At one minute the motor-craft's stern was deep in the water. At another the propeller was whirling in the air and the powerful engine racing madly. Sheets of solid water poured over her bluff bows, until the thick glass panes of the wheel-house threatened to give way under the formidable onslaught. Well it was that Derek knew the channel well both by night and day. All he had to guide him were the leading lights astern. Ahead nothing but inky blackness; to port the breakers threshing against the Tinker Shoal; to starboard more white-foamed masses of water hurling themselves upon the flats of the Dairymaid Sands. An error of eighty yards on either hand would result in disaster both to the R.A.F. boat and her tow, for, notwithstanding her strong construction and uncapsizable design, the life-boat would stand no earthly chance should she be hurled upon the boiling breakers over the sands. Suddenly a light flashed through the darkness away on the starboard bow. "NC--NC--NC" it called, signifying in code language: "In distress; require immediate assistance." "Three hundred yards over the Dairymaid Bank," declared Derek to his coxswain. "Keep her as she is; we can't edge in any closer. I'll slip the life-boat when she's dead to windward." "Aye, aye, sir!" replied the man, wiping the spray from his eyes. The wheel-house window was open, for closed, with the water continually being flung against the glass, the limited range of vision was still further reduced. Plunging, rolling, and staggering, the staunch little craft plugged steadily onwards, the life-boat straining and yawing at the end of three hundred feet of stout grass hawser. With little protection save that afforded by the high, rounded fore-deck, the life-belted and oilskinned crew of the life-boat were literally sitting in water, in spite of the relieving tubes that allowed the boat to free itself of any breaking seas. "Far enough," decided Derek. "Keep her head to wind, coxswain!" Making his way aft the young officer ascended the short iron ladder and looked astern. He had to hold on like grim death, for the lively motion of the motor-craft made it impossible to stand unaided on the slippery deck. Raising one arm Derek motioned to the life-boat to cast off. The coxswain of the latter saw the signal. The motionless crew became active. The hawser was cast off, and oars fell into the crutches. Backing the while, the life-boat vanished into the darkness. Returning to the wheel-house Derek consulted his watch. It was now nearly six o'clock. Already he had been afloat for more than two hours, and the time had passed with inconceivable rapidity. In another hour and a quarter there would be sufficient light to distinguish the position and nature of the vessel in distress.