As a Yachtmaster Instructor I teach the theory of anchoring during shorebased RYA courses and attempt to demonstrate best practice on the water with Westbound Adventurer during practical courses. I believe that most of the business about anchoring can be effectively taught but there is one grey area which causes many of us to loose some sleep and that is the uncertainty about digging it in.
I have no wish to add to the sum of excellent textbooks and articles which already exist so let’s assume that we have the wisdom to select a suitable anchorage, that we have all the appropriate equipment, and that we have the skill to use it.
For 90% of the time all will go well but when the wind rises from fresh to strong we start to have a doubt�.and that doubt means that we don’t sleep well at anchor. If we only anchor for a few nights each year the probability of a disturbed night’s sleep is small and we could perhaps live with that. As a sailing school instructor I spend many night’s at anchor especially during my summer cruises of the Scottish Hebrides. The ultimate test of a skipper’s confidence is very simple: Can you sleep at anchor in a rising gale? I believe I can sleep at anchor in a rising gale because I have taken time to carry out controlled experiments in an attempt to colour in that grey area of �digging it in�.
Some would suggest that gradually building up to full revs astern should take care of most overnight conditions. Some would suggest that for a real blow you should also shock load your cable by motoring forward then charge astern at full revs. I did this some years ago and wasted a lot of time and crew energy resetting a dragging anchor several times when the overnight conditions only required a gentle tug. How hard to dig it in can be educated guesswork!
I wanted to discover what �digging it in� at slow astern, half astern, full astern and full astern with shock load equals in terms of windage on my own yacht at anchor so I invested in an anchorwatch strain gauge device and it took me 2 sailing seasons to build up a profile of the strain which Westbound Adventurer takes at anchor in different wind strengths and conditions. This is simply compared with the measured strain which Westbound Adventurer creates at slow, half and full revs astern and the anchor is dug in accordingly with an extra safety measure added to the equation.
In other words, if the overnight forecast is for a force 2/3 I know that by going astern to half revs I am safe for a force 4/5 then I sleep well in spite of the fact that I could be (unknowingly) anchored in mud or kelp. If the forecast is for a 6/7 I know that I have to go to full revs astern and re-anchor if it drags.
This anchorwatch strain gauge device gives its readings in kilograms but it is actually measuring the tension on the anchor cable which is greater than the windage factor on the yacht. To measure the windage factor you would need to take a horizontal line ashore. The downward component on an anchor cable creates a greater tension in the same wind strength as a horizontal line ashore.
Are you confused? Don’t worry, I was equally confused and had to ask some friends with honours degrees in maths and physics to explain it to me. I was then even more confused so I simply compared a horizontal bollard pull at full revs astern with the pull at anchor full revs astern and guess what my learned friends were correct!
The use of this device is best explained by examining the photos. The load cell on the foredeck feeds the information to the control unit which could sit in the cockpit or be mounted at the chart table. Either way, a source of power is required (12 volts).
I use a back up line on the foredeck for the anchor cable but it is not shown in the photo. The anchorwatch control unit stores the maximum reading and my masthead wind instrument does the same. This means that it was not necessary to stay awake all night in the interest of science!
There are occasions however when we find ourselves in a marginal anchorage and there are no immediate alternatives. This calls for an anchorwatch by the crew and an escape plan. The escape plan should include bringing the bitter end of the anchor cable onto the foredeck and buoying it ready for dumping and collecting. Proper sleep will be unlikely but the crew will be resting and not exposed to the elements. The anchor may drag slowly for some time before there is any danger and a need to clear out.
In Anchoring Extras Part 2 I examine the use of the riding sail, angel and the line ashore.