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# Floating Anchor

Properly shaped concrete mooring anchor floats until it sinks
 (+4) [vote for, against]

How does one replace a heavy anchor that's nearly spent?

Our current 2.5 ton concrete anchor's eyebolt has been submerged in about 4m of salt water for nearly 70 years, and it has worn thin.

To my best guess, it was floated out on a raft of driftwood and was sunk some time in the 1940s.

Rather than trying to repair the eyebolt underwater by hiring expensive divers and underwater welding equipment, I propose to float and sink a new concrete anchor with a seriously large eyebolt (made from ~3cm diameter stainless steel rod, bent in a ring of about 10cm radius,) embedded in a concrete "boat."

"Float and Sink?" you are no doubt thinking (if not saying aloud!) Be patient, I will explain.

Recall that standard concrete has a mass of around 2400kg/m^3 and salt water around 1027 kg/m^3.

If one shapes the cubic meter of concrete into a boat shape of about 3-4m x 1m x 1m depth, it should float. Built at low tide on the beach, it should be able to float as the tide rises, and be towed out to sinking location by a small motorboat.

So it should be possible to build a ~2.5 ton anchor of concrete with a hybrid shape: that of a boat which would float for a short (100m) distance, but when intentionally sunk, would serve as an anchor.

 — csea, Dec 15 2011

Ferro-Cement boats http://www.ferroboats.com/
Been around for about 160 years, but not intended to sink. [csea, Dec 15 2011]

Sounds eminently sensible, practical and doable. But it's Christmas, so [+].
 — MaxwellBuchanan, Dec 15 2011

 Sounds reasonable. How are you planning to sink it? If the answer is "Tipping it over until it starts to take on water" then you'll need a calculation, or something, to make sure it's not embarrasingly stable.

(Or you could give it a mast, and add weight to the top of the mast until it started to tip over.)
 — mouseposture, Dec 16 2011

 Just put a bloody removable bung in it on a cast-in 2" threaded bush. \$10 worth of gal fittings.

 This is a brilliant idea. You should sell them. Putting out private moorings is one of the most time consuming, difficult, expensive, and hazardous things you'll do. This solves all of the probelms in one neat hit. Tow it out with your own boat, remove the bung and presto! Down it goes. I live in a remote area and some of the rivers nearby have rpivate moorings in them and I shudder to think of what dramas those guys went through to put them out. With this concept, you could tow one or more rigs out an indefinite distance to wherever you like and set up a permanent mooring.

[21] - The common term is mooring, not anchor.
 — Custardguts, Dec 16 2011

 ^yes.

[+] what [MaxwellBuchanan] said.
 — FlyingToaster, Dec 16 2011

 Something like a small Phoenix Caisson, as used in the Mulberry harbours then?

 Get it in position, then blow holes in the bottom with a couple of small shaped charges. Should go straight down.

What [MB] said. This is simple, practical, and there are plenty of successful precedents, so it isn't really Halfbaked at all.
 — 8th of 7, Dec 16 2011

 //mooring, not anchor//

 Yes, thanks. The idea was for a semi-permanent (70+ year) attachment to the seabed. Stainless steel chain and snubber would be replaceable. Actual application would be for a floating dock.

Suitable sinking methods left to the student as an exercise.
 — csea, Dec 16 2011

 OK - half-croissant hat in place - here goes.

 Standard boat-shaped concrete tub (or should I say tub-shaped concrete boat? whatever) with wooden rafts attached at the gunwales. Rafts loaded with gravel or cobbles, tub loaded with nothing. Float into position, begin siphoning water into boat... as it goes down, it tilts the rafts, which unload their rock loads into the boat. As the rafts come near to being on-edge, they come disconnected from the half-circle hooks on the boat.

 Is there any current in the area? In that case, have a central pile, with pile-driver weight on top driven by a ramp-cut collar, turned by an anemometer-vaned concrete block caisson. Allow it to nail itself to the seabed.

Is there bedrock on the seabed? If so, invent underwater lava-welding...
 — lurch, Dec 16 2011

Sounds like the perfect opportunity to rent an old diving suit and compressor to conduct repairs/renovation. (And you can use the compressor for the air tools as well)
 — FlyingToaster, Dec 16 2011

I was hoping that this would be an autonomous free-floating platform fitted with large fuel tanks, immensely powerful water-jet thrusters, a GPS system, and unlimited credit with a selection ocean-going refuelling tanker operators.
 — pocmloc, Dec 16 2011

Very good. I'm with [Flying Toaster] on this one though.
 — 2 fries shy of a happy meal, Dec 16 2011

This is simple. Make your boat out of pykrete, and pile the anchor aboard it.
 — RayfordSteele, Dec 16 2011

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