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post #1 of 17 Old 09-12-2018 Thread Starter
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Anchoring, two anchors, wind/tide shift

Situation: A boat has put out two anchors. The wind shifts, the boat spins around twisting one anchor line around the other.

What's at issue? Chaffing? Increased risk of dragging?

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post #2 of 17 Old 09-12-2018
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Re: Anchoring, two anchors, wind/tide shift

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Originally Posted by mrmac View Post
Situation: A boat has put out two anchors. The wind shifts, the boat spins around twisting one anchor line around the other.

What's at issue? Chaffing? Increased risk of dragging?

mrmac
Not a short answer. I have a book out (Rigging Modern Anchors, below) that covers this at length.

https://sail-delmarva.blogspot.com/2...n-anchors.html

First, you are better off with one anchor, most of time. You know his. Someone is going to mention it. We're done with that.

If both are rope, the most serious risk is wrapping rope around the keel, rudder, or prop. The rode can cut and the rudder or drive can be damaged. The solutions are long chain leaders or kellets to hold the rodes down. Yes, the rodes can rub on each other, but that is a lesser problem. But that is also preventable.

If one rode is rope and the other chain, they can rub if the boat spins. This is easily prevented by attaching the second (rope) rode to the chain rode 15-40 feet from the boat so that they form a Y some distance from the boat. A soft shackle or prusik loop work well. The chain between the junction and the roller absorbs the twist, so there is no wrap. The chance of dragging is less than a single anchor because they can support each other (this depends on anchor type, placement geometry and bottom type--too complex to cover here).



If you run both rodes from the lockers and the boat spins you will have a tangle. If both are chain it will be epic and embarrassing. I've watched the antics of people trying to spin boats with a dinghy. Use a separate, short secondary rode.

----

I've done this MANY times in anchor testing. In order to positively secure the boat so that it would not move at all while winching other anchors to failure, towards the stern, I would deploy two large anchors off the bow. I've used this rig through hundreds of tide swings and never had a tangle.
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Last edited by pdqaltair; 09-12-2018 at 08:28 PM.
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Re: Anchoring, two anchors, wind/tide shift

Thank you for responding so quickly and thoroughly.
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Re: Anchoring, two anchors, wind/tide shift

Great info, thanks. Quick question on the prusik loop, would you use prespliced line and pull the 40-60 feet through the loop or could/would you use a hitch or some other knot? Although I guess that leaves extra line so a spliced line would be better just wondering about adjustments. In any case, thanks again.
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Re: Anchoring, two anchors, wind/tide shift

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Great info, thanks. Quick question on the prusik loop, would you use prespliced line and pull the 40-60 feet through the loop or could/would you use a hitch or some other knot? Although I guess that leaves extra line so a spliced line would be better just wondering about adjustments. In any case, thanks again.
I like the soft shackle better. Dead simple.

Typically the prusik loop would be short and attached to an eye in the end of the rope rode with a carabiner, shackle, or something of that nature. A Mantus chain hook or bridle plate will also work. Anything the won't fall off chain. A rolling hitch is not recommended because it does not hold at this sort of angle. I'm sure there are other answers, including some knot I have not tried.




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Re: Anchoring, two anchors, wind/tide shift

I am curious to know what you are using for your prusik. In order to work effectively the cord needs to be noticeably smaller than the rope it is attached to. For example I wouldn't use anything larger than 7mm cord on a 10 mm rope. The 7mm cord is rated at around is rated at around 10-12 kN which is probably a lot less than your anchor rode. Tests have shown that tying knots in rope can reduce its strength by over half its rated value and you obviously have to use a knot to form the loop, usually either a triple fishermen's or a flemish bend.

FWIW a lot of climbers are using Sterling Hollow Blocks instead of prusiks these days. They are rated at around 15kN, and will grab down to 7mm rope. They also have the advantage of being heat resistant so they won't melt on long rappels, but that probably isn't an issue here.
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Re: Anchoring, two anchors, wind/tide shift

A swivel where the three rodes join would be my choice.
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Re: Anchoring, two anchors, wind/tide shift

I use sewn climbing slings. They are cheap and convenient, suitable for boats up to about 36 feet, and by extension, probably any boat using rope main rode.

For chain rode, the secondary rode sling really does not slip on chain (it's bumpy!). A spliced Dyneema loop works great. In fact, a cow hitch (luggage tag) is all you need on chain, but the prusik is a bit more secure.

---

I swivel is perfect for a mooring, but I think the OP was thinking about 1-3 night stays. Adding a swivel is really going to make deploying more complicated and is not really needed for less than a week (if there is enough chain between the roller and the connector).
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Re: Anchoring, two anchors, wind/tide shift

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First, you are better off with one anchor, most of time. You know his. Someone is going to mention it. We're done with that.
First, OP, excellent question--and excellent responses.

Second, I will ask the stupid question: are you better off with one anchor most of the time even with the set up in the diagram?

Third, in the diagram, the secondary rode is optionally longer than the primary (to the connection point). Does that mean you do not necessarily want to pull equally on both anchors, and if so, why?
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Re: Anchoring, two anchors, wind/tide shift

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First, OP, excellent question--and excellent responses.

Second, I will ask the stupid question: are you better off with one anchor most of the time even with the set up in the diagram?

Third, in the diagram, the secondary rode is optionally longer than the primary (to the connection point). Does that mean you do not necessarily want to pull equally on both anchors, and if so, why?
One anchor is generally best.
* Two anchors cause the boat to swing differently than other boats, increasing the odds of a bump in a crowded harbor.
* More likely to have some drag down on you, since your rodes are spread.
* More complicated if you need to leave in a hurry. Really complicated if someone drags and fouls your rode.
* More complicated to set. Power setting each individual anchor also takes time.
* Will tangle if you rotate enough times.
* May increase the stress on each anchor (not equal loads).
* Rodes may trip anchors.

Longer leg. Look closely at the illustration; one is pivoting fluke and one is roll bar.

This is actually a very specific, through rather common case. The most common reason to use two anchors is super soft mud. Otherwise, one anchor is enough and is a better choice. The most common secondary anchors in mud areas are a Fortress or Danforth, because they are so good in mud. However, they are not very good with direction changes. Thus, you set the Fortress farther away, expecting that the smaller general purpose anchor (Rocna, Claw, Delta, etc.) will drag a little, but using its ability to drag stably and reset to buffer the direction of pull on the Fortress, which is at its best when the pull is in a steady direction. In this way, the two anchors are synergistic; the GP anchor adds stability and the pivoting fluke anchor provides the real holding power.

Differing leg length also reduces the chance of the anchors fouling each other. A minor likelihood.

The only reason you would use a symmetrical layout is if both anchors have the same holding capacity and characteristics, which is unlikely. One will always be bigger, and the bigger anchor should be farther away. This is a little like tug of war; you put the big guy at the end of the rope.

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