Square cement block vs pyramid shaped

[mEdensky]

We are planning on installing proper buoy moorings for visiting yachts in front of town Hanga Roa in Easter Island, where anchors have ripped up all of the coral. It's volcanic bedrock and some sand. Getting helix screw equipment out here would probably be quite costly because of the distance, so we were thinking constructing cement blocks.

What is the most efficient shape of these blocks for best holding power? Square, upwards pyramid or downwards pyramid block? How heavy a yacht would each of these blocks hold if they each weight let's say 1000 kg?

[meirriba]

Checking literature. Before checking, my idea would be to consider inverted mushroom shape or C. I do not like shape B, it will be the most easy to drag out.

[jmorrison146]

For the best analysis, the mooring Folsom will want to know the displacement of the boat, the anticipated depth of the water, and maximum current to be encountered.

[meirriba]

Originally Posted by jmorrison146

For the best analysis, the mooring Folsom will want to know the displacement of the boat, the anticipated depth of the water, and maximum current to be encountered.

And anticipated maximum wind.

[valhalla360]

Originally Posted by meirriba

Checking literature. Before checking, my idea would be to consider inverted mushroom shape or C. I do not like shape B, it will be the most easy to drag out.

Mushroom anchors (like C) are great in silt/mud as over time, they will work their way into the ground embedding themselves.

If the location is mostly rock with a little sand on top, I doubt it would embed itself.

Basically, you are looking at purely the weight holding it in place while sitting on the surface of the seabed...in that case B would be the best as it has the lowest center of gravity.

Now, a big question is how much sand do you have available? A helix anchor may be a good option and it doesn't necessarily need significant equipment to install. If you can find an area with deep sand, a couple of experienced divers and a long pole can install them manually.

Concrete isn't the best material. Upwards of 40% of the weight will be offset by buoyancy of the water. Plus maneuvering a 1 ton block will be expensive and require significant equipment. How much holding power it has is very dependent on the seabed.

[Uncle Bob]

Originally Posted by mEdensky

We are planning on installing proper buoy moorings for visiting yachts in front of town Hanga Roa in Easter Island, where anchors have ripped up all of the coral. It's volcanic bedrock and some sand. Getting helix screw equipment out here would probably be quite costly because of the distance, so we were thinking constructing cement blocks.

What is the most efficient shape of these blocks for best holding power? Square, upwards pyramid or downwards pyramid block? How heavy a yacht would each of these blocks hold if they each weight let's say 1000 kg?

Hi, if the bottom is of soft enough material to allow the weight to sink in then I would go with B, but if the bottom is hard then A, but for what it is worth the weight you mention is nowhere enough, concrete loses approx half it's weight when submerged so if the position is exposed the weight should be at least 5000 kg.
My boat is moored in a well sheltered river bay in Sydney, with two train wheels and heavy ground chain, the bottom is soft mud, the boat does not move in weather. Boats around me use 2000 to 5000 kg concrete blocks to achieve the same thing. If the location is exposed you need more weight, not less.

[meirriba]

After some literature perusal:
Soft/muddy bottom that allows the mooring block to sink - best to use inverted mushroom or second best inverted pyramid. These will sink with time and enhance mooring holding power.
Rocky/hard bottom - flat/cubical block (like A or cube). Not only weight matters. The surface area is important as it creates resistance to lifting. So for similar weight, flat is better than cubical.

Material - any material loses weight in water:
Concrete retains about 55% of original weight, steel - about 86%.
It would be beneficial, if using concrete block, to re-inforce its bottom with steel during casting.

If there is substantial current, it is recommended to put two or three weights for each mooring (in a triangle), sort of Bahamian moor to allow for change of direction of the pulling force.
This has additional benefit - allows partial replacement of block by block.

As far as I understand, the weight of the vessel is secondary interest. Most important is the force created by wind/current on the vessel.
One table I have seen suggest a 2.6t concrete block to assure holding of 15m LOA/4m beam vessel in F9 wind.

All the above is not my personal knowledge - only what I have found in books that may not be correct.....

[GordMay]

Greetings and welcome aboard the CF, mEdensky.

As others have noted, concrete loses about 42% of its weight, when fully submerged in seawater, so a mooring anchor designed to weigh 1,000 kg on the seabed, will actually require about 1,744 kg of dry weight concrete.
Concrete block “deadweight” moorings are cheap & relatively easy to construct, but have the least holding power of common mooring types.
However, when they drag, they will resist motion with a constant amount of force (drag more slowly).

The USCG, who no longer use concrete as their main mooring method, have an Aids to Navigation Manual, specifying (page 69) a 12,750 pound “sinker” block, for an 18,500 Lb navigation buoy (with 1.5" chain). ➥ https://media.defense.gov/2018/Jul/1...M_16500_3A.PDF

[DefinitelyMe]

Hi there. I used to build hurricane moorings here in Bermuda for a living. What Gord May has said about the effective mass in water is correct. In terms of effective holding weight per unit volume it is one of the least efficient mooring construction materials. However, it is still my favourite because it is fairly easy to form, sinks down brautifully over time into the substrate and, perhaps most importantly, does not cause other components of the mooring to suffer from galvanic corrosion as so many other materials do.

Regarding shape, I think C will not work in your substrate unless it really is very deep and soft. A or B will work. My moorings use heavy ground chain attached to the weight. I favour round weights because the chain has a tendency to get caught and jammed beneath corners. A circular shape is also easy to form from thin sheet metal and the form is strong. However if you plan to attach a buoyed line directly to the weight then this becomes irrelevant.

I'd use about one cubic meter of concrete (2.5 metric tonnes) for a 40-foot boat, depending on expected weather conditions and how sheltered the bay is ('not very' is the answer to that isn't it?). But pretty much every mooring for cruising yacht usage that I've ever seen is smaller than that; some by quite a bit.

Other great materials for weights include:

- Forklift/crane counterweights
- Big cylinder liners (the kind you might get from a ship's engine or a fairly large power station
- I-beams

[DefinitelyMe]

Originally Posted by meirriba

Checking literature. Before checking, my idea would be to consider inverted mushroom shape or C. I do not like shape B, it will be the most easy to drag out.

With respect, I disagree. B will be the least likely to drag, due to maximal area in contact with the substrate and a tendency to heap up substrate on the leading edge if it does start to drag.

[DefinitelyMe]

Originally Posted by GordMay

However, when they drag, they will resist motion with a constant amount of force (drag more slowly)

That is true, however in general the amount of force needed to break the weight free from the substrate in the first place - especially with concrete weight - is far in excess of what is needed to keep them dragging. Perhaps by many times. Often once they start they don't stop.

[valhalla360]

I don't have a chart but looking at it the anchorage appears to be fully exposed and it's a volcanic island...

If the block starts sliding, is there a significant distance of relatively consistent depth? If it's close to the edge of a drop off, I would hate to have a massive concrete block fall off the edge and take the bow with it if it starts dragging.

[carlosproa]

40 years ago we had a similar situation in Venezuela where we needed to anchor permanently a big floating houseboat made out of wood. This was in Morrocoy where the bottom is mixed sand and corral.
After trying concrete, mushroom anchors which none worked, we settled with Diesel engine block from trucks. We had 4, one for each end of the square house.
As a bonus, the lobster really like the holes for the cylinders as hiding place, so we had 32 options of lobster to choose.
The system worked very good for us for many years