Anchor Chain Length Versus Weight

[mlibkind]

Originally Posted by Don Lucas

it concerned about the weight of the chain verse the size, use the smaller chain and upgrade the the grade of the chain everything is fixable with the proper application of boat bucks!

Practical Sailor recently had a review of the loads on a rode under varying conditions. It turns out that the strength of the chain is probably not the limiting item in the design of ground tackle. The thing you have to worry about, above about 30 kts of wind, is the structural strength of the cleat-boat connection.

[Cheechako]

Originally Posted by mlibkind

It's true that braided line is softer and lies more compact in the anchor locker. However, my experience with a 12-strand braided rode is that it will have a tendency to wrap around the gypsy when raising the anchor instead of coming off smoothly. By chance, just today my brother told me that his friend replaced his 3-strand nylon rode with braided only to go back to the 3-strand after one cruise because of the same problem.

I'm not sure this is true.... unless you lay the rode in by hand meticulously each time....

[SailFastTri]

Originally Posted by mlibkind

It's true that braided line is softer and lies more compact in the anchor locker. However, my experience with a 12-strand braided rode is that it will have a tendency to wrap around the gypsy when raising the anchor instead of coming off smoothly. By chance, just today my brother told me that his friend replaced his 3-strand nylon rode with braided only to go back to the 3-strand after one cruise because of the same problem.

Interesting. I had the opposite problem with 3-strand... it was too stiff and wouldn't fall freely into the locker. The stiffness would cause the stripper to override the rope, unless it was watched closely and pulled down by hand once in a while. I switched to brait over the winter for that reason, but haven't had a problem with it yet.

I guess there are a lot of variables in each setup. We should do what works best for each of our boats.

[Stumble]

Oh goodie, another anchor thread.... But I haven't been in one yet on cruiserforum.net so here goes.

The current idea, and the one I happen to accept is that you will get the most ultimate holding power by going to the largest anchor you can figure out how to get back in the ow roller. There is no such thing as an anchor that is too big.

The reality is that traditionall boats use anchors that are really too small, but they are the largest that can be hoisted onto the deck. This goes back as far as the british navy, using twenty man capstans. But since they recognized that the anchors still weren't big enough they added large chain to get more weight on the bottom. This is only starting to change with the advent of electric and hydrolic wind lasses, but sailors being a conservative lot tend to stick with what worked in the past regardless of why.

Now let's take a boat with an anchoring system budget of say 300lbs. Typically you would expect to see a 60lbs something, combined with 240lbs of chain and line. However the holding power of a 150lbs anchor with 150lbs of chain/line will be orders of magnitude larger, though it does make it problematic to get the anchor up.

As for the amount of ride to put out...
First scope is defined as the relation between the height of the bow roller to the bottom, and the amount of line let out. The leingth of the boat is simply not a factor.

Second, The numbers say that up to 10:1 scope significantly decreases the angle of the anchor to the sea bed. Beyond this while it does help, it changes the angle so slowly that it is rarely worth giving up the swinging room. Though if you had an anchorage to yourself go for it, it won't hurt anything.

The recomendation for a minimum scope is generally going to be 5:1, however this is based upon a traditional sized anchor for the boat. By going to a massively oversized anchor you can get away with less, but again the gains up to 5:1 or so are very significant, very fast, so worth it in most conditions.

A good example of this is the Dashew's Windhorse that sat at anchor in a 60kn blow with 2:1 scope out and never budged. But they also carry an anchor that would normally be sized for a boat 3-4 times their size.

Third. The cantenary effect of chain is massively overblown. For most boats, it will take between 30 and 35kn of breeze to load up a chain rhode to bar tight. Beyond this there is no cantenary advantage. This is leading some people to consider switching to spectra anchor lines, to move even more weight into the anchor (or reduce anchor system weight). I am it willing to recommend this until i see some more testing, but then I'm a sailor, and tend to be conservative.

Finally. Shock absorbers are a great idea in most conditions, but I wouldn't worry about it too much in storm conditions. The shock loads I have experienced in strong blows are actually pretty light. Much more important in my eyes is to massively reinforce the backing plates, and cleats used to attach anchor lines to the boat in the first place.

Most cleats are more than strong enough, and so long as the fasteners aren't rotted they are likely to be more than strong enough to handle the loads. It the fiberglass they are bolted to, particularly if all you have is fender washers will rip out in a hurry.

[sailronin]

Stumble wins with the correct answers!

Boat length has nothing to do with scope of rode....When anchoring 145 foot boat in 25 feet of water I'd have close to 500 feet of rode on the bottom, no way. In real life I'd have about 125 feet (rounded off to a shot and a half) out.

Oceanography and Seamanship by Van Dorn has an excellent explanation of anchoring physics making the point that the key factor is keeping the vector acting on the anchor parallel to the bottom regardless of composition of the rode (all chain, chain and rope or rope and rider) (and yes there are two ropes on a boat, the bolt rope and the anchor rope portion of a rode). The point of his diagram and explanation is that in deeper water less relative scope is required to have the correct loading on the anchor.

Have fun and get a bigger anchor, you'll sleep better.

[Celestialsailor]

Originally Posted by mlibkind

Practical Sailor recently had a review of the loads on a rode under varying conditions. It turns out that the strength of the chain is probably not the limiting item in the design of ground tackle. The thing you have to worry about, above about 30 kts of wind, is the structural strength of the cleat-boat connection.

...and the way the load is spread under the cleat.