Ahoy Sailors, now that you've learned each function of your boat's different sails, it's time to talk about anchoring your vessel correctly. Stay tuned for the next lesson, which will explain how to secure your gally.
When released, anchors dig into the seabed to hold your boat in position. They serve a safety role by keeping vessels out of the surf or off the rocks. They also allow boaters to secure the boat temporarily while fishing, having lunch, or spending the night.
How Anchors Work: When an anchor penetrates the seabed surface, suction generates resistance created by the bottom material plus the weight of the material above the anchor. As the boat pulls on the anchor rode, the anchor digs deeper into the floor, creating additional resistance. Anchors can’t dig in rocky bottoms but rather snag on protrusions and hold precariously.
Setting: Setting an anchor means ensuring the anchor has locked onto the ocean floor. If the anchor is not set, the boat can seem well anchored until the wind comes up, causing the anchor to bounce along the bottom as the boat drags toward a hazard.
As the boat moves backward due to wind, current, or the engine’s power in reverse, gradually pay out the rode (an anchor-rode is the connection system between the anchor and the boat, and its sturdiness during mooring is undoubtedly essential. The critical point is the splice where the rope and chain are connected). Always keep a light tension on the line, but don’t clinch it tight yet. (If you tighten the rode too soon, the anchor will be pulled upward and out of the bottom and not completely set.)
Visualize the anchor rode pulling straight back on the anchor shank as the anchor fluke(s) point(s) dig into the ocean floor. If your anchor rode is all chain or has a chain section at the anchor, the pull will be more nearly horizontal along the bottom. This is how anchors are designed to dig in and hold.
When you have about three times as much anchor rode out as the water depth (a scope of 3 to 1), temporarily cleat or cinch the anchor rode at the bow and let it pull tight. Keep a hand on the rode to feel the tension. The boat should stop, and the rode should feel very tight, indicating the anchor has been set. If the anchor has not set, you will feel the tension in the rode come and go or feel its pull changing as the anchor bounces along the bottom.
If the anchor has been set, continue with the next step of paying out scope. If it has not been set, you can also continue but must be careful to ensure the anchor digs in when you have the proper scope. If the anchor has not set yet with about a three to one scope, many sailors prefer to hoist it now and try again rather than letting out more anchor rode and bringing it all back up to try again later.
The scope is defined as a ratio of the length of an anchor rode from the bit to the anchor shackle, and the water depth under the boat’s bow measured from deck height. Most anchoring texts and anchor manufacturers agree that a scope of 7:1, meaning for every foot of water, you should release seven feet of the anchor. This scope achieves the anchor’s designed holding power, and more scope is better than less. In theory, the 7:1 scope is an excellent yet crowded anchorage; most cruisers scoff at the idea of paying out more than 3:1 or 4:1; there isn’t that much space for boats to swing. You can consider shortening scope in a crowded anchorage when an anchor is securely set.
Once an anchor has been set, it will almost always hold the same amount of tension that was used to set it, even if the scope is reduced. This means that you can pay out the long scope, pull hard on the anchor rode using the engine, and then shorten the scope to reduce swinging room. However, if your boat swings and the anchor has to reset itself, it will have to do so at a reduced scope. This technique is known as Anchoring Russian Roulette.
Resetting: It’s relatively easy to set an anchor when wind and current come consistently from one direction, but if they veer, some perform better than others under varying angles of pull. If the boat swings far enough, any anchor can become dislodged from the seabed. Four techniques can alert you when your boat swings:
- If you have an anchor alarm on your Chartplotter, set it to alert you if the boat swings too far from where it was when you dropped the anchor.
- If you have an electronic compass or autopilot, set the course alarm to alert you if the boat’s heading changes radically.
- If you have alarms on your depth sounder, set maximum and minimum alarms to alert you if the water depth changes significantly, indicating that you are drifting either away from or towards the shore.
- Stand an anchor watch. It is an excellent practice to take bearings on prominent landmarks when you anchor to detect any subsequent change in position.
Anchoring Techniques with Two Anchors Other Articles on Anchoring; Anchoring Bow and Stern:
You may need to limit your boat’s inclination to swing at anchor in tight anchorages. You can locate the boat precisely in the anchorage by dropping an anchor close to the beach and a second anchor in the opposite direction. You can also use the tension on one rode to help set both anchors.
Two Anchors Off the Bow:
Bob Ogg, co-inventor of the Danforth anchor, recommends setting one anchor into the wind or current and a second anchor 180 degrees away. Then take both lines to the bow of the boat. This action allows the boat to swing around in a tight arc while still allowing the boat to pull against an anchor without causing it to reset when the wind or the current changes.
Other Anchoring Considerations:
Assessing Bottom Conditions:
Anchors need to develop enough resistance in the seabed to withstand the environmental forces on the boat, such as the wind and the waves. However, an anchor’s ability to build resistance depends entirely on it engaging and penetrating the seabed.
Fine-grained sand is relatively easy for anchors to penetrate and offers consistently high holding power and repeatable results. Conversely, most anchors will hold tremendous tension in hard sand. The lightweight Danforth-style anchors are the best anchor for sand like the West Marine Traditional and Fortress anchors.
Mud has low shear strength and requires anchor designs with a broader shank-fluke angle and greater fluke area. This allows the anchor to penetrate deeply to where the mud has greater shear strength and presents more surface area in the direction of pull. Mud is frequently only a thin layer over some other material, so anchors that can penetrate through the mud to the underlying material will hold better. Fortress anchors have superior holding power in the mud because they can be converted to a broad fluke angle.
Holding power is more dependent on where you happen to drop the hook than on the type of anchor you have. Plow-shaped or grapnel-type anchors with high structural strength to sustain the high point loads generally work the best. These include the Bruce, CQR, Delta, and the old-fashioned Fisherman-style anchors.
Shale, clay, and grassy bottoms:
Tough bottoms don’t work for all anchor designs. The weight of the anchor, more than its design, might be the most crucial factor in penetration and holding power. CQR and Delta anchors are thought to be good due to their ability to penetrate the vegetation. However, these conditions have a high probability of false setting due to the anchor catching on roots and protrusions rather than something solid or stable.