Copyright NZ H28 Yacht Owners' Association.

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.

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 8^o 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 60^o 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.

Part 2