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To the untrained eye the Davidson propeller looks little different to a conventional propeller but the 75 year-old inventor said he has spent 20 years of trial and error perfecting the design. Mr Davidson, whose father was an engineer, said he spent a lot of his time manufacturing stainless steel components for honey extraction and had operated his own engineering workshop in Timaru for a number of years. As well as extraction fans for the honey pack house, he had designed and built an air fan for a new design of hovercraft based on the same principle as the boat propeller. The big air fan, which provided both lift and thrust for the hovercraft, ran silently even at over 2000 rpm when conventional fans whistle as the air flows over the blade tips at high speed, he said.

About ten years ago Mr Davidson designed and built a propeller for a fishing boat with a slow revving single cylinder diesel engine which only developed 10 horse power. Mr Davidson said the new propeller made a big difference to the pulling power of the boat. “She had so much power at four or five knots that we had to be really carefully when trawling close inshore. If we got snagged on a reef we had to throw the old girl out of gear quick or she would simply snap a trawl rope like a piece of cotton.” Mr Davidson said even early prototypes, which were very similar to the final model, did not produce efficiency gains.

“I am not sure exactly what makes it so efficient but I knew there was something in the basic design which needed to be explored and expanded.” In a series of tests on a 25hp outboard motor Mr Davidson found there was a 40 percent increase in engine efficiency, measured simply by boat speed, over a conventional propeller. There were a number of other, surprising advantages with the new design which including a lack of torque effect; the tendency of spinning propellers to pull the stern of a vessel sideways. This allowed the boat to turn much sharper without the usual amount of sideways drift. A second benefit was the lack of the usual electrolysis effect of spinning propellers in salt water, which causes rapid corrosion of soft metals like brass and aluminum¸ which Mr Davidson said he was unable to explain.

The Davidson propeller is equally efficient in forward or reverse while conventional propellers have little efficiency in reverse. Although he left boat building “a few years ago” Mr Davidson said he had no intention of full retirement and was kept busy running his beekeeping operation and working on the propeller.



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