By Christopher Murman

Most Club Marine readers would be surprised ­ to know that the science of anchors and anchoring has largely been funded by the petroleum industry through extensive research and development, because floating oil rigs must be securely anchored to the seabed for years in order to tap into oil deposits beneath the ocean’s floor.

Over the years, much of this research has filtered down into the small boat industry. The Bruce anchor is a classic example here, while the converse can also be true – the highly successful SARCA anchor has now been upsized for commercial applications.

But anchors come in many different styles and using them properly involves a bit of know-how, so let’s take a brief look at what’s available and how they can be used.

How Anchors Work

The excellent holding power of the power of the Bruce anchor makes it very popular.

As an anchor is dragged along the bottom, its flukes dig into the seabed, dragging the whole anchor and nearby chain under the surface. As the anchor continues to bury itself, the angle of attack of those flukes (relative to the horizontal) declines, until the flukes are approximately horizontal.

At this point, the anchor will not bury itself any further, thus reaching the maximum holding power of the anchor (see Fig. 1). The geometry of the flukes and the angle between the anchor flukes and shank is critical to ensuring the burying process initiates properly – another reason to buy a good quality anchor, instead of a cheapie.

For the anchor to behave as described above, it is critical that the anchor rode (the anchor chain/rope attached to the anchor) always remains horizontal (ie: lying on the seabed). If the rode in this region is lifted above 10 degrees, the anchor will start to pull out of the seabed and will subsequently fail.

Another factor that significantly influences the holding power of the anchor is the nature of the seabed – generally speaking, loose material will have less holding power than firmer material.

When the load on the anchor exceeds the strength of the seabed material, the anchor will rapidly pull out, meaning you’ll need to reset a larger anchor or move to a more suitable location.

The Anchoring Process

While most might expect anchoring to be a relatively simple process, anchoring theory is actually quite detailed and complex. Here are some practical tips to anchoring your boat:

A Danforth style anchor.
  • The anchor achieves its maximum holding power when it has buried itself deep into the seabed
  • Greater final anchor holding power can be achieved if the anchor is partially dug in and then allowed to rest for a short time, before applying a further pull
  • If the anchor has been fully set and the chain is subsequently allowed to pull the anchor at an angle above the horizontal, varying levels of serious disruption of the seabed structure will occur, reducing the holding power
  • To ensure maximum holding value, it is important to never let the chain near the anchor lift more than 10 degrees above the seabed
  • When anchoring in shallower water, a larger scope (the ratio of the length of anchor rode in use to the depth of water) is required than when anchoring in deeper water.
  • The following example shows calculated values for the same vessel with the same anchor, with a 10mm all-chain rode to produce the same holding load value:
  • The forces acting on the anchor are many and complex – windage, currents, snubbing forces etc. The practical lesson here is that all vessels impart differing amounts of each type of load on the anchor rode. For instance, sailing catamarans typically have a larger windage factor than a monohull yacht. However, a large powered vessel will have approximately the same windage factor as a large powered catamaran of the same length. Multihulls are typically lighter than a monohull of the same length, hence the inertia forces generated will be smaller. Multihulls tend to veer more than a monohull when at anchor.

How Much Rode to Veer

The anchor rode will straighten out as the vessel pulls on it, due to wind, current, pitching and veering etc. The greater these forces, the flatter the angle of the rode relative to the surface of the water. If the slope of this angle is observed from the bow, it is possible to estimate the angle of the rode (relative to the horizontal) on the seabed. Thus it is possible to gauge if the anchor rode is still lying on the seabed or if the anchor is in danger of breaking out and dragging.

Figure 2 provides a graph for an all-chain rode. It is worthwhile to undertake the following exercises in order to gain a fundamental understanding of the anchoring process. The graph is used as follows:

  • With the known scope, enter the graph from along the bottom (horizontal axis)
  • At the known scope, move up vertically to the first curve
  • From here move horizontally to the vertical axis and read off the slope angle of the chain at the sea surface (horizontal distance divided by the vertical distance)
  • This indicates the maximum slope angle of the rode (above the water) for the anchor chain to remain on the seabed
  • Repeat steps one to four, but instead move up to the second curve (the higher one) to find when the rode is being lifted 10 degrees above the sea floor
  • As an example, let’s assume the scope is four (ie: the length of chain that has been paid out is four times the depth of the water)
  • Move along the horizontal axis at the bottom of the graph to find the number four
  • Move up the vertical line to the first curve
  • Travel horizontally along and read off the number, which is approximately two
  • This means that the slope of the chain at the surface must have a horizontal distance (from the bow to the point where it enters the water) twice that of the vertical distance. Since you know the height of the bow of your vessel, it is easy to calculate
  • To check if the anchor rode is being lifted to 10 degrees above the seabed, repeat steps one to four, but instead move up to the second line (the higher one) and read off the value of three. This means that when the horizontal distance is three times the vertical distance at the water’s surface, the anchor rode is being lifted 10 degrees above the seabed. This would strongly suggest that it is time to release more rode or move to a more sheltered location

