Imray Cruising Guide: Anchoring techniques

Adapting to seabed types 

Anchoring securely requires an understanding of how different seabed types impact anchoring technique.  
 
The Isles of Scilly - a sailor's paradise where anchoring is vital due to the lack of marinas and limited number of moorings – provides a perfect case study. Cruising through this archipelago will challenge you with a variety of seabed types, from sandy stretches to rocky outcrops, making it an ideal location to sharpen your anchoring skills.  
 
 
1. Sandy seabeds 
 
Characteristics: Sandy seabeds provide excellent holding ground for most anchors. In areas like Tresco Channel, sand offers great security, as the anchor can dig in deeply, creating strong resistance. 
 
Best Anchors: Plow, Scoop, or Danforth anchors are ideal for sandy seabeds because of their ability to bury themselves in the sand and hold firmly. 
 
Technique: Drop your anchor slowly, allowing it to settle into the sand before gently reversing your vessel to help it dig in. It’s vital to ensure a generous scope - five to seven times the depth of the water in settled weather - for the best holding power. Keep an eye on tidal ranges because this will affect the depth and scope required. 

2. Rocky seabeds 
 
Characteristics: Rocky seabeds, found in parts of St. Agnes, pose a challenge for anchoring due to their irregular, hard surfaces. These conditions make it difficult for many anchors to get a proper grip, leading to potential dragging. 
 
Best Anchors: Grapnel or Plow anchors are typically best for rocky seabeds as they are designed to catch on the rough surface, though care is needed to avoid damaging the anchor. 
 
Technique: Lower your anchor slowly to avoid damage from sudden impacts. You might also want to consider using a trip line to retrieve the anchor if it gets stuck between rocks.  

 
 
3. Weedy seabeds 
 
Characteristics: Weedy seabeds can be particularly challenging, as dense vegetation might prevent the anchor from penetrating to the seabed. This is common in more sheltered bays like Old Grimsby Harbour, where eelgrass and kelp can be found. 
 
Best Anchors: Sharp, pointed anchors like Delta or Grapnel anchors are best suited for weedy seabeds, as they are designed to cut through the vegetation and reach the seabed below. 
 
Technique: Ideally you want a heavier anchor or one with a sharp point to slice through the weed. It’s important to ensure that your anchor has fully penetrated the weed and reached the actual seabed. Failing to do so may leave your vessel vulnerable to drifting. Patience is key in these conditions, so give the anchor time to set properly. 
 

4. Shingle seabeds 
 
Characteristics: Shingle can be a mixed blessing when it comes to anchoring. Found in some coves around Bryher, shingle provides relatively good holding but may require more time for the anchor to settle. 
 
Best Anchors: Scoop or Plow anchors work best in these conditions, as they can wedge themselves between the stones and provide a good grip. 
 
Technique: Drop the anchor slowly to allow it time to settle into the stones. Once it’s in place, slowly reverse the vessel to set the anchor. As with sandy seabeds, you’ll need to ensure you’ve laid out enough scope to account for tidal changes. 
 
 
Anchoring with confidence 
 
Anchoring is an essential sailing skill, and the variety of seabeds found around the Isles of Scilly make it a great training ground. By mastering these techniques and selecting the right anchor for the seabed, you’ll ensure that your vessel stays safe and secure, no matter what the weather or tides throw at you. For a more detailed exploration of anchorages and navigational challenges, the Isles of Scilly pilot book provides invaluable guidance to explore this unique archipelago.