Anchoring Protocol revised

[rabbi]

Originally Posted by StuM

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.

Ooops. If that is the definition then we are comparing apples to oranges...

I always though scope is measured from anchor shackle to the point where the chain exits the water. Because deck height has significant impact and is very different depending on the boat.


Why should scope be different for a low profile 20ft cat versus a high deck 40ft trawler yacht?
Also based on your definition scope would be very much affected by deck height for shallow waters but not much for deeper water.

[Don C L]

Ah, you are saying the weight of the chain in the catenary will help determine how much scope is needed. Except that is not how it works. Sure, in common usage, you may not lift that chain up to stretch the catenary, even with a good breeze. But once you do, it is the angle, determined by scope, that will determine the anchors ability to hold by continuing to bury itself, or be at risk of failure because the angle is high enough to begin to coax the anchor out rather than dive. And as I said that is made worse buy any swell that enters the anchorage as well causing the bow to ride up and fall back causing higher angle shock loads on the rode.

[conachair]

Originally Posted by rabbi

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.

http://www.moondogmoving.co.uk/catenary.html

So lifting the last link with equal force could be 3:1 in 20m water but over 6:1 in 5m water.

The equation used to create thag plot might not be bang on but should be close enough to be of interest.

[rabbi]

Originally Posted by Don C L

Ah, you are saying the weight of the chain in the catenary will help determine how much scope is needed. Except that is not how it works. Sure, in common usage, you may not lift that chain up to stretch the catenary, even with a good breeze. But once you do, it is the angle, determined by scope, that will determine the anchors ability to hold by continuing to bury itself, or be at risk of failure because the angle is high enough to begin to coax the anchor out rather than dive. And as I said that is made worse buy any swell that enters the anchorage as well causing the bow to ride up and fall back causing higher angle shock loads on the rode.

I fully agree with you that ultimately its the angle of pull that determines if the anchor digs deeper into the substrate or just scratches on the surface.
That is for our anchor at scope 1:3 (measured from where the chain hits the water to the anchor shackle). In shallow water of say 2-3 meters we can lift the chain in WOT reverse so no catenary.


However I have yet to see winds that lift 60 meter of 10mm of chain in 20m water so that there is no catenary left. There is always some, and this will ensure that the pull on the anchor is almost horizontal.
The forces to stretch all (!) catenary out of a chain would be in the tons, more than the bridle or the padeyes at the beam can hold, maybe more than the chain is rated for (~5ton typically).

Not saying these winds don't exist, but I have never anchored in more 50kn. This was in 12m of water with around 50m chain out, and the forces were not large enough to lift the last few meters of chain clear off the bottom, so the pull on the anchor was always horizontal.

[StuM]

Originally Posted by rabbi

Why should scope be different for a low profile 20ft cat versus a high deck 40ft trawler yacht?

Because the critical factor is the angle when the rode is taut between anchor and where it enters the boat or is attached (depending on setup - in a cruising boat, that is usually the bow roller, rather than a bit)



[

Also based on your definition scope would be very much affected by deck height for shallow waters but not much for deeper water.

Correct. One metre from waterline to bow roller in 3 metres of water depth has much more effect on entry angle and therefore anchor security under load than one metre in 20 metres.






Boat US:

[skipmac]

Originally Posted by Don C L

BTW, who are you super divers who routinely dive to 20m to check anchors?

Well it was routine for me 30 years ago. Best I can do now is a quick bounce to 15 meters but I'm pretty sure with a little practice I could do better. I have done almost 100 meters but I was cheating and used tanks.

My son in law, who is seriously into spear fishing, does most of his hunting around 20-22 meters. Not sure how deep he could go on a bounce.

[rabbi]

Originally Posted by StuM

Because the critical factor is the angle when the rode is taut between anchor and where it enters the boat or is attached (depending on setup - in a cruising boat, that is usually the bow roller, rather than a bit)



[


Correct. One metre from waterline to bow roller in 3 metres of water depth has much more effect on entry angle and therefore anchor security under load than one metre in 20 metres.

I understand that point!

However I think that any definition of "scope" that depends not only on the length of rode but also on the looks of the boats is flawed.

To me scope 1:3 should result in the same length of chain no matter how ugly / boxy the boat is.

And twice the water depth should result in twice the chain length for the same scope.


