For those of you who may not know me, I manage Mercury Racing’s propeller department. We offer a wide variety of high performance outboard and sterndrive props. I oversee production and manage our prop offerings. My most enjoyable responsibility is helping customers solve their unique propeller issues.
I’ve been getting great feedback regarding our Bravo I XS outboard propeller. The latest is from Allison owners who say it is an awesome prop for their XB-21 tournament bass boats. XB-21’s rigged with an OptiMax 250 Pro XS do best running a 27-inch pitch Bravo I XS. Those who power-up to the OptiMax 300XS say their ride is dialed-in using a 29-inch pitch Bravo I XS. XB-21 owners have found odd-pitch Bravo I XS props provide the best hole shot and bow lift.
How They’re Made
So, I bet your wondering, “Did I just read that right? Bravo props for outboards? I thought they were for Bravo One sterndrives?” Well, you’d be correct on both points. We designed Bravo I XS (new in 2010) and Lab Finished Bravo I (featured 1998) prop for low-emissions, 2-stroke OptiMax outboards. And they are derived from Bravo I props for sterndrives.
Standard Bravo I propellers are cast in even, two-inch pitch increments (22″ to 36″). To use the Bravo style prop on 2-stroke outboards, we needed to expand the range of pitches. We added odd-pitch sizes in one-inch increments. We do that by increasing blade cup height on even-pitch castings — making them behave like “taller” pitches. For example, the 27-inch Bravo I XS (part number 48-831914L80) begins life as a 26-inch pitch Bravo I XS casting. Additional cup on the trailing edge of the blades differentiates a 27-inch pitch XS from a 26-inch pitch model. The part number for a 26-inch pitch Bravo I XS is 48-831914L60. The base part numbers for 26 and 27-inch models are the same. L80 designates higher blade cup height.
Another birthmark differentiating a Bravo I XS from standard is the addition of four, one-inch vent holes on the prop hub. The holes add prop blade ventilation. The engine spools up faster for enhanced hole shot.
One more difference is the tuned exhaust tube: we’ve shortened the tube for less drag at wide open throttle (WOT), picking up another one mph over a stock Bravo I.
You may not think a slight change in blade cup height would have a big effect on performance – but it does. In the case of an Allison XB-21, the performance difference between a 26-inch and 27-inch pitch Bravo I XS is 150 rpm at WOT. If a 26-inch pitch allows an engine to rev to 6000 rpm at WOT, then a 27-inch pitch would typically lower rpm to 5850. The reduction of 150 rpm provides “a window of performance opportunity.” Its like free horsepower! — or a second chance to dial-in the boat for maximum overall performance. A slightly bigger, cupped, 27-inch pitch prop might stay hooked up longer, allowing a tick more trim for additional bow lift and offer higher speeds. It may also allow increased engine height of 1/4 to 1/2-inch. If the motor can be raised, without sacrifice safety or performance, one will usually regain 50 to 75 rpm, resulting in a higher top-end speed and better overall boat performance.
Cup plays an important role bringing a boat on plane and affects bow lift. Running an odd-pitch Bravo I XS, with its tall blade cup, can actually scrub top-end speed on boats that have natural bow lift. For example, air-entrapment tunnel hulls from Simmons Custom Boats typically run best using even-pitch Bravo I XS props.
The Bravo I XS is also popular on Bullet Boat’s 21 XD with OptiMax 250 Sport XS or OptiMax 300XS power. Bullet owners tell me a Bravo I XS provides great planing and mid-range punch while giving up little top-end speed. Tournament anglers say weight doesn’t seem to affect performance as a Bravo I XS “…carries the load, no matter what we throw in the boat.”
The Bravo I XS is available from 22″ to 31″ pitch, see your authorized Mercury Dealer to order.
There you have it. We sweat the details to ensure people enjoy high performance on the water. I’ll be back with more tips, helpful hints and insights regarding high performance props.