Like many fencing techniques, the use of box strainers was developed in New Zealand. As in a conventional strainer, the box strainer relies on the principle of the triangulation of forces, but in the case of the box strainer, the force is taken by the strained wire.
1. The box strainer is as strong as a conventional strainer in firm soil. In weak or very shallow soils the box strainer may perform better than the conventional strainer.
2. The posts in a box strainer do not need feet, and where possible, are best driven into the ground. The fitting of the horizontal stay and the tensioned wire takes longer than fitting a strut.
3. The components of a box strainer are only slightly more expensive than those of a conventional strainer, because although two posts are needed, they are of smaller diameter.
4. The box strainer is not suitable for areas where vandalism is a problem, as the whole assembly will fail if the wire is cut.
The box strainer is a more complex structure than a conventional strainer, with more to go wrong, and its use is probably best restricted to situations where a conventional strainer is difficult to build. Single and double box strainers are widely used for high tensile fencing. Box strainers can also be used for high tensile deer fencing, although the Forestry Commission prefers the conventional strainer with retaining wire.
Where ground conditions are difficult, or very high strains have to be held, a double box strainer can be constructed. This has over twice the strength of a single box strainer. Although the optimum depth is 1m (3'), it is possible to construct a double box strainer in only 300mm (1') depth of soil, for example over bedrock.
Construction Of Box Strainers
1. Knock the two posts into the ground, the distance apart of the horizontal stay.
2. Cut recesses of about 15mm (1/2") near the top of each post. Place the stay in position.
3. Drill a hole through the post into the stay, and then hammer in a 200mm (8") length of 12mm (1/2") mild steel bar. Repeat at the other end of the stay.
4. Follow the procedure for straining and fixing a retaining wire, as described above.
Alternative Box Strainer Design
A 'windlass' can be used to tension the wire where mild steel wire is used for the loop. This can be tightened further as necessary if the wire slackens. Preferably use a piece of hardwood, about 50 x 50 x 450mm (2 x 2 x 18"), inserted between the wires. Twist until tight, and then lock behind the stay. Fasten it with wire to discourage vandalism and to prevent stock interfering with it.
The same principle is employed in the box strut, which is a very useful device for fence corners, slopes, and for repairing and strengthening posts and strainers. The following diagrams show some of these uses. An incidental advantage, for farm use, is that the horizontal stay provides an easy way of getting over fences, without damaging yourself or the fence. However, this may be a disadvantage if it provides unwanted access.