Mooring chain: galvanized vs SS??

[smckean (Tosca)]

I have a mooring ball attached to a heavy concrete block in 5 feet of water at minus 3.0 foot tide; the highest tides we see are about plus 9.0 feet. I have 2 or 3 feet of chain through a U-shaped tube molded into the concrete block. That piece of chain attaches to 20 feet of 3/4" nylon rope via a SS thimble and shackle. At the top of the nylon line there is another 5 feet of chain up thru the mooring ball via another SS thimble and shackle. This gives me about 2:1 scope at the highest of tides....which is not really enough but I am constrained by other nearby mooring balls.

I use 3/8" galvanized chain. I've been surprised how quickly this chain rusts and then flakes off to near nothing. (Once by luck I found one link down to the width of a piece of spaghetti!). The top chain lasts maybe 2 years; the bottom chain maybe 5 years. I think the degradation is mostly due to mechanical wear and the resulting spalling of the rusting metal.

So my question is: maybe I should go all stainless. It's less than 10 feet of chain, so not too expensive. But I've read that SS has its disadvantages as part of a mooring system. I know one thing the SS thimbles and shackles show no wear at all.

My main interest is not the initial strength of the system (or should it be?), but rather avoiding a profound weakness that could lead to catastrophe if I allow the chain to go too long without a visual check (which requires a dive).

 

[Doolittle]

I am not all that familiar with mooring buoys other than the one covered in barnacles that scratched the heck out of my boat several years ago. I am assuming the chain at the top is to keep the mooring line submerged so it does not interfere with the swing of the boat while on the buoy. Could you run the nylon line up through the buoy and just hang a downrigger ball off the line about 5 feet down? If the chain weighs roughly 2# per foot, then a 10# ball should be adequate. Just guessing here but wanted to throw that out there.

 

[thataway]

The quality of the SS chain is an issue. If it is ISO rated, it should be good. But most SS will be foreign made (China). I would be concerned about poor chain quality--as we see in some of the SS shackles.

SS G40 3/8 is $12.68 + shipping, SWL is 3700#. SS G 40 5/16 SWL 2400#. $9.05 a foot plus shipping.

USA Galvanized 3/8 BBB going to be half the price and probably no shipping. and SWL 3000#

Many commercial grade moorings pull the chain and replace yearly.

See the Practical Sailor article on mooring chains. The SS looked good after several years, the galvanized not so. The higher rated galvanized chains seem to have poor adhesion of the zinc galvanizing due to higher carbon in the steel of the G30/G40 or G 70 chains.

You are going to have to pull and inspect the mooring each year. But I would opt for the SS (3/8 G 40). It is probably much stronger than what our mooring ballast can hold.

 

[westward]

How is the chain attached to the concrete block? I also moor on a buoy at our Summer home and have installed the following:

150# navy anchor, attached to a large diameter Galv. swivel/shackle. This interfaces with a thimble-ended length of 1" Samson. The rope has clamp floats attached at +8', then again at +15' off the bottom so as to keep it floating and not abrading the sea bed ( a state DNR requirement and it prevents wear on the rope). The upper end of the rope has another galv. thimble, which attaches to ~ 10' of 3/8" proof coil galv. chain. This chain comes up through the center tube of my mooring ball and is held from dropping by a 1/2" galv. ring.

This setup allows relatively easy monitoring and replacement of the chain, which wears the most in the upper zone of the water, but pretty evenly/predictably. I last installed in 2015 and it's still sound. (your post will prod me to check things out when I'm up this weekend . Based on your reported rate of chain wear I'm thinking it's being caused by mechanical wear, as you suspect. Normally high quality galvanized lasts longer. In this regard, I avoid Chinese chain and attachments in favor of the higher quality domestic products.

Stainless lasts and resists abrasion but is more noble than galvanized. In the presence of saltwater and O2, the stainless will tend to electrically eat through galvanized parts if attached to them. This won't happen overnight, but is worth noting.

Good luck in resolving this. Sound sleep is correlated with a sound mooring!

 

[smckean (Tosca)]

westward,

How is the chain attached to the concrete block?

As I mentioned, the concrete block (1000 pounds!) has a U-shaped 2" PVC pipe tube embedded in the block....note both ends of the pipe exit the top of the block. This allows the bottom chain to loop thru this tube exiting at the top making it easy to replace. The 2 ends of the chain are held together with a SS shackle. I imagine there is a reasonable amount of chafing where the chain exits both ends of the pipe. This arrangement avoids an eyelet (or whatever) embedded in the concrete which can't be replaced. There is nothing in/on the block itself that can wear out.

Based on your reported rate of chain wear I'm thinking it's being caused by mechanical wear, as you suspect. Normally high quality galvanized lasts longer.

I suspect I do have a rapid rate of chain wear since the mooring is in a fairly open cove with no protection to the SE. Winter storms produce 2 to 3 foot swells (sometimes bigger) that pound the hell out of this mooring.

Stainless lasts and resists abrasion but is more noble than galvanized.

