am not sure any anchor manufacturer would say 3:1 is ok for their anchor. There are certainly times I have a hard time pulling my anchor out of the sand even when I am almost on top of it, but I'd never sleep well with 3:1 and any kind of a serious wind. If someone had 3:1 upwind of me, I'd be concerned. It is not just the wind(age) but if any swell is involved and the rode starts to yank at the anchor, the scope or lack thereof will become evident.
BTW, who are you super divers who routinely dive to 20m to check anchors?

As said 1:3 is on the edge for shallow water. But in 20m water those 60m chain is definately enough in normal conditions. Would not even stretch the catenary of 60m chain in 30kn.


Diving 20m, certainly not me. But to visually check the anchor to see if its burried and how much it has dragged to burry itself I don't need to get that deep - around the islands in the Med there is crystal clear water.