Recently there is a trend to install engines with a small number of cylinders on ships in order to save energy and space and to gain simpler maintenance. But the engines with a small number of cylinders which have long piston stroke and run slowly have a tendency to cause serious vibration problems. Particulary some observations of fore-and-aft vibration of an engine frame, which are not found in the engines with a large number of cylinders, have been reported. The fore-and-aft engine vibration is excited by the longitudinal vibration of a shaft system and the fluctuation of propeller thrust. In this report, dynamic response of shaft systems with 4-, 5- and 6-cylinder engines is studied in order to see it's effect on the fore-and-aft engine vibration. Main conclusions obtained are as follows : 1) A significant longitudinal displacement of the shaft arises due to coupling between torsional and longitudinal vibrations when the shaft diameter is increased to make the natural frequency of the torsional 1-node vibration higher than the exciting frequency. 2) The shaft of decreased diameter with the 5-or 6-cylinder engine induces little longitudinal displacement at the thrust bearing due to torsional vibration. 3) When the diameter of shaft is increased to avoid resonance of the torsional 1-node vibration, more significant exciting force of the for-and-aft engine vibration is induced in the 5- cylinder engine than in the 4-cylinder engine. The following two reasons may be considered : a) The crank shaft of the 5-cylinder engine induces larger longitudinal displacement associated with torsion. b) The shaft system with the 5-cylinder engine has usually closer natural frequency of the torsional 1-node vibration to the exciting frequency.