Installing a helix (corkscrew) mooring.

[Franziska]

@fatherchronica

Let me help you with the units. We may not be so advanced in Europe, but we have a clever system too

:-)

1000mm = 100cm = 1m = 3.28ft

[RaymondR]

You could just build one of these.

[valhalla360]

Borrow 3 dingy anchors, manually set them in a triangle pattern around the site. Place them so you will be pulling perpendicular to a line extending towards the screw.

By pulling on the line, you can develop enough force to turn the lever arm on the screw. By moving from anchor line to anchor line, you can rotate thru 360.

[a64pilot]

Ever seen a post hole digger? The motor driven, hand held one, very similar to an ice drill for those of you up North.
I’d assume one would make short work of a helix anchor, they put in Mobile Home helix anchors in a hurry, usually have about 10 on each side for a Mobile Home properly anchored.

You may however have to hammer a section of pipe in to act as an anti rotation brace to use the motorized post hole digger.
Depending on where you are of course, but in the US down South, they are easily rented for the day.

Found this on YouTube, this is a big boy machine bigger than what you need, but illustrates the point.
https://youtu.be/Tv1zKr0vTIc

[Jim Cate]

Originally Posted by a64pilot

Ever seen a post hole digger? The motor driven, hand held one, very similar to an ice drill for those of you up North.
I’d assume one would make short work of a helix anchor, they put in Mobile Home helix anchors in a hurry, usually have about 10 on each side for a Mobile Home properly anchored.

You may however have to hammer a section of pipe in to act as an anti rotation brace to use the motorized post hole digger.
Depending on where you are of course, but in the US down South, they are easily rented for the day.

Found this on YouTube, this is a big boy machine bigger than what you need, but illustrates the point.
https://youtu.be/Tv1zKr0vTIc

How does this work under water?

Jim

[AndyEss]

Originally Posted by datapete568b

Has anybody considered using a jet pump? Most dock builders jet poles
into the bottom with water pressure from a pump . water pressure clears
the sand and mud creates a hole , the helix falls into the hole, nature does the rest (or you can use the jet) to fill in the hole. Works great. (not in coral)

Pete

Dock piling is loaded in compression- anchors are loaded in tension.
Jettiing a hole to advance a compression pile is a pretty lousy way of contruction, but it can result in some compressive capacity, along with a bit of resistance to bending moments.
Jetting in a helix anchor, and letting natural sedimentation fill in the hole might be similar to drilling a hole in your bulwarks, dropping in the end of a shroud, and filling the hole with sawdust (no epoxy). The likelihood of that shroud carrying any tension is nil.
Instead of going to all that effort and cost to jet in a helical anchor, one might just throw the helical anchor overboard like a usual anchor, and hope for the best. At least you won't have many illusions as to its holding power.

[David M]

I have divers install helix moorings a few times per year for research instrument installations.

Once the thread is started a ways, the diver pulls up on the helix with the bar that is used to turn the helix. The bar is usually a 4 foot long stout length of rebar. Pulling upwards gives the diver traction on a sandy or muddy bottom. Use chain and not line and calculate for the weight of the chain versus the buoyancy provided by the float. (about a 2:1 ratio). Make the scope about 2:1. Use a plastic float with internal foam (commercial fishing supply store) and not a fender. I have seen them hold for many years even in strong current.

No offense intended to others, but some are making this much more complex and expensive than necessary.

[valhalla360]

Originally Posted by AndyEss

Dock piling is loaded in compression- anchors are loaded in tension.
Jettiing a hole to advance a compression pile is a pretty lousy way of contruction, but it can result in some compressive capacity, along with a bit of resistance to bending moments.

Dock pilings have negligible compression loading but jetting is perfectly acceptable for compression loads assuming they are designed for point loading (not skin friction).

The main load consideration for docks is bending forces...as long as the jet hole doesn't get out of hand (ie: too large), it's also fine.

I agree with you comments about jetting a screw in.

[RaymondR]

Drivel piles gain most of there axial load capacity from skin friction rather than end bearing. Consequently they should be driven rather than inserted into pre formed holes unless they are to be cemented in place.

However pre jetting a hole to assist in driving helical, screw installed anchors is a valid installation method provided that sufficient undisturbed soil is retained to provide a cylindrical, virgin soil shear plane for the flights to act against.

[AndyEss]

Originally Posted by RaymondR

Drivel piles gain most of there axial load capacity from skin friction rather than end bearing. Consequently they should be driven rather than inserted into pre formed holes unless they are to be cemented in place.

However pre jetting a hole to assist in driving helical, screw installed anchors is a valid installation method provided that sufficient undisturbed soil is retained to provide a cylindrical, virgin soil shear plane for the flights to act against.

