Technical article by Brian Greer (reprint)


Much of the fun in owning a cruising yacht comes from being able to explore new anchorages.

The shallow draught and docile manner of the H28 makes her ideal for this purpose; but, as Robbie Burns once warned us, things do not always go the way we planned. Every yachtsman, through lack of local knowledge, or error of judgment, is certain to go aground sooner or later.

This article studies some of the principles of anchoring and offers some tips on getting off again if you come to rest unintentionally.

 

 

ANCHORING THE PROBLEM
 


Most publications on anchoring, (often sponsored by firms interested in sales of ground tackle) base their logic on static loads, multiplied by large safety factors, and resulting in weights and strengths of equipment greater than necessary to withstand the predicted forces. This may be fine for large boats; but the cruising H28 sailor does not always have such a herculean crew. Moreover, experience has shown the brute force methods do not always work, and it may be that a little knowledge of the dynamics involved will allow the skipper to outwit nature where it is not practicable to over whelm her.

The main difference between a yacht moving upwind under sail and one lying at anchor is that the steering and propulsive forces acting near the mast are replaced by tension on an anchor rode, pulling diagonally downwards, near the bow. Given stable conditions a yacht will normally lie quite peacefully at anchor. As we all know, however, wind and tide are never steady, and as these forces become more severe and contrary, the once docile yacht starts to surge about like a caged lion.


THE THEORY

There are three motions experienced by a yacht at anchor:

a. Pitch and Heave (the rise and fall of the bow). This motion is damped by the weight (in water) of that part of the anchor rode clear of the bottom.

Provided enough scope is laid out to prevent the shank of the anchor from being lifted more than about 8o from the horizontal, this motion has little effect on anchor holding power.

b. Surge (fore and aft movement). Because a yacht is designed to move forward (or backward) easily, there is not much resistance to this motion apart from tension on the anchor rode. As with pitch and heave, surge forces can be countered by increasing this tension, so the two motions can be treated together.

 

c. Sway and Yaw (the familiar swinging of an anchored vessel). A yacht, having her centres of wind and wave pressure forward of the centre of lateral resistance (the underwater shape), is dynamically unstable when anchored by the bow ,and will sheet from side to side as much as 60o from the wind direction. The typical period of oscillation is 1 to 5 minutes. Although generally accepted as a fact of nature, this swinging demands a lot of space in crowded anchorage. Not only that, but an anchor is basically a unidirectional device; so alternate swinging over a wide arc can break it out, or cause it to saw its way through soft mud for fifty metres or more in the course of a night. It is this yawing motion that is the primary cause of anchor failure.

SOME SOLUTIONS
 


Correction of Pitch and Surge

The traditional solution to this problem is to let out more chain. In the shallow anchorages (3-10 metres) frequented by H28s, however, veering more chain does little to improve the damping of surge motion. There is not much working stretch while the chain lies on the bottom, and virtually none when it lifts clear. Sudden gusts can bring the vessel to the limit of its tether with a jerk, perhaps lifting and breaking out the anchor.

The development of nylon rope, on the other hand, has brought entirely new dimensions into anchoring theory. Nylon gains its elasticity from stretch rather than from catenary action, so the load changes on the anchor are much less violent. In fact, nylon can recover fully from having been stretched by as much as 50% more than its original length. Naturally, this advantage over chain becomes more pronounced in shallow anchorages, where the proportion of chain that can be suspended in the water is reduced.

Another interesting disparity between chain and nylon rodes is illustrated at Fig. 1. It will be seen that when using chain, movement away from the anchor tends to raise the anchor shank; whereas, with nylon, being stretchable, the converse occurs!

 

 

Fig. 1

A classic example of ineffectiveness of veering more chain in shallow water is the experience of W.S. Kals who, in his 9m sloop "Boheme" was driven aground and wrecked in the Bahamas by a fierce squall gust, while riding to a 35lb anchor and 30m of 9mm chain in TWO METRES of water. He might well have avoided this mishap had he included 20 or 30m of nylon warp in his anchor rode.

This does not imply, of course, that nylon is the answer to anchoring problems. Chain has distinct advantages in that:

a. it will not chafe on the bottom;

 

 

b. it is essential to safe anchoring in abrasive areas, such as coral, or in deep water anchorages, and

c. its relatively large weight/length ratio allows for a great reduction in the scope needed for a given depth.

