Marine oil suction bell-mouths of the controllable gap type have been invented by the author.One of the types embodying this invention was manufactured on a full scale basis, and was tested twice on board both the “Tozyo Maru” (41, 000 DWT tanker) and the “Cactus Maru” (50, 600 DWT tanker), compared with the ordinary type oil suction bell-mouths (called OBM hereinafter) for cargo oil and stripping oil. This paper deals with the results of the experiments on the controllable gap bell-mouth (called CGBM hereinafter). The mechanism of the CGBM consists of the fixed bell-mouth proper and the movable shutter, with which the gap between the lower end of the shutter and the bottom of the tank is controllable from the upper deck by means of a reach rod, in accordance with the level of oil in the tank. The invention, furthermore, includes various types of CGBMs, i. e. the method of raising and loweriug the bell-mouth proper without the shutter, as the movable part, and the wiring, hydraulie or automatic float system, instead of the reach rod system, as the gap control device, etc. The results in comparison with the OBMs for cargo oil and stripping oil were as follows : 1. When the sea water in the tank is discharged through the CGBM and a cargo oil pipe of 10inches in nominal diameter by the cargo oil pump of 1, 000 m³/h in capacity : (1) the suction pressure is 0.13-0.16 kg/cm² (about 20 per cent of the suction pressure) less than the one when discharged through the OBM for cargo oil and the pipe, (2) the pump capacity is expected to be increased 4-8 per cent more, (3) the air-draw is less apt to occur, and (4) the quantity of ballast water, which is impossible to be sucked, becomes less. 2. When the residual ballast water is stripped through the CGBM and a cargo oil pipe of 10 inches in nominal diameter by the stripping oil pump of 150 m³/h in capacity : (1) the suction pressure is less than the one when stripped through the OBM for stripping oil and a stripping oil pipe of 6 inches in nominal diameter, (2) the pump capacity is 1.3-1.5 times more, and (3) the quantity, which is impossible to be stripped, becomes less.