Results of Tests with regard to Depth of Water, Speed of Launching and Construction.

By CAPTAIN HOWARD F. J. ROWLEY, C.B.E., R.N., Chief Inspector of Life-boats.FOR many years past the practice of launching Life-boats off Slipways has been accepted as the best method of getting heavy Life-boats into the water.

In fact, it may be said that the Slipway has had to be adopted at all places on the coast where adequate protection cannot be found for heavy Boats to lie afloat.

In the beginning of the present century a minimum depth of 2 feet 6 inches of water at the seaward end, or toe, of the Slipway was considered sufficient, but, in succeeding years, owing to the rapid development of the Motor Life-boat and the increase in its size and weight, this depth has had to be increased to 3 feet, 4 feet, 4 feet 6 inches and 5 feet. In fact, 6 feet has often been required in exposed places where heavy seas, and a consequent fluctuation in the depth of the water, are likely to be encountered in heavy gales.

Obviously this deepening at the toe has made it necessary to extend the Slipway further seaward, and this again has added very much to the cost, particularly for underwater work.

To avoid this cost, the toes of Slipways have, wherever possible, been terminated with a vertical drop, instead of being carried down to the sea-bottom, provided, of course,that the depth of water required was forthcoming. In some places these' Slipways with vertical drops have been proso constructed that the water lappedthe toe at low-water at ordinary spring tides, at others they have been continued until the water submerged the toe to a foot or more ; but at exceptional spring tides these toes might be so far uncovered as to give a vertical drop to the water level of some three feet. This abrupt termination produced a certain amount of doubt and apprehension in the minds of some of the Honorary Officials and Coxswains. Their fear has been, first, that the rudder might not clear the Slipway when the Boat dropped off the vertical end, although it is triced well up above the keel level; secondly, that the Boat would strike the sea bed if travelling down the Slipway at a great speed.

It was generally thought, however, that a 45-foot Boat, leaving the Slipway at a moderate speed of 18 feet per second down a 1 in 5 gradient with a 3-foot vertical drop, would clear. On the other hand, it was felt that the possibility of the rudder striking the keelway would increase in proportion to the retarding effect of the water at the moment of impact. Thus, should the bow of the Boat be water-borne on the crest of a wave before the stern post cleared the toe the danger would be greater than when launching in smooth water, and as the water level rose from 3 feet below | the toe to 3 feet above it (at which point these' the danger would cease) the risk of j striking would be increased in direct proso | portion to the increase in wave motion.It was with this problem before them that the Committee of Management approached the National Physical Laboratory at Teddington in October, 1924. As a result, experiments were undertaken by the Laboratory in the William Froude National Tank in February of the following year, in order to trace the paths taken by the forward and after ends of the keel of a Life-boat during launching, with a view to finding whether damage to the Boat was likely to occur, either through the forefoot striking the ground or the heel dropping back on to the launching ways, after prematurely leaving them, before the vessel had cleared the Slipway. For the exhaustive and valuable tests which were made the Institution is greatly indebted to the Laboratory.

The special points studied by the Laboratory were :— (1) Declivity of Slipways.

(2) Speed of launching.

(3) Depth of water over toe of Slipways.

(4) Eoughness of water.

The tank in which these experiments were made had the following dimensions :— Length . . 64 feet.

Breadth . . 5 ,, Depth . . 3 feet 6 inches.

The model was made from the lines and drawings of the 45-foot Cabin Motor Life-boat, Manchester & Salford (O.N. 689), stationed at Douglas, in the Isle of Man. The bottom of the tank was made to correspond with the fall of the ground at the foot of the Slipway at Douglas, and the Slipway ibself was a scale model of the actual Slipway at that Station, the scale of the Boat and Slipway being one-eighth full size.

