All about anchors….
Ensuring that your anchor, rope and chain are correct will provide secure safety in the water.
The anchor cable between the boat and the anchor on the bottom forms a curve called a catenary. Ideally the catenary is such that the bottom angle is zero and the curve is sufficient to absorb shock loadings from wind and waves.
Two aspects affect the catenary.
The length of cable paid out relative to the depth of water -called the scope (normally about five times the depth of water) and the proportion of chain in the cable.
It is good for the entire anchor cable to be chain especially in rocky or coral areas but sometimes the weight of the chain can be prohibitive, such as in racing sailboats or high performance planing powerboats. Certainly the bottom section of the anchor cable should be chain so as to reduce the wear of the cable dragging over the bottom as the boat moves around in the wind and also to maximise the catenary.
The two most popular high holding power anchors are the Danforth or sand type and the CQR or plough type.
The sand type
This gives very good holding power for its weight and is easy to stow. Its disadvantage is that it does not penetrate through weeded bottoms well, and its performance is very sensitive to zero bottom angles so more cable needs to be paid out on average.
The plough type
This gives good holding on most types of bottom but is slightly heavier than the equivalent sand type and is more cumbersome to stow.
Anchor Tackle Selection Guide
The following table is our recommended table for anchor tackle. Care should be taken when using this table as boats with more windage or displacement may require heavier tackle in rough conditions. The rope diameters and cable lengths are taken from the old Victorian Marine Board Regulations for boats of a given length in metres. We have given approximate equivalents for boat length in feet for those that still work that way. The old Victorian Marine Board only advise 2.5m minimum of chain. For those boats over 6m in length that may find themselves wanting to anchor in rough conditions we recommend the Australian Yachting Federation (AYF) recently re-named Australian Sailing regulation for 10m of chain. We have chosen anchor and chain sizes from or suppliers as appropriate. Variations between our recommendation and the AYF regulations are annotated at the bottom of the table. The galvanised chain should have shackles of one size greater (e.g 8mm chain should have 10mm shackles). Shackle pins should be wired in with monel wire to prevent pins working loose. We sell this wire in 3m lengths.