Öz Maximum free space optical communication distances betvveen mobile on-sea terminals, by night, are numerically evaluated for 2.4 to 57.6 kbps bit rates and clear and hazy weather conditions. Advantages o f a medium power laser diode at the transmitter are pointed out. Effects of other design param etas such as the use o f an opticai filter, the size and position o f the iaser spot cm the receiver four- quadrant detector are analysed.