Life Without Cavitation

The fewer vibrations a boat subjects you to, the more pleasant it is to be aboard. And while waves are certainly a big part of that equation, the boat’s power system is responsible for creating constant quaking and shaking every inch of the cruise. That’s one of the reasons why you can spend a day buzzing around on placid, calm water yet still come home feeling worn out.

The surprising thing? A lot of those vibrations don’t come from the engine—they’re actually generated by propeller cavitation. Those bajillions of bubbles in your prop wash, formed via vortices at the tips of the spinning propeller blades, are a visual indication of this phenomenon. Despite being on boats for my entire life, and for well over 30 years on a professional basis, I always took those bubbles for granted and had no clue that the propeller was a vibration-creating culprit. I had always blamed the engine.

At least, I did right up until the moment we got on plane on a 30-foot center-console rigged with a pair of 300 hp outboards swinging Sharrow propellers. There were far fewer vibrations, far less whitewash appeared behind the boat, and we heard far less noise than we had just a few minutes earlier while running the exact same boat with traditional three-blade stainless-steel props.

Even more surprising was that we were on plane in the first place. With the outboards spinning those regular propellers, at 3,000 rpm, the boat had mushed along at 14 mph with its bow kissing the sky. It just didn’t have the oomph to get over the hump and break on plane. Turning 3,000 rpm with Sharrow props, on the other hand, we cruised on a level keel at 24 mph.

The difference in fuel economy was equally eyebrow-raising, at 0.9 mpg for the regular props versus 1.4 mpg for the Sharrow props. The speed and efficiency difference remained downright extreme up to the 4,000 rpm mark, where it shrank a bit but maintained at least a 7 percent advantage—all the way up to 5,500 rpm.

The tech that makes it all happen is the elimination of the propeller’s blade tips. Rather than having blades like every other prop that’s ever pushed a boat, the Sharrow Propeller has loops springing from its hub. With one end attached to the front of the outer hub and the other to the back, the blade tips are completely eliminated. And those blade tips just so happen to be where most of that vibration-inducing, efficiency-eating cavitation is formed.

Added bonus: Having spinning loops rather than blades also nets you a handling advantage. Turns are noticeably tighter and crisper. You’ll appreciate the difference the most, however, when operating in reverse. I wasn’t able to measure the exact turning radius out on the water, but when I cranked the wheel over and pulled the throttles into reverse, my gut feeling was that I could back the boat in a circle half the circumference it had required with regular props. Oh, and the usual shaking and shimmying that goes along with a healthy application of reverse power? That was MIA.

The exact degree of performance and efficiency advantage you’ll enjoy when spinning Sharrows versus regular props differs from hull to hull. In fact, each individual propeller is itself a bit different because they’re optimized to match up with specific boat/motor combinations and then finished to within one- to two-thousandths of an inch at the company’s Detroit, Michigan, machine shop. That’s why you can’t simply walk into a store and buy one a Sharrow Propeller off the shelf. Fortunately, the company has tested plenty of boats with these props, and there’s data for all different sizes and types of rigs at their performance reports webpage, which can give you an idea of the sort of change in performance you’re likely to see with your own specific boat. And, lest one wonder about the veracity of those company-generated reports, note that when I gathered my own performance numbers on that 30-footer, they matched Sharrow’s published report to a tee.

So, should you get Sharrow props for your boat? Naturally, it’s your call, but if you get a chance to ride on a boat that’s rigged with them, that call will become a whole lot easier. Because once you experience life without cavitation, you won’t want those bajillions of tiny bubbles sapping efficiency and ruining your good time ever again.

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