Anchors let ships, boats, and some submarine support vessels hold position by transferring forces from wind, waves, and currents into the seabed. Different anchor shapes work better in different bottom materials such as mud, sand, gravel, rock, or clay. The main job of an anchor is not just to be heavy, but to dig in and create enough holding force to resist drifting.
Understanding anchor types helps mariners choose safer equipment for the vessel size, weather, and anchoring location.
A typical anchoring system includes the anchor, a shank, flukes or arms, and a chain or cable called the rode. When the vessel pulls horizontally, the anchor rotates, digs, or hooks into the bottom, increasing friction and soil resistance. Stockless anchors are common on large ships because they stow neatly in hawse pipes, while plough, fluke, claw, and mushroom anchors are chosen for different seabed conditions.
The angle and length of the rode are critical because a low pull angle helps the anchor stay buried instead of being lifted out.
Key Facts
- Holding power depends on anchor shape, seabed material, pull angle, and anchor size.
- Scope = rode length / water depth, and common safe scope values are about 5:1 to 7:1 in moderate conditions.
- Stockless anchors are compact ship anchors that work well for large vessels but may have lower holding efficiency than some modern small-craft anchors.
- Plough anchors roll upright and bury in sand, mud, or clay, making them useful for changing pull directions.
- Fluke anchors have broad flat flukes that give high holding power in sand or soft mud but perform poorly in rock or heavy weeds.
- Horizontal pull increases burial and holding force, while vertical pull tends to break the anchor free.
Vocabulary
- Anchor
- A device lowered to the seabed to hold a vessel in place by gripping, digging into, or adding resistance against the bottom.
- Rode
- The chain, rope, cable, or combination of these that connects the vessel to the anchor.
- Scope
- The ratio of rode length to water depth, used to control the pull angle on an anchor.
- Fluke
- A broad blade or pointed part of an anchor that digs into sediment to create holding force.
- Seabed
- The ocean floor material where an anchor lands, such as sand, mud, gravel, clay, weed, or rock.
Common Mistakes to Avoid
- Choosing an anchor only by weight is wrong because shape and seabed grip often matter more than mass for holding power.
- Using too little scope is wrong because a steep rode angle can lift the anchor out instead of pulling it deeper into the seabed.
- Assuming one anchor works equally well everywhere is wrong because sand, mud, rock, weeds, and clay require different gripping mechanisms.
- Backing down too suddenly after dropping anchor is wrong because many anchors need a gradual horizontal load to set and bury properly.
Practice Questions
- 1 A ship anchors in 12 m of water and uses a 6:1 scope. How many meters of rode should be paid out?
- 2 A small research vessel needs 84 m of rode for safe anchoring at a 7:1 scope. What is the water depth?
- 3 A vessel must anchor in a sandy seabed with shifting wind direction, while another must anchor near exposed rock. Explain which anchor features would be useful in each case and why.