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post #13 of Old 05-17-2013
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Re: Mantus Anchors NEWS: anchor sizing tool

Originally Posted by blt2ski View Post
Not sure if you get or can find the latest "pacific Yachting" out of BC. In it was an article comparing a rope/chain rode to an all chain in lower water depths. Under 30' IIRC. It pointed out a rope/chain with 7-1 will almost ALWAYS do better than an all chain with the recomended 4 or 5-1. As in this example, hurricane force winds. the rope/chain will have a lower pull angle on the anchor, vs the all chain. Mainly due to both options will have almost no rode on the ground, with a direct pull on the anchor itself...


Ahhh.... now we get into more advanced anchoring theory and practice. I'm not at all surprised that the article showed that chain on shorter scope is no good compared to a longer rope in a hurricane. Guidelines, like "5:1" or "7:1" are good for a start, but there's more to the story.

Lets start with rope, which, in water, we can assume is weightless for our purposes. Therefore, there is no (very little) weight that has to be lifted off the ground and thus there is no caternary effect. Very little wind (or force) is required to pull all the rope off the ground into a straight line making a perfect hypotenuse of a triangle. Thus, the only way to change the angle is with more rope.

Chain, famous for it's weight, has a catenary to it so that when you pull on the boat end, the catenary (or sag in the chain) translates to a lower angle of pull.

All old news right? So far so good.

Now the guidelines for chain start to fall apart when we consider that the deeper we are, the more that 4 or 5:1 actually weighs. Consider chain at 1lb/ft with 10ft depth. At 5:1 that's 50ft of chain, and 50lbs. Compare that with the same 5:1 in 50ft of water, which would be 250ft of chain or 250lbs.

How much more force does it take to stretch out a chain rode that weighs 50 or 250lbs? Not sure, that's getting into more advanced physics than I can easily calculate, but it does take a lot more.

In hurricane force winds, there's enough force to make either the 250ft or 50ft chain straight as a bar. So, the weight is mostly negated. In the magazine example you're really comparing a bar-taut chain to a bar-taut rope. The rope makes a better angle with that triagle because it is longer because it it 7:1. Rope wins in that one, but if the wind was less than a hurricane, the catenary of the chain would still be in effect and they would be equal.

For example if it takes 30kts of wind to make 50lbs of chain bar taut, then 5:1 isn't enough for 30kts in 10ft of depth. That same 5:1 might be plenty of weight to still have a low pull angle if applied to an anchorage that is 50ft deep where you would have 250lbs of chain out. It might be still at a good angle of pull up to 45kts.

The short version of all this is that while I think 5:1 is a good guideline for chain, you can't skimp at all when it's shallow but you can at depth. I've anchored on 3:1 chain in 80ft of water but will use 6 or 7:1 in 20ft. Also, one's errors in calculation for tide height, bow roller height, and depth sounder depth all magnify in the shallower waters, which is another reason why more rode is better the shallower you are.

The other lessons?

1: Avoid hurricanes.

2: Anyone who only uses 4:1 of anything (except depleted uranium chain) in hurricane force winds deserves whatever they get. Another example, I'm afraid, of a magazine anchor test not being particularly applicable to real life.


PS To stay on topic I shall add that I'm sure a Mantus, sized appropriately with the calculator in this thread, would have done great.

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Last edited by MedSailor; 05-17-2013 at 11:18 PM.
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