Airship Anchors

The problem of anchoring a ship, that is, securing her without the assistance of men on the ground, is one which is mainly of importance if it is necessary to prevent an airship drifting when broken down. Various forms of grapnel have been used from free balloons for many years, but an airship, which is many times greater weight, is found to acquire such momentum when drifting that she will pull out or break any ordinary grapnel. The problem of getting hold of the ground from an airship above is much more difficult than appears at first sight. An ordinary grapnel will be dragged a considerable distance before it catches a tree or anything giving a suitable hold.

A proposal was made that the airship should fire a form of harpoon into the ground and ride to that as an anchor. This question was again raised in 1916, and rough designs were therefore prepared to determine the best form of harpoon which would sink into the ground and then open so as to exert considerable resistance to being pulled out. The principal difficulty lay in obtaining sufficient penetration, and experiments were, therefore, carried out to determine the form of head which would give the best penetration. Several samples were dropped, and at the conclusion of the tests attempts were made to pull the dummy anchors out of the ground. This proved a very difficult business, and the idea of a solid grapnel which would penetrate the ground sufficiently far to jamb itself securely obviously presented itself. Considerable success was obtained. The head of the anchor was made of cast iron with a long, tubular shaft, and the wire was secured to a point close under the head. When this anchor had penetrated the ground to a considerable distance and the ship had drifted so that the pull came fairly oblique, the wire cut into the ground and tended to pull the whole grapnel sideways. To such an action a very satisfactory resistance was obtained.

It was still, however, found that a heavy ship drawing her trail rope suddenly taut against a grapnel such as this, either parted the trail rope or ran considerable risk of damage to the mooring point of the ship. It was, therefore, necessary to devise some suitable means of gradually absorbing the energy of the drifting ship without producing any excessive impulsive tension on the rope.

The problem of anchoring over the sea is a comparatively simple one. An ordinary drogue, formed much like a parachute, has quite a satisfactory effect in reducing the speed of a drifting airship down to two or three knots. It was thought that if an anchor was dropped so as to be on the farther side of the drogue, the anchor would secure itself satisfactorily to the bottom of the sea, and the drogue would then act as a weight to resist the upward component of the pull of the ship. Under these circumstances, however, it is found that the drogue which is kept moving through the water can easily be arranged to keep itself full.

It is not easy to pull a drogue such as this out of the water when it is desired to get under way again, and a slip has, therefore, been arranged whereby the drogue is secured to the end of the trail rope and can be spilled from the ship.