Why only half the peak voltage in my half wave rectifier?
You haven't said that there's any reservoir capacitor in your circuit.
So your voltmeter is doing its best to display a DC voltage, when it's actually getting half of a sine wave.  The other half is blocked by the diode, so the voltmeter is getting 0V on the negative half cycle.
So if we take a secondary voltage of 14V, knock off about 0.7V, and then halve it because half the time the voltmeter is receiving 0V, then the result will be something like 6 to 7V.