with the size prop then your motor can handle.  If you are under speed there is nothing you can do but put in more powerful motors.
Cavitation is the other problem that can cause ship speed to decrease (or remain the same) when power is increased.  This condition occurs when the props are spinning too fast and operating very inefficiently, causing a loss of performance.  When cavitation is present increasing the voltage will either not effect the ships speed or actually slow it down and decreasing the voltage may speed the ship up.  To check for cavitation look for a visible stream of bubbles coming from the shafts.  It is common for some cavitation to occur when starting from a dead stop, but no bubbles should be streaming from the props once the ship is at speed.  To correct this problem you will need to slow the props down by either reducing voltage or by selecting a lower blade angle on the prop.  Once you get cavitation to stop hopefully your ship is at the desired speed.  If it is still to slow you will need to add a larger diameter prop
The purpose of the drive trains is to transfer power from the motors to the prop shaft(s) in a reliable, efficient manner.   As the above examples show there are several factors one must consider before deciding which drive systems is best for a given ship.  These include but are not limited to possible motor locations, motor speed, torque, number of shafts, and reliability.  As for driving motors there are really only two methods in rc combat warships.  These are direct drive and rpm reduction drive.  Reduction drives are either gear or belt and pulley.