What have I started? Now, to end it:
Weight of a small ship: 5,000,000 kg
Weight of a Stone: 0.5 kg
Acceleration by human throwing: 50 m/s² (roughly estimated)
Frequency 1 stone/second
Force of a stone: 0,5 * 50 = 25 N
Acceleration of the ship by one throw: 25N/5,000,000 kg = 5x10^-6 m/s² = 0.000005 m/s²
Distance between planets by example of earth-mars: 3x10^8 km
Time it would take to bridge that distance using the stone-drive: 6.9*10^7s = 2.2 years (assuming atmospheric deceleration)
Except for possible sore arms quite reasonable, eh?
Well....
Necessary amount of stones in the cargo hold: 6.9*10^7 = 70 million
Weight that has to be swept under the carpet: 34,500,000 kg
Solution: place portal under carpet. All is perfect!