Anchor chain size advantages

[thinwater]

Originally Posted by conachair

And here's the maths...
https://www.desmos.com/calculator/lszqjjyer4

So for 8mm chain with a force of 100Kg (which ain't a lot! )..

a scope of 4:1 in 10m water will have the chain just lifting off the seabed at the anchor, or about 3.8:1 in 12m water (just under 40')

Feel free to check the equations anyone.

Yes. And for the record, I use a long snubber all the time (on a cat it is a bridle). That said...

The force probably requires a sustained 25 knots of wind, but it depends on the boat. Gust will be greater, but much or that is damped out by the keel. Brianlara was talking about a Triton, which probably does not see 250 pounds of wind load until 42 knots. Additionally, he was talking >6:1 scope.

It has been shown (on the water testing)that occasional lifting does not really disturb the anchor.

There is nothing magic about the chain just coming off the bottom (unless you are using an in-line tandem on rocks or cobbles). There is still a lot of curve left at that point.

Another interesting number is the sustained wind at which the chain becomes effectively straight. At that point the cable only has about 23' of "stretch" in it, and by 500 pounds (60 knots for a Triton), only about 6 inches. At 42 knots, that 3' plus the motion of the chin through the water is plenty to damp out waves. However by 60 knots the chain is pretty straight and damping will become minimal. Also, the chain will be giving a lot of lift.

Thus chain can help a lot in most weather in deep water (25 knots sustained is pretty brisk), but in really strong conditions, you still should have some scope 5:1 in 30' is only 150'.

So Brian was telling the whole truth, for his situation, using the same "simple maths" we have been discussing. For a larger boat... well then the math changes. But he didn't claim knowledge beyond his experience.

How many folks here know the wind load for their boat (no fair looking it up just now)? I've measured mine in a range of conditions with many rode and snubber types. It's a pretty important number. How about reverse thrust for testing that anchor? Do you know what windspeed that equates to?

[Olddave]

Hi Andy,
Sounds good, I am in the process of moving our windlass and chain locker aft by 600mm. That will be just enough to stack a reasonable length of chain without having to assist it. We have the same length of chain as you but will be up-ing it to 300ft. The more the merrier.
I have been surprised at the amount of evidence showing that most of our chain is coming off the bottom during a blow.
Enjoy. Dave

[conachair]

Originally Posted by thinwater

There is nothing magic about the chain just coming off the bottom (unless you are using an in-line tandem on rocks or cobbles). There is still a lot of curve left at that point.\

Might be worth remembering that once you've got to that point, there is practically nowhere for the force to go apart from straight to the anchor.
For example from here - Cable Sag Error (Catenary Curve Effect) Calculator
30m straight run of 8mm chain in water, to go from 200Kgf to 800Kgf is a move to just 21 inches.
Which is why we all use snubbers and don't rely on and damping from the chain catinary when things get really bad

[svrevelations]

I'm drifting a little from the topic, but;

1. What does "G4" mean?
2. Where can I read more about anchor chain specifications?

[double u]

...& there is actually going to be some catenary left even at the breaking load of the chain...