18t is about the maximum the airplane can carry with a max ZWF of 48.307 kg.
DOW of a B737-300 is around 31 t (a little less) , with a load of 18 t, ZWF will be 49 t.
1.500 NM trip in still air is around 3:40 hrs @ 280/.74, tripfuel @ 9,5 t.
Add min 2 t alternate & holding fuel, so the trip requires around 11,5 t fuel min.
TOW will be around 60,5 t, LW around 51 t.
So, basically yes, a trip of 1.500 NM with 18 t load should be possible but you have to have a more detailed look into the exact weights as this trip is operating at the max ZFW of a B737-300. I have used only rounded numbers above.
If you really need this weight of load, have a look into the B737-400F.
Most of the time, weight is not the problem but the volume of the cargo.
There are different certified versions in respect to weights of the B737-300F available, the standard max certified LW of a -300 is 51,7 t but there are also versions with 52,8 t max certified LW available.
Last edited by TheWanderer; 10th Jul 2010 at 11:53.