Mate here's my 2 bobs worth - caution I do not have a text book with me at the moment and this is from memory.
In the jet example, the thrust available curve is often presented as 'flat' as the speed increases.
Think of the thrust required curve as a drag curve. So as this curve decreases towards a minimum value then starts increasing again, the minimum value of thrust(drag) will occur at the max excess thrust or the L/D max position. This is best angle speed. Tangent this minimum curve and you get Vy - best rate - always higher in value.
In the prop case the power available curve continues moving up (increasing) before it sarts to reach a maximum value then starts to decrease over the increasing speed value range.
Consider the same power required curve underneath - again read this as a drag line and you have max angle again at the bottom of this curve BUT L/D max occuring at the speed where the greatest distance exists between the Power Aavail and power required curves - same logic with the jet - but this is best rate and is also the tangent point of the curve - again just like in the jet case.
I hope this helps
