Screw Anchors, Steel Anchors and Concrete Blocks: How Each One Holds
A concrete block holds by its weight, a steel anchor by dragging the blade at its tip (its fluke) into the bottom, and a screw anchor by the soil above its plates. Here they are side by side, including where a screw anchor is not the right choice.
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Side by side
Three anchors, one mooring leg at a time
Mussel and shellfish farms are usually moored on concrete blocks, fish-cage grids on steel anchors of 500 to 5,000 kg. Here is how both compare with a screw anchor, also called a helical anchor.
| Aspect | Concrete block | Steel anchor (500–5,000 kg) | Screw anchor |
|---|---|---|---|
| How it holds | By its weight in water and its friction on the seabed. | It is dragged along the bottom until its fluke, the blade at its tip, digs in. | It is screwed to a set depth, where its helical plates bear against the soil. |
| Holding for its weight | Low. It loses about 43% of its weight in seawater. | High for a horizontal pull, once it has set well. | Its own weight plays no part in holding. |
| Upward pull | Resisted only up to its weight in water. | Holds best when the pull is near-horizontal. An upward pull can break it out. | Resisted by the soil above its plates, so the line can meet it at an angle. |
| In a storm | Can slide or shift once the pull exceeds its friction on the bottom. | Can drag if the pull exceeds what its fluke holds. | Holds as a fixed point. There is nothing resting on the bottom to drag. |
| When the load direction swings | Resists a pull from any side, but only up to its weight and friction. | Set facing one way. If the pull swings round, it can break out and drag before it re-sets. | Holds the same whichever way the line pulls. There is no fluke to turn. |
| Scope (line length ÷ depth) and ground chain | About 5:1, with chain lying on the bottom. | About 5:1, with heavy chain lying on the seabed to keep the pull low at the anchor. | About 2:1, with no ground chain. |
| Space on the lease | Legs at about 5:1 take up area that could hold lines. | Long chain legs take up area that could hold cages. | Legs about a third as long, so more of the lease or mooring field holds lines, cages or boats. |
| Where it ends up | Where it was lowered, until a storm shifts it. | Wherever it sets after dragging in, so its final position varies. | At the coordinates you give us. |
| Getting it there | Cast and cured ashore, shipped out and lowered by crane barge, all before the season opens. | 0.5 to 5 tonnes of steel per anchor, plus heavy chain for every leg. | Compact steel parts, turned in from a work boat sized to the job. It takes load on the day. |
| Capacity known at install | Only its weight is known. | Only if it is tensioned to a test load after it sets. | Installation torque is recorded for every anchor and converted to a calculated capacity. A pull test confirms it where the project calls for it. |
| Seabed footprintWhat it means for seagrass | The block’s full base, plus the scour of its chain. | Ground chain lying on the bottom, and the scour around the anchor. | The diameter of its shaft. |
| Where each one fits | Any seabed, including rock where nothing can be screwed in. | Standard equipment that any anchor-handling vessel can lay, recover and re-lay. | Sand, silt, mud and mixed seabeds, not rock. It needs specialist installation equipment. |
Where the pull meets the seabed
A steel anchor holds by digging in as it is dragged, so the pull has to reach it almost flat along the bottom. That is what the heavy ground chain is for. Lift the pull and the anchor can break out. A screw anchor is already set at depth, so the line can run straight to it at an angle, and upward pull is taken by the soil above its plates.
Illustrative, not to scale. Your site survey and grid loads set the real layout.
How this is drawn
One mooring leg from each system to the same surface float, at the same water depth. Line angles and chain length are drawn for clarity, not taken from a design. A real cage grid has several legs at every corner.
Which one for your site
- Rock, or a seabed nothing can be screwed into
- A concrete block. A screw anchor needs soil to bear against.
- Anchors you lift and re-lay every season
- A steel anchor. Any anchor-handling vessel can lay, recover and re-lay it.
- Sand, silt, mud or a mixed seabed, where the mooring has to stay put
- A screw anchor: a fixed point at the coordinates you set, on a shorter line, with no chain on the bottom.
- Not sure what your seabed is
- Send what you have. An engineer tells you which one fits your site, including when it is not a screw anchor.
Send the site. We’ll tell you which anchor fits.
Depth, seabed type and loads, or whatever you have so far. An engineer replies within 24 hours with questions and a first read; the full proposal follows within two business days.
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