Making an anchor for a mooring line

[wetb4igetinthewater]

The two anchors work in opposite directions, one for the flood, one for the ebb.

The riser comes up in the middle of the two anchors and, when your boat's moored to the buoy, it'll pull against the ebb or flood anchor depending on the direction.

Anchor's tend to work in a single direction you see

Oh, I get that. But I can't say we really care. We don't need to keep the boat in one place. Though I could easily do that as I'm cutting the 55 gallon drum in half with a grinder and can make a bottom for it. I can experiment with the amount of chain to use. I'd definitely want a buoy to keep off the bottom.

Hard bottom (rock or coral) use a core drill to set the anchor point. Drill into the bottom with the core drill until the bit get far enough in you can just see the top, then leave the bit in place and bond it into position using polyester or epoxy fiberglass resin. The resin is denser than seawater, resistant to mixing, and can be poured in with just a hose and funnel from the surface mixing support.
For small boats on hard bottom boats a 316 stainless steel eyebolt of an inch or so in diameter and long enough to engage enough rock epoxied into a hole drilled in the rock will last a long time if the mooring line has suitable chafe protection installed.

There's some rock and then if you keep going into deeper (not much deeper) water, there is more and more silt on top of the rocks and sand. Another neighbor put in an entire barrel, and that has sunk into the bottom a fair bit. It hasn't rolled. Mind you, nothing is attached during the worst storms of winter.

On soft or sand bottoms there are several easy solutions if you have access to a marine or industrial scrap/boneyard. Ships are required to replace their anchor chains when corrosion reaches a defined level, generally about a 10% reduction in cross section. It is illegal to "de-rate a used chain for use on a smaller vessel, so that can be available for free or scrap steel pricing. Even smallish ships use chain large enough to be effective anchors for boats under 100' long if enough of it is spread out on the seafloor. Chain is easy to load onto a small boat and deploy by allowing it to run free off a plywood guide (I've done this off a 12' Boston Whaler) onto a sand bottom. A buoyant section of line or submerged buoy needs to lift a few feet of chain off the bottom to reduce damage by scour, with the rest of the line being either near neutral (spectra) or slightly negative (nylon). When working off Andros we did all chain and line moors with no buoy such that the top of the moor was submerged by 15 to 20 feet, with a submerged negatively buoyant painter 30 or more feet long to hook to the visiting vessel. Hookup was done by sending a snorkeler down to retrieve the line.

I will be removing the chains in winter and inspecting them. Stuff does grow on them, even oysters.

Inland or near industrial areas old worn out railroad wheel sets are handy and relatively inexpensive. Price is generally that of scrap steel, with each wheel set yielding 2 anchors. The wheels are roughly 500# each and work well with a heavy duty 316 stainless hoop or shackle welded to half the axle for mooring attachment. They self embed in sand or mud through natural scour and are highly resistant to dragging if the attachment point is on the axle end. Other things that work well are engine blocks with chain passed through the cylinders (pick your engine size to get the weight you want,) and scrap iron filled concrete blocks. Pure concrete is too light underwater to resist much water movement or ship dragging.

Yes, there are railroad wheels that have gotten buried. I have no idea how they got there, but yeah EXTREMELY heavy.

Another option is a propane cylinder full of lead cast around a suitable size of chain. That is real dense and embeds due to scour quickly.
The life of any mooring is related to the relative movement of parts and the corrosion rates. Both steel and 316 stainless steel dissolve at the rate of about 0.008" per year in still well oxygenated salt water. The primary difference is the stainless allows the corrosion loss to be readily seen and measured. There is also the erosion corrosion effect on stainless where the protective film of corrosion is removed by relative movement of the components. You'll either need to use heavy enough sections to handle that or design it to minimize relative movement between adjacent hard parts.

Good to know. I'm 52 and hope that this anchor will outlive me or the time I'm still living in the United States as I plan to move back to Europe.

 

[Vicko]

Half a standard oil drum will weigh around 300 kg, meaning that if there is enough swell the boat could lift it off and drag, so use pleanty of chain and you will be kind of fine-ish.
If you do use 2 keep them in a line connected with heavy chain, that way there is very little chance of lifting them both off the bottom.
For a safe vertical mooring we usually calculate 3x the weight of the boat.
EDIT: if there is muddy bottom just drop it in there, the suction will keep it in place.

 

[Wibble]

Oh, I get that. But I can't say we really care. We don't need to keep the boat in one place. Though I could easily do that as I'm cutting the 55 gallon drum in half with a grinder and can make a bottom for it. I can experiment with the amount of chain to use. I'd definitely want a buoy to keep off the bottom.

If only I had a picture to save this thousand words...

It's not about keeping the boat in one place, it's about one anchor working for the flood tide and the other anchor working for the ebb. The connecting chain between them is effectively your chain scope, so should be heavy. The riser can be thinner chain or even a rope.

 

[Fred Tagge]

If you go with the galvanized shackle, spend the additional money to get a good made in USA one, like a Crosby. In my experience, the chinese ones fail at an alarming rate in saltwater.

