Wednesday, 30 June 2010

Anchor's Aweigh.

I have been doing some casual research on the "t'interwebby" on the types of anchors that would be suitable for use with our new boat when travelling on tidal rivers.  Selection depends on the size of your boat, the length of your boat, the weight of your boat as well as the speed of the current flow and the depth of the water. There is an additional issue around the type and nature of the bed of the river.

The problem is there is no real "one size fits all" boat anchor.

Then there is the type of anchor, the thickness and length of the chain, the thickness and length of the warp to choose. You could also factor in a second anchor (which will let the specific single anchor weight be reduced) if there are problems with deploying and recovering heavy anchors. Plus the grab load can be shared across two mooring points on the boat. The reason for the chain is to make sure the anchor lays correctly flat to the river bed and the angle is right to dig in and take hold.

I found an interesting quote here. "For an anchor to "hold" it requires a horizontal pull along the bottom of the waterway. Ideally you would have enough anchor "line" paid out to allow for roughly three times the depth of water. The third of the paid out line nearest the anchor needs to be laying on the bottom, to provide the horizontal pull at the anchor stock. Having chain in this section helps considerably, as it tends to lay on the river bed. The effectiveness of an anchor is dependant on this deployment. They are designed to dig in harder, the more horizontal pull is provided. A small anchor, correctly deployed on a correct length line, with a chain length to the anchor stock, will be many more times effective than a heavy anchor on a short line, or one that does not have a suitable amount of chain to ensure the pull at the anchor is along the sea or river bed."

Then there is the problem of deployment and recovery after use. My fact finding is based around selecting an anchor that can be set and recovered by hand. Suitable for a 16 ton 50' standard beam narrowboat.

So what we have to do is come up with a compromise.

CQR (Clyde Quick Release)/Plough
The CQR "Clyde Quick Release" is a plough styled anchor. The CQR anchor is so named due to its resemblance to a traditional agricultural plough, many manufacturers produce a plough-style design, all based on or direct copies of the original CQR (Secure), a 1933 design by mathematician Geoffrey Ingram Taylor. As I work in a University with mathematicians I have learned not to take their theory at face value. Owing to a now well established history, ploughs are particularly popular with boaters. They are generally good in all bottoms, but not exceptional in any.
The Bruce or Bruce Claw
The "Bruce anchor" This claw shaped anchor gained its reputation from the production of large scale commercial anchors for ships and fixed installations such as oil rigs. The Bruce and its copies, known generically as "claws", have become a popular option for smaller boaters. Claw-types set quickly in most seabeds and although not an articulated design, they have the reputation of not breaking out with tide or wind changes, instead slowly turning in the bottom to align with the force.

The Danforth Anchor
The Danforth Anchor, this type of anchor have two long flukes that pivot and bury the anchor when under tension. They hold best in firm sand, gravel or mud. Danforth type anchors are not recommended for rocky bottoms were they cannot penetrate, and soft clay bottoms were they may not hold well.
The shank is the stem of the anchor in which direction is pulled to set (bury) the anchor. Weak shanks will bend when the boat pulls from another direction.The crown connects the various parts of the modern anchor. Also know as the hinge in the case of the CQR. The stock turns the anchor into an attitude that enables the flukes to dig into the sea bed. Danforth and CQR anchors are examples of anchors with a stock to help guide the points into the seabed. The tripping ring is used for the optional tripping line: by pulling the tripping line, the anchor will break out. The flukes will be buried into the seabed. The very tip of a fluke is sometimes called the bill.
The IWA Thames Tideway Handbook has a useful table of information that can be used as a rough guideline. 

Length        - Craft type       - CQR/Bruce       -  Chain       - Warp  
20ft (7m)    - narrow boat   - 4.5kg                 -  6.2mm      - 10mm
30ft (9m)    - narrow boat   - 9.0kg                 -  8.0mm      - 14mm
40ft (12M)  - narrow boat   - 11kg                 -  8.0mm      - 16mm
52ft (16M)  - narrow boat   - 16kg                 -  10.5mm    - 18mm
72ft (22M)  - narrow boat   - 18.5kg              - 10.5mm     - 20mm

The next problem is the length of the chain and then the length of the warp to choose. The minimum chain length would seem to be 5 metres and the warp around 15 meters.

A rule of thumb seems to be 10 metres of chain and 20 meteres of warp is a good compromise for most situations. You could however double the chain and warp lengths for a bit of overkill. I will also be attaching an anchor float to the line so that if we ever have to cut the line from the boat. We can then go around and attempt to recover the anchor. When Ken and Lynda the previous owners took us for the grand tour of our new boat, there was an anchor in the front locker. After feeling the weight of it. I am about to order a truss in readiness, just in case I ever have to deploy it!

It is a good idea to make sure the anchor warp has been secured onto the boat. Boats have been known to deploy an anchor that was not secured to the boat!

P.S.  It is not a good idea to deploy an anchor on a canal. You might disturb the clay lining and create problems with leakages or even breaches.


1 comment:

  1. Some good points well made there. Another one to add is make sure it is secured onto the boat!!

    We have 10kg Delta anchor with 100ft of chain (no warp) fastened to the boat with a webbing strap incase of emergency this can be cut to free the boat from the anchor and chain. This set up along with the electric winch is adequate for our 25ft boat and our cruising range.

    Another point to consider is that you must be able to handle the anchor safely so it cant be too heavy for you to lift comfortably. The anchor should also be sited where you can easily reach it in case of emergency.

    We had to use our anchor in anger at the weekend when we had a small mishap on the tidal Trent just above West Stockwith, it held very well in the conditions.


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