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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Reduction in anchor scope with chain use
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Thread: Reduction in anchor scope with chain use Reply to Thread
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Topic Review (Newest First)
10-17-2012 10:37 PM
PCP
Re: Reduction in anchor scope with chain use

Quote:
Originally Posted by klem View Post
...

Line effectively provides no catenary so the angle of pull on the anchor is determined by the scope. In calm weather, chain provides catenary and the angle of pull on the anchor will be parallel to the bottom. However, in conditions that actually test the holding power of the anchor, the chain will be stretched out straight and the angle of pull will be determined by the scope, there is effectively no catenary in these conditions. Therefore, the holding power of the anchor is determined by the scope and is independent of what the rode is made out of unless you have ridiculously heavy chain (I am thinking of captaaron and his 5/8" chain on a <30' boat).

Keeping the loads down means providing some shock absorption in the system. Line is a good shock absorber because it is relatively stretchy. In calmer conditions, chain is an okay shock absorber because it has catenary. In stronger conditions, the chain is effectively a steel rod and it has virtually no shock absorption meaning that the loads are higher on the anchor. When you are in conditions bad enough to be testing the anchor's holding power, line will yield lower loads on the anchor.

In conditions that test your anchor, you do not need any less scope with chain than line. Since chain is a terrible shock absorber, you really need a good snubber in bad conditions. Chain still has other benefits such as chafe resistance, easier setting, etc so we shouldn't go out and ditch all of our chain but this rule of thumb is totally bogus.
On windy conditions I use two mooring lines connected to the chain by shackles. The lines have big rubber shock absorbers, the kind that have the line passed several times around. That system provides a good shock absortion especially with waves, making also the live aboard more comfortable on those conditions.

Regards

Paulo
10-17-2012 09:45 PM
klem
Re: Reduction in anchor scope with chain use

In the end, the goal is to have the anchor stay in the same place, we can all agree that dragging is bad. For a given bottom and boat, the only 2 things that you can control are the angle of pull on the anchor and the shock loads. The lower the angle of pull, the more holding power the anchor will have and if the shock loads are lower, you need less holding power.

Line effectively provides no catenary so the angle of pull on the anchor is determined by the scope. In calm weather, chain provides catenary and the angle of pull on the anchor will be parallel to the bottom. However, in conditions that actually test the holding power of the anchor, the chain will be stretched out straight and the angle of pull will be determined by the scope, there is effectively no catenary in these conditions. Therefore, the holding power of the anchor is determined by the scope and is independent of what the rode is made out of unless you have ridiculously heavy chain (I am thinking of captaaron and his 5/8" chain on a <30' boat).

Keeping the loads down means providing some shock absorption in the system. Line is a good shock absorber because it is relatively stretchy. In calmer conditions, chain is an okay shock absorber because it has catenary. In stronger conditions, the chain is effectively a steel rod and it has virtually no shock absorption meaning that the loads are higher on the anchor. When you are in conditions bad enough to be testing the anchor's holding power, line will yield lower loads on the anchor.

In conditions that test your anchor, you do not need any less scope with chain than line. Since chain is a terrible shock absorber, you really need a good snubber in bad conditions. Chain still has other benefits such as chafe resistance, easier setting, etc so we shouldn't go out and ditch all of our chain but this rule of thumb is totally bogus.
10-17-2012 09:44 PM
ScottUK
Re: Reduction in anchor scope with chain use

My friend came into an anchorage one time and came up to an anchored boat with an old salt on it and asked how much chain did he have out. The response was 'all of it... it ain't doing me any good in the box'. I think of it as a classic line.
10-17-2012 07:04 PM
PCP
Re: Reduction in anchor scope with chain use

Quote:
Originally Posted by Mike Banks View Post
...
My boat weight five tons, and with stores and fuel more than six when cruising. My anchor is sixty pounds and I use at least thirty metres of 13mm short link chain, to which is lock-spliced one hundred metres of 20mm hard woven nylon rode. Even so, I would hate to have to ride out a storm at anchor. ..
The type of anchor is also very important. A modern high performance 16kg anchor is enough for a 6 tons boat . A 20kg anchor is for the ones that cruise extensively and expect to have now and then bad weather and high winds while on anchor. A 30kg anchor is just too much weight on a bow for a 5T boat.

