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I

n the previous issue we looked at the fuel consumption and range of early offshore powerboats, and began our examination of powerboat efficiency, by using transport effi-ciency and miles per gallon to compare boats. We saw how longer more slender hulls could be moved farther faster for the same amount of fuel. Comparing different hull forms normalized for displacement, we also saw that driving boats at lower speed/ length ratios also improved effi-ciency. We’ll con-clude our discus-sion of powerboat efficiency here by looking at the ef-fect of improving propulsion ma-chinery and also at the effect that overall size has on efficiency. We’ll also consider how efficient slender hull forms affect seakeeping, com-fort, and accom-modations.

The Effects of Larger Diameter Propellers

Taking our 67-foot Ironheart , we can get an idea what addi-tional performance can be garnered from increasing propel-

ler diameter, which means reducing shaft rpm. If we as-sume a standard 3:1 gear on Ironheart ’s 419-hp engine, we’d find a 4-blade propeller of 40-in. diameter by 36-in. pitch, for an approximate propeller efficiency of 65%, If w could install a larger 5:1 reduction gear (and a much larg propeller) we could then install a 56-in. diameter by 67-in pitch 3 blade, with an approximate efficiency of 73%. Th

reduction in horsepower fro the increase in efficiency can found from:

Resulting HP = original HP x standard efficiency ÷ n efficiency

In this case, 0. ÷ 0.73 = 0.89 89%

Accordingly, where we nee 113 hp for 13. knots, we wou only need 101 hp; and for 16.9 knots we would require 3 hp rather than 419. Obviously, this would increase transp efficiency and thus fuel economy.

What Efficiency Means, How It Can Save Money and How it Makes for Better Boats

By Dave Gerr, © Dave Gerr, 2011

Summer Moon II —82-ft. Voyaging Motorcruiser

Summer Moon II —82-ft. Voyaging Motorcruiser

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