I think the problem with too little clearance is vibration (noise?)
created as prop blades pass by the narrow spot. The vibration is less
with more blades and/or more clearance.
--- In AtkinBoats@yahoogroups.com, "adharvey2" <adharvey@...> wrote:
> In the article accompanying the plans for Sand Dollar, Wm. Atkins
> "There is ample clearance for a propeller diameter of 15 inches. I
> think it may be of interest to explain that the tips of the blades can
> clear the bottom of the horn timber by very little and still perform
> efficiently. There is little, if any, advantage of two or three inches
> space between the tips of the blades and the hull.