Of course a modern anchor has an holding power about 3 times bigger, sometimes more than an old anchor.

Regards

Paulo
10-17-2012 02:45 PM
DRFerron
Re: Reduction in anchor scope with chain use

As a CG Aux instructor I agree that you should question any statement made by your instructor that you feel is incorrect. If you feel comfortable doing so and can do it in a way that isn't confrontational, I'd say to do it in class. The instructor should be prepared to either back up his or her statement with facts or concede that the statement was incorrect. At the very least, if the instructor sticks by his or her statement, at least the other students may understand that something is off and have it in the back of their minds to research it on their own.

Unfortunately, I know too many instructors for which a private chat would not make a difference. Occasionally my flotilla has had instructors for whom we get complaints for various reasons (fortunately mostly about telling war stories instead of going over the material) and we do our best to deal with it.

I don't like the feeling that because we are volunteers we should be given a pass for offering up bad information. Volunteers or not, we're presenting ourselves to the public as being more informed and trained than the non-boating student sitting in front of us. It does no one any good to have bad or potentially unsafe information presented as fact.

In my classroom I welcome lots of questions and questioning. It tells me that the students are paying attention and thinking things out rather than simply getting through the material so they can pass the test. It also creates a fun atmosphere.
10-17-2012 12:01 PM
sailak
Re: Reduction in anchor scope with chain use

Quote:
Originally Posted by chucklesR View Post
When you KNOW he/she is wrong speak up in class, but be prepared to bring in proof next class.

If they are putting out info that is UNSAFE it deserves to be called out.
I had already challenged them last night...really don't want to become "that guy".

We're in Alaska so cold water survival is a hot topic. One of the instructors last night said "If you go into the water head first without a PFD you're not coming back up" meaning you'll drown. I pointed out that I have dove into the water and survived and that there are more people who go overboard and survive than there are people who go over and die.

I plan to speak to him in private next week.
10-17-2012 11:53 AM
chucklesR
Re: Reduction in anchor scope with chain use

Quote:
Originally Posted by sailak View Post
Okay, you guys have confirmed what I knew all along about this. When he said that in class I whispered to the wife that there was no way in hell I'd anchor with that little rode/scope and questioned if this guy had ever anchored in anything but calm conditions.

The wife and I have been very disappointed in the class we're taking. In nearly every presentation there have been blatant errors in the information they are putting out. I realize they are volunteers (Coast Guard Aux) and didn't expect them to be professional instructors but I did expect to gain some useful knowledge and for the instructors to at least be competent.
When you KNOW he/she is wrong speak up in class, but be prepared to bring in proof next class.

If they are putting out info that is UNSAFE it deserves to be called out.
10-17-2012 11:49 AM
chef2sail
Re: Reduction in anchor scope with chain use

Cantanary angle is of utmost importance in anchoring even in a situation where you have to be short scope such as a crowded anchorage, a kellet could/ should be used to improve this angle. Less strain on the anchor on the bottom.

Dave
10-17-2012 11:02 AM
BubbleheadMd
Re: Reduction in anchor scope with chain use

I use my extra length of oversized chain to help lower the angle of attack which aids in faster, better setting of the anchor, not to reduce the amount of scope I use.
10-17-2012 11:00 AM
BlndSqrl
Re: Reduction in anchor scope with chain use

Dale,

The instructor seems to have failed to understand one of the main reasons why anchors drag. The bow rising in waves or chop lifts the anchor out of the bottom and dislodges it. You use the chain so that the angle of the scope is reduced, forcing the boat to lift the chaing long before it lifts the anchor. It also keeps the pulling angle on the anchor very low.

I would ask for a refund on that class.
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