A Story about Props

QuestionMy grandkids are already planning to invade our home this summer for weeks of skiing, tubing and “stuff.” The older one asked if my boat had a “speed prop.” I’ve already thought of two other “S” words that I’d want to hear before “Speed,” but I said I’d check into it. I believe you covered it before, but I can’t find the article. The boat is an 18-foot Carolina Skiff with a Yamaha 115 four-stroke.

Samuel Battle, Columbia, S.C.

AnswerIf you’re a grampy, you remember the halcyon days of Elvis, Darin, the Everlys and so forth. Life was wide open — or not. In those days, the predominant engine was the Mercury, to which most owners affixed a two-blade brass/bronze/aluminum prop, also known as a speed prop. Slicked-back ducktail-coiffed boat operators would race each other up and down the lake all day long. At 15 cents a gallon for gas, the sun never set. Skiing was fun, too, but the two-blade prop just couldn’t pull the skier out of the water, so a prop change to a three-blade was in order. Problem resolved. Race, change prop, ski, change prop, race, change…

However, there was no thought given to engine longevity through comprehending power ratings and curves, nor arriving at the conclusion that changing props took up precious hours of beach-blanket bingo. I missed the day somebody said “Eureka!” and understood the world was changing on the water. I believe some manufacturer finally had enough of engines self-destructing and labeled each with a power rating at a certain rpm. If you look at it as a Bell Curve, the engine would increase its power to Point X, and any rpm increase beyond that resulted in less power. That set parameters for engine longevity, a compromise between “grunt” and “go,” and more time messin’ around on the beach.

When the prop manufacturers became involved in this morph, the heavens opened up and the Lord himself smiled. Not only did operators become more aware of pitch and diameter relative to the power band of the engine, they became aware that fine-tuning props made a heck of a difference. Blade stiffness and thickness, blade ventilation, rake — they all contributed to performance. If your engine was turning 6000 rpm but was rated 115 hp at 5250 rpm, then you installed the prop that brought your rpm down by 800 to about 5200 rpm with your normal boat load.

If you had an 18-inch three-blade prop that let the engine turn 6000 rpm, then increasing pitch to 20” would roughly achieve that goal. It would also theoretically advance the hull 2-inches per prop revolution, increasing boat speed and power translation.

Take your boat with your normal load aboard for a run at full throttle. You should be showing roughly 5500 rpm. If you are, you have a great prop for normal use. Ask your dealer if the prop is ventilated. If it is, the kids will do just fine skiing with heavy acceleration. If it hasn’t been ventilated, ask him to perform that simple operation. It’s an easy way to achieve increased performance under all circumstances.

Oh, and tell the grandkids the boat has the correct prop, uses gas and they can contribute to expenses as part of the growth/learning curve.

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