72 Horsepower DC Motor
I am saying that your yacht does not have a 72 HP DC motor. Yachts, unlike, say, jetboats and outboards, have what's called a displacement hull. This means that it's good in choppy water and it needs very little power to go slowly. In fact, more power won't help speed much. If you look at the wiring and ballpark the current at your known voltage of 72 V then the kW will be apparent. You can ballpark the efficiency of the DC motor at, say, 90%. Remember that 1 HP is 735 watts (horsepower according to DIN 66036) and then you are done. I would also study the size and hence kWh of the heavy onboard battery pack and use this to support a much lower answer.