Let S be the number of people on the first hundred years’ ship.
The second hundred years’ ship had twice as many as the first, so it had 2S people.
The third hundred years’ ship had twice as many as the second, so it had 2 * 2S = <<2*2=4>>4S people.
All the ships had S + 2S + 4S = 7S = 847 people.
Thus, the ship that the monster ate in the first hundred years had S = 847 / 7 = <<847/7=121>>121 people on it.
#### 121