A look at anchors and anchor rodes
By Paul McNeill
Two anchors and cables should be carried on board but what size and types of anchor and what lengths and diameters of chain and warp? If you have a lot of time at your disposal you could research the following:
- Test carried out on the USA early 1990’s collated by Robert Smith and published ISBN 0-9651350-0-4
- Tests in 2003 and 2009 by French magazine Voiles et Voilers and published in November 2009 edition of UK’s Yachting Monthly
- MGN 280 Table 20.1 available via MCA website. This specifies minimum requirements for small commercial vessels.
- Test results by individual manufacturers of anchors. Caveat emptor!
The above list is by no means exhaustive but I suspect that 95% of cruising yachtsmen have neither the time nor the inclination to fully research the complex issues of anchor design, catenary and scope. My job as a sailing instructor is to attempt to simplify things so that those on the learning curve can “just do it!” This may be mission impossible when it comes to anchoring but I will give it a try over the next 3 articles.
It’s easy to say the “more the merrier” or the “bigger the better” when it comes to choice of hardware but only a few yachts are large and heavy enough to follow these maxims. Most of us have to make careful choices based on available space, weight and practicalities of handling our ground tackle. Most cruising yachtsmen are very safety conscious and I hope the following comments will reassure you that what you have on board already is perfectly adequate.
In the last 10-15 years a considerable number of new anchors have appeared on the market and independent tests indicate that they should be taken seriously. (Not all tests carried out have been equally rigorous or comprehensive.) The bottom line is that you can achieve greater holding power relative to size and weight than some of the older generation of anchors which have been on the market for a long time. This does not mean that you should rush to change the old faithfulls which have served us well for many years. If you are thinking of extending your cruising grounds and spending more time at anchor you might want to take a look at some newer models. I am not going to identify which anchors I currently use as I don’t want the good to become an enemy of the better but I can offer some comments about size.
Westbound Adventurer is a 10 metre Sigma 33C probably on the smaller side of average if there is such a thing as an average size cruising yacht. Displacement when fully loaded is probably about 5 tons. I have a 15 kg. “storm” anchor which is used in good holding in winds of F7 or more. I would use it in winds lesser than F7 if I had any doubts about the quality of the holding on the seabed. I also have a 10 kg “working” anchor which is used most of the time. Ease of handling is important as far as RYA Training is concerned especially as Westbound Adventurer spends 80 – 120 nights at anchor each year plus many lunchtime stops at anchor. I also have a third light anchor and cable for certain training purposes but there is no need for most cruising yachts to have a third system. I believe that 15 kg is as heavy as most people can haul up manually given that there will always be the extra weight of chain relative to the depth. Beyond 15 kg an anchor windlass becomes highly desirable if not essential. Some will regard it as highly desirable on all occasions irrespective of weight. If we can have furling sails and lazy jacks to make life easier why not an anchor windlass? There is no windlass on Westbound Adventurer but I will not hesitate to get one fitted when I retire from RYA Training.
The 10 and 15kg anchors are kept in the anchor well along with the main cable. This allows the anchors to be changed on the foredeck if required. I keep the second anchor cable and the third system in a locker amidships as it is not desirable to have too much weight up forward even if there is space.
The Anchor Cable or Rode
The issues of Catenary and Scope are distinct but not separate.
There are many experienced yachtsmen who prefer to have all chain cable and lie at anchor with a few metres under their keel. This allows the weight of chain to form a curve or catenary. When the wind increases this catenary is the first defence in the system and can act as a stabiliser to the motion of the boat at anchor. In this context it is possible to be securely anchored with a scope of only 1 to 3. I believe however that this will only be the case in light to moderate winds.
When it blows hard enough for the cable to be fully stretched the catenary has no further purpose. It has disappeared. The security of the system is now dependent on minimising the angle between the seabed and the chain closest to the anchor. This is achieved by increasing the scope. If in doubt let it out! When it blows up your cable does nothing for you in your anchor locker.
