A small gap between the top of the rudder and the hull surface should reduce distortional forces on rudder, depending on laminar flow characteristics of the particular hull.
Typically I install the rudder with about 1/2" between the top of the rudder and the hull.
From the constructive point of view, a root gap is almost unavoidable.
But from the hydrodynamic point of view, the root gap is detrimental to both lift and drag characteristics of a rudder.
It puts in direct communication pressure and suction sides of the foil at the root, allowing the waterflow to leak and create vorticity (and hence to waste energy and increase drag) and to diminish the pressure difference between the two sides (and hence to decrease lift).
In other words, it diminishes the effective aspect-ratio of the rudder - with all the related and well-known consequences to rudder lift and drag curves.
Just to give you a numerical example:
You said that you have 1/2" on your rudder? If your rudder's mean chord is (for example) 25", the effective aspect-ratio will be some 10% less than the aspect-ratio of a rudder with no gap. It means more induced drag and less lift for a given deflection angle.
The positive side of the story is that your rudder will have a delayed stall, which can also be a desireable feature.
So if you cannot avoid to have a root gap you should try to keep it to a minimum necessary. If you need to minimize the loads acting on the rudder stock, or if you need to have a more stall-resistant rudder, then you have other more efficient ways of reaching the goal, like changing the rudder area, shape, foil sections etc.
That being said, I was reading through David Gerr's Boat Mechanical Systems Handbook and he seems to suggest that the interference of the hull increases the efficiency of the rudder and can dramatically affect the CL calulations, is that the case or does the gap nullify the effect?
I have not read that book so I don't know what was Mr. Gerr referring to in particular, but I'll tell you how I see it.
The hull "interference" acts in two ways, basically:
1) the hull boundary layer slows down the flow around the rudder root
2) the hull volume displaces and distorts the water flow around it, accellerating it (in the region outside of the boundary layer) to speeds which are somewhat higher than the free-flow velocity.
Knowing that, a higher inflow velocity to the rudder means that you can have the same lift force with a smaller angle of attack. A smaller angle of attack means less induced drag. That's why the efficiency increases. Of course, this influence will greatly depend on where the rudder is placed below the hull.
The effect of the gap will be as seen above.
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