brushless for outboard of a 4 meter boat
Torque (couple?) is proportional to current. With only 30 amps available, the motor should not be loaded with more than about 45% of rated torque. Limiting the voltage to 24 V (65% of rated) will probably limit the speed to 65% of rated speed. Limiting the speed will probably reduce the torque in proportion to the square of speed 0.65 squared is 0.42 or 42%. That should be enough for the motor to handle with the power supply limit. That is just a guess based on the limited details given. If you can find data on the motor, controller and propellor, a better estimate could be made.