Which is not the case based on your definition where freeboard has large impact in shallow water and little impact in deep water.

This definition may very well be the common understanding and thus correct.
Still its flawed to me. Even if I'm just a "one-person-minority"

[skipmac]

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.

Well as Stu and others have noted, this makes much more sense. In moderate to shallow water your 3:1 is more or less the same as my 5:1.

I have changed my mind and decided you aren't foolhardy, delusional or insane.

[rabbi]

Originally Posted by skipmac

Well as Stu and others have noted, this makes much more sense. In moderate to shallow water your 3:1 is more or less the same as my 5:1.

I have changed my mind and decided you aren't foolhardy, delusional or insane.

OK, with my definition of scope being based on water surface and not freeboard I seem to be a one-person-minority .
This is just a PC word for "insane", "stupid" or just plain wrong.

[skipmac]

Originally Posted by rabbi

OK, with my definition of scope being based on water surface and not freeboard I seem to be a one-person-minority .
This is just a PC word for "insane", "stupid" or just plain wrong.

No, no. You're being much too hard on yourself. Just say you march to the beat of a different drummer.

[rabbi]

Originally Posted by conachair

http://www.moondogmoving.co.uk/catenary.html

So lifting the last link with equal force could be 3:1 in 20m water but over 6:1 in 5m water.

The equation used to create thag plot might not be bang on but should be close enough to be of interest.

Interesting plot.
Even more interesting would be a plot showing the angle of pull depending on chain length, water depth, and force

Because its not really important if the last ink lifts half a milimeter. Only the angle of pull is important.

[rabbi]

Originally Posted by skipmac

No, no. You're being much too hard on yourself. Just say you march to the beat of a different drummer.

Sounds also a bit strange - but OK

[skipmac]

Originally Posted by rabbi

I understand that point!

However I think that any definition of "scope" that depends not only on the length of rode but also on the looks of the boats is flawed.

To me scope 1:3 should result in the same length of chain no matter how ugly / boxy the boat is.

And twice the water depth should result in twice the chain length for the same scope.


Which is not the case based on your definition where freeboard has large impact in shallow water and little impact in deep water.

This definition may very well be the common understanding and thus correct.
Still its flawed to me. Even if I'm just a "one-person-minority"

I have a friend that installed a very heavy duty SS eye on the bow of his boat just at the waterline, spliced his snubber onto the eye and keeps the loose end of the snubber on the bow. When he anchors he lets out the chain to appropriate scope, attaches the snubber to the chain and lets out more chain until the chain is slack and the load taken on the snubber. For him the water depth is exactly the measurement he uses to calculate scope.

The only down side, other boaters riding by are very confused when they see the slack chain hanging off the bow and nothing taut to hold the anchor since the snubber is right at the water. He has had occasional knocks on the hull to warn him of an anchor problem.

Lets him anchor with less line to get the correct scope and he finds it greatly reduces sailing at anchor.

[StuM]

Originally Posted by rabbi

Interesting plot.
Even more interesting would be a plot showing the angle of pull depending on chain length, water depth, and force

Because its not really important if the last ink lifts half a milimeter. Only the angle of pull is important.

But angle of pull only changes when the last link lifts.

Until the last link lifts, the angle of pull is horizontal. Once it lifts, the angle of pull starts to change.

[Don C L]

Originally Posted by rabbi

I fully agree with you that ultimately its the angle of pull that determines if the anchor digs deeper into the substrate or just scratches on the surface.
That is for our anchor at scope 1:3 (measured from where the chain hits the water to the anchor shackle). In shallow water of say 2-3 meters we can lift the chain in WOT reverse so no catenary.


However I have yet to see winds that lift 60 meter of 10mm of chain in 20m water so that there is no catenary left. There is always some, and this will ensure that the pull on the anchor is almost horizontal.
The forces to stretch all (!) catenary out of a chain would be in the tons, more than the bridle or the padeyes at the beam can hold, maybe more than the chain is rated for (~5ton typically).

Not saying these winds don't exist, but I have never anchored in more 50kn. This was in 12m of water with around 50m chain out, and the forces were not large enough to lift the last few meters of chain clear off the bottom, so the pull on the anchor was always horizontal.

I've actually been curious about the math on that and how to calculate the force required to pull a given chain at a given length and angle up to taught. Anybody have a link to the formula?