Jetting relieves all horizontal stresses by introducing a void within the soil mass. Soil has zero shear resistance if there is no confining stress.
Jetting a tension pile/helical anchor is absolutely the worst possible way of installing the component.
As for pile installation, the balance between skin friction and end bearing capacity depends on how the pile was installed and the type(s) of soil the pile goes through. Soil is notoriously inhomogeneous. Quite often, before full end bearing is mobilized, side shear has peaked, and can even degrade.
About the only ground engineering components installed by jetting are dewatering points (obviously rarely used in marine applications).
An analogy with screw fasteners (into wood, fiberglass, plastic and metal substrates, all of which have tensile, compressive and shear strength) would be to drill an oversized hole (how do you control hole size while jetting in soil?) and backfilling it with sawdust, and expecting the screw to hold.
Soil has no tensile strength and has zero shear strength at zero confining pressure.

[Boatguy30]

120 cm is a very tiny anchor.

All the Florida mooring field ones are about 6m long and jetted in.

[Empty Pockets]

Originally Posted by Boatguy30

120 cm is a very tiny anchor.

All the Florida mooring field ones are about 6m long and jetted in.

20 feet?

[RaymondR]

Originally Posted by AndyEss

Jetting relieves all horizontal stresses by introducing a void within the soil mass. Soil has zero shear resistance if there is no confining stress.
Jetting a tension pile/helical anchor is absolutely the worst possible way of installing the component.
As for pile installation, the balance between skin friction and end bearing capacity depends on how the pile was installed and the type(s) of soil the pile goes through. Soil is notoriously inhomogeneous. Quite often, before full end bearing is mobilized, side shear has peaked, and can even degrade.
About the only ground engineering components installed by jetting are dewatering points (obviously rarely used in marine applications).
An analogy with screw fasteners (into wood, fiberglass, plastic and metal substrates, all of which have tensile, compressive and shear strength) would be to drill an oversized hole (how do you control hole size while jetting in soil?) and backfilling it with sawdust, and expecting the screw to hold.
Soil has no tensile strength and has zero shear strength at zero confining pressure.

You don't relieve confining stresses when you drill or jet a round hole, it's resisted by the circumferential compressive loading which occurs. The same phenomenom is responsible for the skin friction.

[AndyEss]

Originally Posted by RaymondR

You don't relieve confining stresses when you drill or jet a round hole, it's resisted by the circumferential compressive loading which occurs. The same phenomenom is responsible for the skin friction.

You know, for the last 40-50 years, the world geotechnical engineering community has been trying to figure out how to drill (jet? Holee schmolies!) those "perfect holes" you talk about.

Exotic drilling muds, cased holes, hollow stem augering, you name it, has been tried. Most professionals have given up on trying to drill that perfect hole, and most of the industry has switch to in situ testing, where directly pushed in instrumented probes are used to test the soil. No drilling at all because of all the difficulties in getting "undisturbed" soil to retrieve and test in the laboratory.

Drilling your perfect hole in soil (below the water table) can rarely be done by the best professionals in the business. I don't believe two minimum wage marina laborers who were given the responsibility of installing helical anchors using jetting are going to meet the grade.

You are right, soil arching can and does occur - except never in a sand below the water table. And we're talking about installing marine anchors - by definition deep below the water table.

Sure, you can bury helical anchors by jetting. The issue isn't being able to bury the anchor, but what pullout forces they can develop. Another commentor mentioned Florida marina experience with helical anchors installed by jetting.
They put in 20 ft long anchors, relative to the 3.3 ft long anchor the OP is asking how to install.
I bet a 20 ft long anchor despite being set in a jetted hole, would develop some fair amount of pullout resistance with time. That would depend on that perfect hole of yours losing its arch, and then collapsing in around the helical anchor. You seem to need that perfect hole staying perfectly open, but yet it needs to collapse into that jetted hole to re-create even some minimal frictional pullout resistance
So, if you can afford over-designing everything by 600% (and paying for it) go right ahead.
If the OP installs his 1 meter long helical anchor using jetting, I bet a big dinghy moored to the anchor pulls it out, much less the OP's boat.

[Fundy Sailor]

A diver installed my 5' helix a few years back using about a 5' bar through the eye. Took him all of 10 minutes, maybe. I suspect the bottom in this case is pretty much loon sh*t, so added about 40' of 3/4" chain which mostly lies on the bottom an acts as the main anchor. Another 40' of 3/8" chain up to the ball. If the wind gets up enough to drag the big chain out straight, then the helix holds it in place. I'm glad the system wasn't put to the test in the first few months while the ground was still 'disturbed', but suspect that after a few years of settling back into place, it would take a significant pull to get that thing out.