Comparison of Knox Anchor vs. conventional anchor performance 


This chart shows directly obtained experimental data. It shows the development of the holding forces of the major conventional anchor designs as they are pulled through a sandy seabed. 

  • The Delta gives modest performance with a maximum holding force to weight ratio of 10.
     
  • The Bruce design shows a maximum holding force to weight ratio of only 5.
     
  •  The CQR, in this case the 21.5 kg model, performs in an unexpected way. Its holding force develops initially as does that of the Delta, but after ploughing about 4 m, its holding force falls off to a very low value coming loose from the seabed. If ploughing continues the holding force builds and falls again in a periodic cycle. The 6.7 kg CQR behaves similarly. In our opinion this behaviour is dangerous, and we believe that current CQRs should be replaced. 

In summary, these conventional anchors give poor or moderate performance. The CQR in particular will roll out of the seabed when overstressed.
 

Measurement units

To assess relative anchor performance, the anchor's measured holding force is divided by its weight force to give a 'holding force to weight ratio'. This allows anchor design performance to be accurately compared even between anchors of different weights. If an anchor has a holding force to weight ratio of 10, it can hold a force equivalent to a load 10 times its weight without moving in the seabed.

Comparison of Knox Anchor vs. other new generation anchors


The chart compares the development of the holding force of the Knox Anchor with those of a range of other new generation anchors in the weight range from 11 to 16 kg. The holding force to weight ratios of the new generation anchors are very similar and appear to reach maxima between 20 and 30. The maximum holding force to weight ratio of the Knox Anchor is around 40, 30-100% greater than that of the other new generation anchors we have tested.

While all the new generation anchors have comparatively stronger holding forces than conventional anchor designs, our tests show the Knox Anchor can hold the most force per unit weight of any other design.

Performance testing

The Knox Anchor's design applies the learning accumulated over 20 years of anchor testing by Professor John Knox in Scotland. Prof. Knox developed a rigorous testing method to assess anchor performance and has written many published articles on the subject which can be found here.

All relative performance results used on this website come from the test method outlined here, applied scientifically and fairly. We are currently undergoing third-party testing for the Knox Anchor to independently verify the conclusions we present.

Our tests of holding force have been carried out in shallow tidal pools at Longniddry beach on the South side of the Firth of Forth, Scotland, where the seabed is medium-hard sand.

Beach tests have been verified through field testing at anchorages on the west coast of Scotland, in a wide range of different seabeds.
 

Test procedure

  •  The test anchor is placed on the seabed surface in shallow water. It is pulled by a winch firmly anchored on the shore in stages of 0.5 to 1 metre at a rate of 1 to 3 cm/second. 
     
  • After pulling for between 0.5 and 1 metre, the winch is locked and the tension allowed to fall. It falls rapidly at first, but quickly reaches a steady value. This steady value is the static holding force which the anchor can withstand without moving in the seabed at that stage in the test.
     
  • This procedure is repeated until the anchor has ploughed say 5 to 10 metres. As the test proceeds the static holding force increase gradually until reaching a plateau value. 
     
  • The plateau value is the maximum holding force of the anchor. If the force applied exceeds this, a good anchor will plough slowly through the seabed.