Yeah, I'm aware. Right now I have a mixture of galvi and SS, but that's because I didn't want to spring for SS chain. The thimbles and shackles are SS since I find the threads on the pins of a galvi shackle corrode and can't later be removed (or worse they fall out). The bulk of the galvi chain is so much larger than the SS in the shackles and thimbles that I figure electrolysis of the chain is minimal.

 

[westward]

Sandy,

Here’s what my spot check showed after 5 years in place:

-Upper 10’ of 3/8” galv. Chain was fine, but starting to wear thin at the lower end connection to the upper rope. Still 3/16” of steel left at thinnest point.
-upper galv. Rope thimble completely gone! Galv. Shackle against bare rope, but rope in surprisingly good condition at the connection, minimal chafing/ rope loss
- rope appears in good shape, heavily covered with kelp, mussels, etc.

- shackles were seized with SS wire, no apparent Electrical damage to surrounding galv. Steel

I’m thinking the thimble, being the thinnest contact point, wore through and fell away after a few years. I asked the mooring vendor about this 5 years ago when I purchased it, and he assured me galv. Wouldn’t be a problem ( he was the vendor for State marine parks buoys). Now I know otherwise. I have to assume the lower end thimble is also gone.

I’m gonna replace from the anchor up, using SS for all the thimbles as well as the upper chain/ shackles/ terminal ring. SS against large diameter galv. Should be less destructive than the inherent susceptibility to abrasion of the galv. Steel parts.

In my case I’m able to pull the whole thing by flotation, starting on a low tide. I need to get on it while the rope is strong enough to pull an embedded navy anchor. I also don’t need to be excessively concerned about chain strength due to the particulars of our cove.

 

[westward]

Connectivity issues now, but scratch what I said about using SS chain. Check the Rocna knowledge base for details.

 

[thataway]

You might consider using Monel metal wire for seizing wire. It is far likely to corrode or fail. Not particularly expensive.

 

[westward]

I think I actually did use monel wire. I remember researching it at the time, in my usual OCD manner :wink:

 

[westward]

What is the minimum recommended scope for a permanent mooring buoy, in a well-protected, crowded mooring field? (Assume all ground tackle is sufficiently sized for the boat).

 

[thataway]

What is the minimum recommended scope for a permanent mooring buoy, in a well-protected, crowded mooring field? (Assume all ground tackle is sufficiently sized for the boat).

Mooring systems are much different than anchoring systems. (There can be storm moorings made using standard type of 3 anchors put in a triangular configuration, and there scope would be an issue).

Most moorings are one of 3 type: a mushroom anchor in soft mud.
Blocks of heavy material-old railroad car wheels, cog wheels, blocks of concrete and even old engine blocks are popular.
A pyramidal or navy type of anchor. With these generally the heavy chain is 1 1/2 to 2 x the maximum depth of the water. The lighter chain, or rope if no light chain, is the max depth of the water. Governmental entities often stipulate the size of the mooring and the length of chain etc.

For example for a 22 foot boat common specs would be: Mushroom anchor: 200 lbs Helical screw type 4,000 lbs holding power, Pyramid anchor, concrete block or railroad wheel:300 lbs Heavy chain and shackles: 1/2"; light chain 3/8 or 5/8" rope.

In heavy conditions mooring blocks can be moved by a large vessel. We moved a mooring block which we were told was 10,000# with our 62' ketch during swell conditions at two harbors.

Many personal moorings may be lighter those which were specified. (Chapmans' has a list of mooring sizes).

 

[westward]

Thanks Bob. What are your thoughts about Mooring line scope for a permanent buoy in a tight mooring field? The minimum scope recommended with a secure weight on bottom

 

[smckean (Tosca)]

Westward,

What you are seeing is almost exactly what I have seen.

-Upper 10’ of 3/8” galv. Chain was fine, but starting to wear thin at the lower end connection to the upper rope.

I will assume that my mooring system gets a lot more mechanical jostling from wave action than you experience. I have the same results as you except mine are far worse. The 3/8" galvi chain is good overall except where the links touch....obvious wear at each of those touching points. Like you, the worst is where the galvi chain ends at the shackle connecting to the rope thimble. I've had that link go to almost zero.

-upper galv. Rope thimble completely gone! Galv. Shackle against bare rope, but rope in surprisingly good condition at the connection, minimal chafing/ rope loss

I had the same thing happen. I too was surprised at lack of chafing of the rope. However, I decided that I lost the thimble not because it wore thru, but rather that the thimble just fell out of my spliced loop. When I replaced the thimble, I added a very tight whipping on the rope tight against the thimble. That seems to have solved the problem.

- rope appears in good shape, heavily covered with kelp, mussels, etc.

Mine always looks like that. Once a year, I go out in the dinghy at low, low tide; get as much of the rope into the dinghy as I can; and use a putty knife to scrape along the rope. Does a pretty good job.

 

[thataway]

Thanks Bob. What are your thoughts about Mooring line scope for a permanent buoy in a tight mooring field? The minimum scope recommended with a secure weight on bottom

The minimum would be the Max water depth plus half of that for heavy chain. (That assures that some chain will be on the bottom, and then max water depth for the light chain. All of the moorings should be to the same specs--and that ay with currents and heavy winds the spacing between boats, should remain the same.