During the rough weather tests a wave-maker was used, the waves created being 13 feet 2 inches long by 10 inches high, and 5 feet 1 inch long by 8 inches high. To obtain the corresponding wave for the full-size Life-boat, these measurements must be multiplied by eight, so that in the first rough water experiments the wave motion corresponded to actual seas 105 feet long and 6 feet 8 inches high. In these rough water tests each experiment was repeated in such a manner that the model enteredthe water half a wave period later, so that if the bow of the Boat plunged into a wave crest in the first experiment, it entered the water at a wave trough when the experiment was repeated.

In all 160 experiments were made, accurate and continuous records being kept of the paths taken down the Slipway and through the water of both ends of the keel of the model Boat.

From these records the exact position of the keel relative to the Slipway or ground, and also its speed at any desired instant during the launch, can be obtained.

It would be impossible here to describe the results in detail, but they have conclusively shown two important things :— (1) Provided the front half of the Boat is fully water-borne before the heel leaves the Slipway, there is no danger of any kind to be anticipated from the vertical drop.

(2) So far from the danger of the Boat striking the ground when entering shallow water from a steep Slipway being increased by the speed at which the Boat travels down the Slipway, the exact opposite was found to be the case.

If the depth of water into which the Boat is being launched is not more than four feet, the experiments revealed a possible danger of the forefoot striking the ground ahead of the Slipway, when the gradient is steep and the launching speed slow. For instance, where the speed at entering the water was a maximum of 28 feet per second for the actual Boat, the forefoot was found to get very close to the ground before the bow commenced to lift, but where this maximum speed was increased the bow lifted sooner, pointing to the desirability of attaining a high speed of launching at all times, even for a Low Water launch into a calm sea.

The heel of the Boat, it was found, never approached so closely to the ground as the forefoot, even when it dropped off the ways, and the inference drawn was that in smooth water at low tide the forefoot is more liable to damage through striking the ground than the after-end.

Very useful information was also obtained with regard to the amount of water shipped. None was taken in during any of the experiments in smoothwater. In rough water very little was shipped at the highest launching speeds at lower declivities, but, as was to be expected, more was taken on board in rough water at the steeper declivities and at the higher launching speeds.

Water was occasionally taken inboard amidships at the steeper declivities and at the lower launching speeds when the model entered the water at the moment when a wave crest was at the Slipway and at amidships relatively to the model simultaneously.

If we translate the general conclusions into actual figures we get the following instructive results :— (i.) A Slipway gradient of 1 in 5, the standard now usually adopted, is about the most suitable for all conditions of launching.

(ii.) It is advisable that the Boat should be launched as high up the Slipway as possible, and that rollers should be inserted in the keelway above high water level for gradients of 1 in 5-5 and steeper—and between high water and low water at ordinary spring tides as well for flatter gradients—in order to increase the speed.

(iii.) For Stations in exposed positions in the open sea the Slipway toe should be carried down to a depth of 4 feet below low water at ordinary spring tides, but in harbours and other positions where it is not likely that waves more than 2 feet in height will be encountered at low water it is not essential that the keelway should be continued beyond2 feet 6 inches to 3 feet below low water at ordinary spring tides.

(iv.) Provided there is sufficient depth of water ahead it is not necessary to carry the toe of the Slipway right down to the sea bed.

(v.) A depth of 6 feet of water below low water at ordinary spring tides at 25 feet beyond the Slipway toe is desirable.

The experiments clearly showed that there is no danger in launching at low water at either Gorleston or Blyth, at which Stations such dangers were first feared, provided the maximum speed of the Boat down the Slipway is obtained.

Further, generally speaking, at all Stations on the coast the launching conditions may be accepted as satisfactory, and there is no need for any alteration in the existing Slipways.

At Margate and at St. Abb's it has always been an accepted fact that the Boat could not be launched at low water of exceptional spring tides without striking the ground beyond the toe.

At Bembridge the rocks have already been trimmed, and the hollows filled up with concrete forming a continuous keelway to a sufficient depth of water.

At some Stations the depth over the toe and 25 feet beyond is less than the depth just given as necessary, but the Life-boats at these Stations are of a smaller type than the 45-foot Cabin Watson Boat, for which the experiments were made, and the depth at these Stations is quite adequate for the size of Boat..