Nevertheless, the conclusion to be reached from all this is that a mixture of chain and nylon will gain the advantages of each, and it can be shown, mathematically, that the optimum is equal lengths of chain and nylon. For the H28, anchoring in favourable conditions, the scope (or ratio of anchor rode to water depth), should be about 4:1. Thus, to anchor in say 5m you should let out about 10m of chain and 10m of nylon rope. If the conditions become marginal, (for example, if the wind forecast for an exposed anchorage were 60kts, with gusts to 85kts), then the minimum effective scope rises to more than 9:1. You would then require 23m each of chain and rope. Should you not have that amount of chain available, the same elasticity can be achieved by increasing the proportion of rope; but the length of rope required rises fast. The man with only 10m of chain on board would have to add 120m of nylon to be confident, under such conditions, that he was not going to drag his anchor! That may be all very well if you have plenty of room, But if you are anchored in company, and 140m away from your anchor, you are likely to spend most of the night fending off other boats as you sail about on your long warp. Surely there must be a more subtle solution. There are two:

1. Add Another Anchor Shackle a second anchor onto the chain of the main anchor, (see Fig. 2). The distance between the two anchors should be about 10 metres. In theory, this method should almost double the holding power of the first anchor. There are disadvantages though. You cannot drop both anchors together, extra ground tackle is required, maybe leaving the skipper without an emergency anchor aboard, and the gear is difficult to deploy and retrieve in heavy weather.

2. Weight and Anchor Rode The catenary weight of chain can be simulated by sliding a weight down the rode, (see Fig. 3). In the case described above, 23m of 9mm chain weigh 47kb. If you have only 10m of chain you are 20kg short of the required weight. By shackling that amount (of chain, ballast, anchors etc.) to the warp, and letting it out a distance equal to the average depth of water, you can achieve the same result without any of the disadvantages of double anchoring

 

 

 

Fig 3

 

 

 

 

SOME SOLUTIONS Continued
 

CORRECTION OF SWAY AND YAW
 


The remedies outlined above may have controlled the motions of pitch and surge but we are still faced with the basic cause of anchor dragging; the swing induced by the imbalance of forces when anchored in the conventional way.
The Heretic View

The obvious way to overcome the imbalance of bow anchoring is to anchor by the stern. Wind and wave forces now act behind the CLR and the vessel becomes self stabilising (see Fig 4). The bridle should be brought aboard abeam the propellor line, protected against chafe, secured to the nearest stanchion and taken round the mast or bollard.

Traditionalists will immediately point to problems of wind and weather from a direction never envisaged by the designer. Provided the rudder is secured amidships, however, the H28, with her soft waterlines aft and overhanging stern, will ride stern seas comfortably and lie more quietly and safely than when yawing and swinging to a bow anchor. The main difficulty arising from stern-to anchoring comes from the need to protect the companionway from draught and rain. Moreover, should a wind change in the night turn the anchorage into a lee shore, the skipper will have the added task of having to turn his boat before getting underway

 

 

 

Fig. 4
Some Conventional Alternatives

Increasing the scope will not correct swing; it may even accentuate it. Nor is fore and aft anchoring appropriate, unless the swing is induced by strong tides or changing currents. Notwithstanding, for those who favour more conventional methods, there are several other ways to minimise swing, all of which take advantage of the principle of elastic resistance rather than physically limiting the range of movement, thereby perhaps placing dangerous loads on the ground tackle.

1. Rig a Staysail If the wind is not too strong, hank a storm jib to the backstay and sheet it tight amidships. (see Fig. 5)
2. Rig a Sea Anchor If you have searoom astern, deploy a small sea anchor, or an old tyre casing from the stern on a long nylon line. If there is any sea running, aim to have this drag about half a wavelength astern, so the maximum drag will occur when the yacht is in the trough and most likely to sheer away.

 

This method, of course, suffers from the disadvantages of multiple anchoring mentioned earlier, and if the wind shifts, you are back to one anchor.

Of the three types of motion discussed above, swinging is without doubt the most serious. Fortunately, it is also the most easily prevented. If fact, suppression of swing is the key to emergency storm anchoring - and it makes life a lot more comfortable for ordinary anchoring as well. As can be seen, successful anchoring does not involve massive ground tackle and multiple anchors, but rather the application of simple logic and the use of modern technology.

 

 

Fig. 6

c. Rig a Forward Bridle A bridle on the bow rode, brought aboard just forward of the mast, may provide a restoring torque, and at the same time, allow the bow to rise more easily to steep seas.

d. Set a Second Bow Anchor This is a favourite remedy in nautical books, particularly for strong winds in shallow anchorages. The anchors should be set 45o apart and centred on the wind direction, (see Fig. 6).