In the world of outboard fishing boats, propeller selection is always a hot topic of debate. That's especially true now that big-time salt water tournament circuits like the Southern Kingfish Association have flourished. Competitive fishermen constantly seek a superior hole shot and higher speeds in their quest to gain an edge over rivals.
Even if you're not a gung-ho tournament type, trying to find the perfect prop for your boat is a never-ending problem. Selecting the correct pitch is essential, but many people pitch their props to run fast with a light load. When you load the boat down with thousands of pounds of fuel, ice, tackle and people, it may be over-propped, causing the engine(s) to lug and potentially shortening their service life.
Many of us have experienced the frustration of trying to run a heavily loaded boat offshore in rough seas. The small diameter of outboard props makes them less than ideal for pushing a heavy boat up the face of large waves, so the driver must constantly adjust the throttles to keep the boat at a steady speed. If you have to run a long way in rough water, it can be an exhausting proposition.
Here's the Pitch
For a moment, though, imagine not having to worry about your throttle setting in rough seas, or whether or not your boat has a heavy or light load. Imagine having the same experience with your outboard that you now take for granted with the automatic transmission in your car or truck. Such a radical new possibility is at least on the horizon due to an extraordinary new propeller now in the experimental stages at Mercury Marine.
Mercury introduced this breakthrough to the boating press last December at its annual dealer meeting at Disney World in Orlando, Florida. Although the prop has no official name as of yet, the engineers who designed it call it the "SmartProp" to go along with Mercury's innovative line of SmartCraft engine-monitoring systems.
The SmartProp is a variable-pitch propeller, which is certainly not a new concept in and of itself (airplanes have used variable-pitch props for years). And there have been previous attempts at developing variable-pitch props for marine use. Mercury had one a few years back, but none were anywhere close to what the SmartProp offers.
Earlier versions of variable-pitch props had two fixed pitch settings - basically a "low" and "high" gear, to use an automotive analogy. But while cars change the gear ratios within the transmission (it would be problematic to change the size of your tires while rolling down the highway), the gears in a marine drive system are fixed, so the only way to alter the power ratio is by changing the pitch of the propeller.
The two-pitch props relied on a heavy-duty spring mechanism that would shift from a low pitch, which was useful for getting on plane, to a higher "cruising" pitch at a higher speed. The shift came as a result of inertia at a certain rpm level as the throttle setting was increased. Likewise, when decelerating, the springs would shift the prop back to the lower pitch setting. These props never really worked too well and met with limited market success.
A New Proposition
As we said before, SmartProp is a whole new animal. The pitch in this prop is controlled hydraulically and is infinitely adjustable within a specified range. It is currently being tested on MerCruiser Bravo sterndrives, with incredible results. We got to play with one on a runabout boat at Disney World, and were blown away by the possibilities of such a technological advance.
Carl Schott, Mercury's Senior Manager of Hydrodynamics, is in charge of the SmartProp program, and he was the one who took us for a ride aboard the boat in Orlando. He controlled the prop pitch at dockside with a slide switch that allowed him to maneuver the boat at idle without ever shifting the transmission. When the switch was in the middle of the graduated scale, the prop had no pitch and simply spun without providing any thrust. By sliding the switch down, the prop pitched itself in reverse, causing the boat to back away from the dock. When Schott slid the switch forward, the prop developed forward thrust and off we went! It was an amazing performance.
Schott says the SmartProp offers several different ways of controlling pitch. "The pitch can be adjusted in all operating conditions under control of the engine and its ECU computer," he explained. "The SmartProp offers pitch settings from 15 inches in reverse to 32 inches in forward, and we can command any pitch in that entire range any time we want."
Here's how it works: The mechanisms that move the blades are housed in the gearcase. An electric hydraulic pump (basically a modified trim tab pump) and valve assembly provide the hydraulic pressure and are controlled by the engine's ECU, and an accumulator helps build up the necessary pressure. A hydraulic slip ring puts hydraulic pressure on the rotating prop shaft, and there is a bearing assembly between the piston in the actuator and the prop. The actuator is fixed and doesn't rotate.
This complex engineering, combined with the control of the ECU, allows SmartProp to operate in three distinct modes: Continuous, Discrete Shift and Manual. The manual mode is the one described earlier, where you manually control pitch with a sliding switch. You can choose any pitch in the aforementioned range by merely moving the switch left or right.
Continuous Mode and Discrete Shift are infinitely more sophisticated. In the continuous mode, you choose a desired rpm and the ECU automatically adjusts the propeller pitch to maintain that rpm. Future versions may also be able to do the same thing using boat speed as the determining factor. The previous example of running a boat in heavy seas would be taken care of. All you need to do is set the throttle at a comfortable setting and the ECU will maintain your rpm by adjusting pitch. No more applying throttle to climb waves, and backing off when running down the backsides.
The same goes for a Bimini start in a tournament situation. In the continuous mode, you could firewall the throttles and the ECU would increase pitch to keep the engine from reaching its rev-limiter until peaking at the maximum rpm that's appropriate for the load you have onboard. The pitch is automatically taken care of no matter what size load you're carrying.
The Discrete Shift mode operates like the automatic transmission in your car in that it shifts up and down through a predetermined series of "gears," in this case pitch settings. You have a low-pitch setting for climbing out of the hole, a mid-range setting for acceleration and a high-pitch setting for maximum speed and efficiency at cruise.
Wave of the Future
It's important to note that SmartProp isn't on the market yet and has only been tested on Bravo sterndrives because the Bravo gearcases are bigger than outboard gearcases and all of the necessary equipment fits into them. But Mercury is undoubtedly looking down the road toward applying this technology to the outboard market.
"The load numbers we've been seeing were significantly lower than what we had estimated," Carl Schott said, "so we're hoping that the parts can be downsized and potentially fit into an outboard gearcase." Schott adds that the durability of the system has been outstanding so far. The SmartProp is really just an extension of the SmartCraft system that will place all engine functions under the control of the engine's computer (ECU). This is an industry-wide trend and it promises to bring us lots more sophistication down the road.
"It's about time someone did this in the boating industry," Schott said. "It will make your boat run a lot more like your car, and allow you to tune the whole drive system." That is undoubtedly true, and a good thing. But I'm going to miss all of those dockside arguments about who has the best props!