How much clearance should I leave between the hull and the propeller blade tip?

The question of available space and tip clearance is a bit complex, and requires some additional information, and ultimately a judgment call.

Minimal tip clearance has little effect on the performance of a propeller. The real reason for tip clearance is to reduce noise and vibration created when the vortices coming off the blade tip reach the hull. Due to the shape & efficiency of Flexofold blades, we have lower tip vortices than any other low drag propeller to begin with.
 
There are 3 factors that come into play in deciding how much tip clearance to leave:

  1. What is the hull shape? A flat hull above the prop will create more resonance (like a drum skin), while a V-shape or rounded hull form will reflect more of the tip vortice
  2. What is the hull construction? A modern, light laminate will resonate more than an older, heavier laminate. A cored hull may increase or decrease resonance, depending on the core material.
  3. How tolerant are the boat owners of noise and vibration under power? If you’re used to a relatively vibration prone 1 or 2 cylinder diesel, you may not notice the noise created by low tip clearance.