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Here is a photo of
Reciproca's propeller, made of free machining brass. The very small developed blade area (about 1/3 of the area of my first propeller) is a result on calculations available in
Propeller Handbook. I was very doubtful as to whether it would work, so I whipped up the first draft of it in steel so as to not waste much money on materials. Once in the water though, there was no looking back, it worked very well. The small area makes for pretty weak bollard pull, so she doesn't tow very well, and a headwind really slows her down. In calm conditions though, she really goes, no wasting of power on dragging oversized blades through the water.
Here is a photo of the forms I used to make the blades. The one on the left is the quick & dirty form I used for the experimental steel blades. The one on the right is the cast iron form I had cast to give a smoother shape to the brass blades. I started with 3/8" thick brass, band sawed to shape, and shaped with a hand grinder into the proper profile for a blade. The blade blanks needed to be heated with a torch,and hammered while still hot to take on the needed twist. The still roughly rectangular stubs on inside end of each blade were then silver soldered into corresponding slots milled into the hub.
All that being said though, with Michigan Wheel offering 12"x18" props, I would buy one of those before attempting to make another homemade prop. If needed, it is easy to remove excess blade area from a commercial prop using a hand grinder with a "cutoff: wheel, then fairing the surface into the new profile.
It was not easy to convince Allnutt. All his shop training had given him a profound prejudice against inexact work, experimental work, hit-or-miss work.