It’s important to note the following points here:

  • If the slope of the anchor rode is below the first curve, the chain will never lift off the seabed
  • If the slope of the anchor rode is above the second curve (the upper curve) the chain (at the anchor) is already off the seabed and the anchor is being pulled at an angle above 10 degrees, in which case it would be prudent to release more rode
  • If the slope of the anchor rode is between the two lines on the graph, the rode on the seabed is being lifted slightly, in which case it would be prudent to release more rode
A CQR or Plough anchor on a Whittley cruiser.

Sizing Anchors and Rodes

The size of anchor and rode required depends on the type of vessel, and some of the factors to be considered include windage, displacement (total mass of the vessel), the type of sea bottom and usage of the vessel.

The following provides a guide and should be used purely as a starting point ­– if in doubt, always choose a larger size.

Heavy vessels (such as trawler-style launches etc) or multihulls may need to choose heavier gear, as the aforementioned figures are more suited to typical pleasure craft. When selecting a rope rode, it is important to select a rope with the following qualities:

  • It shouldn’t float as it will become fouled around the propeller or rudder
  • It should have high elastic qualities (ie: it will stretch easily) so that it will absorb some of the snubbing loads
  • It should be highly resistant to UV and abrasion
  • It should coil easily
  • It should be able to be tied off easily (ie: it can be knotted and tied to the bollard or cleat)
  • It should be able to be spliced

Most people recommend nylon rope as the best available for anchoring applications. There are cheaper alternatives, but it is always cheap insurance to invest in the very best of anchoring equipment, as one day it may save your vessel ­– and potentially your life.

Anchor Types

There is a wide array of anchors available today and it can be a little confusing for the newcomer to know where to start. The following is a brief description of the main types.

Admiralty (also known as a Ketch anchor)

  • Low holding power (when compared to some other commonly available types)
  • Can be difficult to manhandle on the bow
  • Can be dangerous in shallow water, as one fluke always protrudes
  • Not often used these days

CQR (also known as a Plough anchor)

  • Good holding power
  • Ideal for mud or weedy bottoms
  • Difficult to stow
  • Will not dig in on a rocky seabed
  • Should be used with a buoyed trip line to ensure easy recovery
  • Favoured by many sailors

Danforth

  • Excellent holding power
  • Easy to stow
  • Favoured by many sailors
  • Should be used with a buoyed trip line to ensure easy recovery

Bruce

  • Excellent holding power
  • Difficult to stow
  • Should be used with a buoyed trip line to ensure easy recovery
  • Popular with many sailors

SARCA (Sand And Reef Combination Anchor)

  • A new development in anchoring design
  • Excellent holding power
  • Suitable for a wide range of seabeds
  • Has a self-tripping mechanism built in, thus is always recoverable Less common types

Mushroom

  • Rarely used these days
  • Used for sand and mud seabeds only
  • Medium holding power

Fisherman’s

  • Good holding power in rocky or foul bottom seabeds (not suitable for other types of seabeds)
  • Not often used these days

Special thanks to Colin Thorne M.Eng.Sc.(Hons), FAIE for his assistance with this article. Please remember there are many rules of thumb when it comes to anchoring, and while the points outlined in this article can be useful, especially for the less experienced, they are generalisations nevertheless and should be used with due care and caution. Look for a special feature on anchoring practice in the next issue.

Commonly Used Anchoring Terms

Anchor aweigh: The anchor has been freed from the seabed and is ready to be hauled aboard (ready to weigh anchor)

  • Broken out: The anchor has pulled out from the seabed and is no longer holding
  • Anchor Rode or Anchor Cable: This term used to collectively describe the anchor chain and rope. The term anchor cable is more often used on ships rather than smaller vessels
  • Dragging: The anchor is sliding across the seabed, but has not dug into the seabed
  • Foul anchor: The anchor is caught on an obstacle on the seabed
  • Let go: To release the anchor over the side
  • Moused shackle: To wire the pin to the shackle so that it cannot come undone
  • Scope: The length of the anchor rode divided by the depth of the water
    (see Fig. 3)
  • Snubbing the rode or cable: To stop the rode running out by using the brake on the anchor windlass
  • Veer the rode or cable: To pay out more anchor rode
  • Weigh anchor: To haul the anchor and rode aboard.

 

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