The Iris and Daffodil were both well known to Liverpool folk, being used for conveying passengers across the River Mersey many times daily. They were extremely handy craft, could each carry fifteen hundred men if required, and drew very little water, but they possessed two serious disadvantages. Firstly, their decks were so low as to necessitate the use of long storming ladders for reaching the parapet. Secondly, their steaming qualities were comparatively poor, judged from the point of view of the operation for which they were required. Just picture their ordinary daily employment for a moment. Waiting alongside one of the piers at Liverpool till their usual quota of passengers had embarked, they would make the short trip across the river to the Birkenhead shore and then wait once more. During this second period of waiting the steam pressure would be increased in the boilers in readiness for the next short voyage across the river. Compare that employment with a trip of nearly one hundred miles across the open sea. It will then be evident that the task allotted to these two ferry vessels was by no means simple from the engineering point of view alone.
All this, however, was carefully thought out, and it was decided that their advantages outweighed their disadvantages. Both craft, by nature of their work, were designed to stand heavy bumping alongside piers; their draught was small, and, as already stated, they were easy to handle. After minor alterations they proceeded to the port of assembly in charge of their naval crews and adopted the title H.M.S., much to the amusement of those of us who made their acquaintance for the first time. It is rumoured that one of these two vessels arrived at her destination with her anti-submarine escort in tow, which thus early showed that proud spirit to which she so justly proved her right on St. George's Day, 1918.
The first duty of the Daffodil on arrival at the Mole was to be that of pushing the Vindictive bodily alongside. The former vessel was to place herself at right angles to the latter, bows against the latter's side, and to continue pushing until Vindictive, which would previously have anchored, was secured by means of the grapnels. Daffodil was then to drop alongside Vindictive and her parties were to climb over the latter and up to the Mole. The Iris was to go alongside the Mole ahead of Vindictive, to anchor, to grapnel the parapet, and to land her storming parties by means of ladders against the wall, her decks being too low to allow the use of large gangways as carried in Vindictive.
In the event of Vindictive being sunk, Iris and Daffodil were to storm the Mole as best they could and do everything possible to knock out the three-gun battery or divert its fire from the blockships.
It was believed that the Mole garrison consisted of about one thousand men. But what of reënforcements arriving from the shore? Access to the Mole would entail the crossing of the viaduct by such reënforcements. Therefore the viaduct must be destroyed. Consideration on this point led to a decision to utilise one or more submarines filled with explosives and to blow them up under the viaduct, so as to cut the latter in twain.
This particular phase of the operation had not been included in the original Plan evolved at the Admiralty. As previously stated the first edition could not be expected to cover every single investigation of every point in the problem. The attack on the viaduct, after a large amount of experimental work ordered by Vice-Admiral Keyes, took the following shape.
Two submarines, each carrying several tons of high explosive, were to accompany the expedition. They were each to carry a crew of two officers and four men, who, after securing their craft underneath the viaduct, were to light the time fuses and then to take to the boats. Each submarine carried a small motor-driven dinghey for this latter purpose.
So much, then, for the blockships, storming vessels, and submarines at present.