For those with an all chain cable, a length of nylon needs to be added to the system when it blows really hard. Even in light to moderate winds many yachtsmen will add a rubber snubber or a short length of nylon warp over the bow roller to eliminate the nocturnal grunting of chain on the bow roller. The occupants of the forward cabin will appreciate this.
In very strong winds when an all chain cable becomes fully stretched great shock loads are put on the weakest links in the system. Attach nylon warp to the chain outboard of the bow roller and secure the inboard end of the warp to a strong point on the foredeck or take the warp to a primary winch in the cockpit. Nylon is very strong and it stretches which is just what is needed to absorb the shock loads created by a boat surging at anchor in a strong gust. The longer the length of nylon the more energy can be absorbed. It is prudent to insert chaffing protection for the nylon warp.
It is theoretically possible for security at anchor to be achieved by catenary alone but the amount of chain required would make this totally impractical for most yachts.
How hard does it have to blow for an anchor cable to be fully stretched? The variables involved are:
- the strength of the wind
- the windage factor of the boat both head on and after it tacks through the wind at anchor
- the sea state within the anchorage
- the weight of chain deployed.
The amount of chain to be kept on board should be considered in terms of its weight as well as its length. Weight is determined by the diameter of each link. I would suggest that the maximum diameter of chain that is practical to handle manually is 3/8” or 10mm. If you have a windlass this will determine the diameter of chain used. On some models it is possible to change the pall to suit the chain diameter. The length of chain for your anchor cable depends on the sort of depth you want to anchor in, the size of your boat and its arrangements for stowage. Many cruising yachtsmen will carry 40-60 metres of chain on their main cable but considerably less on their second cable. 40 metres of chain allows a scope of 1 to 4 in 10metres of water or a scope of 1 to 8 in 5 metres of water. Some will argue that the more chain you can carry the better, but is this always the case?
My own solution to the balancing act of security at anchor versus weight and sailing performance is different to the practice of many cruising yachtsmen who would typically have a larger yacht with fewer crew and more space for heavy ground tackle.
With a draft of 1.3m I anchor in shallow water (5 – 7 metres at the top of the tide) with a cable of 20 metres of 10mm chain spliced into 20 metres of nylon warp. (20 metres of 10mm chain is the same weight as 32 metres of 8mm chain often used on yachts of this size) For an overnight anchorage I have all the chain in the water and create a minimum scope of 1 to 4 by letting warp out if in 6 or 7 metres depth. My second cable is identical to the first but chain and warp are shackled together rather than spliced. This allows the chain of the second cable to be added to the first cable on the very few occasions I anchor in deeper water. RYA Day Skipper Course Notes recommend a minimum scope of 1 to 4 for an all chain cable and this is excellent advice while at anchor in light to moderate winds. As the wind increases we need to increase the scope by letting out more cable but this does not have to be more chain.
A fully stretched cable runs more or less straight from the anchor to the bow roller so the geometry is the same if the anchor cable is all chain, half chain and half nylon or all nylon. It is not desirable to have an all nylon anchor cable because of the risk of chaffing on the seabed and the motion of a yacht at anchor with an all nylon cable. It would also be difficult to set an anchor with an all nylon cable. It may be desirable to have an all chain cable (to which nylon warp is added in very strong winds) but it is not practical to do so on many yachts due to space, weight and sailing performance.
The bitter end of the anchor cable should be arranged so that it can be brought onto the foredeck still secured below. This allows the system to be buoyed and dumped in a hurry if required. How long would it take to haul up the anchor if you had an MOB at night drifting downwind from your anchored boat?
To fully explore the delights of the Scottish Hebrides we need the confidence to anchor in gale force winds. The good news is that summer gales are usually well forecast and short lived. Your first experience of gale force winds at anchor will cause you some anxiety. It is only by doing it for real that you loose your natural anxiety and gain confidence in your ground tackle. Read “Digging in for a Blow”
Paul McNeill is a Yachtmaster Instructor and Principal of Westbound Adventures Sailing School which operates on the Clyde and in the Scottish Hebrides during the summer months. Paul is also a member of the Royal Institute of Navigation.