Originally Posted by qcamel
I think the point being made is that if you move the engine backward to find cleaner water then you would need to move it back more than a couple of inches to have any effect...
If you have 10" setback, then the areated water will be hitting the prop 0.01234 seconds later at 40kts. Now travel behind any boat at 40kts and see how long the water remains disturbed and areated for. I'm not convinced that the setback will make a tangible difference, even at 4kts, but I could be wrong. Naval Architect or Fluids Dynamics Engineer please!