Chinese chain is built to tension standards, not chemical. When I worked designing buoys for NOAA the purchasing agent "got a deal" on a shipload of Chinese mooring chain. After we started using it we had several buoys go adrift quickly. Since a deep water mooring is several million dollars not counting the labor to assemble or install it this became an issue rather quickly. I had the NASA material lab at Stennis Space Center do a chemical analysis of the chain. The best description of it was "used Buick." High copper, high aluminum fairly low carbon and elevated silicon. The alloy met tension specs, but it was about the worst alloy possible for submersion in seawater. We couldn't use it. The Naval Oceanographic Office ended cutting most of it up into short pieces to use for disposable self embedding anchors for anti-mine warfare operations, and we had to buy new chain.

Never trust any Chinese specification sheets. They say whatever it needs to say to sell the product, not what the product actually is.

 

[wetb4igetinthewater]

Half a standard oil drum will weigh around 300 kg, meaning that if there is enough swell the boat could lift it off and drag, so use pleanty of chain and you will be kind of fine-ish.
If you do use 2 keep them in a line connected with heavy chain, that way there is very little chance of lifting them both off the bottom.
For a safe vertical mooring we usually calculate 3x the weight of the boat.
EDIT: if there is muddy bottom just drop it in there, the suction will keep it in place.

The suctions is basically what works. You have to have a short line, a very high tide (+12 feet) and strong winds (which does happen). At that time, we will have 1-meter swells. Not significant compared to the Pacific, but still results in hard jerking by runabouts on the anchor weight.

If only I had a picture to save this thousand words...

It's not about keeping the boat in one place, it's about one anchor working for the flood tide and the other anchor working for the ebb. The connecting chain between them is effectively your chain scope, so should be heavy. The riser can be thinner chain or even a rope.

So what is the problem of the same buoy working on both? The speed of the water during flood/ebb is minimal. The real issue is wind at high tide. We maybe have 1 meter swells at high tide in summer.

Never trust any Chinese specification sheets. They say whatever it needs to say to sell the product, not what the product actually is.

And it is getting harder and harder to get non-Chinese goods. As I inspect/remove during the winter, I'll at least be aware of any degradation. I need to ensure that the stainless steel "loop" set in the cement anchor is American made.

 

[Vicko]

If you go for stainless loops but galvanized chain you will also need protection from galvanic current. Cheaper to go oversized but galvanized.

 

[Wibble]

So what is the problem of the same buoy working on both? The speed of the water during flood/ebb is minimal. The real issue is wind at high tide. We maybe have 1 meter swells at high tide in summer.

If the wind blows it sideways, both anchors bite.
If the wind blows along the ground chain, one anchor bites (and the other's slack).

The chain between the ground anchors works as their scope. As per a standard anchor+chain where there's a curve (catenary) in the chain which holds the anchor flat in the mud/sand/whatever with the anchor flukes dug in. If there's a strong gust of wind, the chain allows some give, but it's long enough to keep the anchor flat and thus dug in.

 

[wetb4igetinthewater]

If the wind blows it sideways, both anchors bite.
If the wind blows along the ground chain, one anchor bites (and the other's slack).

The chain between the ground anchors works as their scope. As per a standard anchor+chain where there's a curve in the chain which holds the anchor flat in the mud/sand/whatever with the flukes dug in. If there's a strong gust of wind, the chain allows some give, but it's long enough to keep the anchor flat and thus dug in.

During the summer, the wind never blows that way. This is a fjord and the wind either blows offshore (from the east) or there's a marine push (from the west). Southerly winds are a problem in winter when there's a king tide and we get debris over the bulkhead. But at that time, the buoy and chain will be in storage.

If you go for stainless loops but galvanized chain you will also need protection from galvanic current. Cheaper to go oversized but galvanized.

Planning on everything stainless.

 

[sea_ledford]

Chinese chain is built to tension standards, not chemical. When I worked designing buoys for NOAA the purchasing agent "got a deal" on a shipload of Chinese mooring chain. After we started using it we had several buoys go adrift quickly. Since a deep water mooring is several million dollars not counting the labor to assemble or install it this became an issue rather quickly. I had the NASA material lab at Stennis Space Center do a chemical analysis of the chain. The best description of it was "used Buick." High copper, high aluminum fairly low carbon and elevated silicon. The alloy met tension specs, but it was about the worst alloy possible for submersion in seawater. We couldn't use it. The Naval Oceanographic Office ended cutting most of it up into short pieces to use for disposable self embedding anchors for anti-mine warfare operations, and we had to buy new chain.

Never trust any Chinese specification sheets. They say whatever it needs to say to sell the product, not what the product actually is.

That sounds about right. Probably the same "deal" our contract got on China Star chain and shackles. I had a marker buoy 3/4" stud link chain wear though in less than 6 months from rubbing on concrete. The concrete was undamaged. Don't know how long it lasted, but it was gone at the 6 month inspection. That cost about $25K to replace. Good thing we spent $500 on the chain instead of $1500.

 

[Fred Tagge]

That sounds about right. Probably the same "deal" our contract got on China Star chain and shackles. I had a marker buoy 3/4" stud link chain wear though in less than 6 months from rubbing on concrete. The concrete was undamaged. Don't know how long it lasted, but it was gone at the 6 month inspection. That cost about $25K to replace. Good thing we spent $500 on the chain instead of $1500.

We had buoys moored in 18-20,000 ft underway before the ship that deployed them got back into port due to this problem. Do you have USCG history?