During the Olympic 5.90 Speedster test, on the transom of which a Mercury four-stroke 150hp engine was mounted, we had the opportunity to compare two outstanding propellers: the Bravo I FS 15.25" x 21" and the Bravo I XC 15.30" x 20".
We should remind that both of these propellers are pro-finished Bravo I ones, i.e. standard Bravo I propellers in which very specific and targeted modifications have been made.
The wide range of Bravo I FS has been enriched lately with new numbers of pitch, giving us the ability of greater precision concerning the WOT rpm. Thus, among other numbers, Bravo I FS is also available in pitch sizes: 20.5", 21.5", 22.5", 24.5", 25.5", 26.5" and 28.5".
The sea trials of the mentioned propellers took place on the same day, in the same sea area, with the same loads, in the same course and keeping the same angles of trim, so as to evaluate them as realistic as possible.
Observing the above table we see that Bravo I XC:
- gets the rib on plane at lower rpm
- offers a shorter time to plane by about one second
- is considerably faster by 1 - 5.5 knots at low and medium rpm despite its shorter pitch.
- has impressively lower slip values across the whole rpm range, due to which the higher cruising speed is achieved
- achieved almost the same top speed
- gives less rpm at WOT, by about 350 despite its shorter pitch, while it was not losing its grip even when it was operating at a higher engine position by 2cm.
By comparing the behaviour of the two props, Bravo I XC:
- provided impressive stern lift, keeping the hull much more higher above the sea level, which we felt strongly from the first moment of our test
- was keeping the rib on a level ride attitude, even at the lower speeds
- offered a quicker throttle response and faster accelerations throughout the engine rpm range
- could run closer to the water surface without losing its bite, allowing us to trim the engine more out or to run at elevated transom heights
- provided much better handling and greater stability even at high trim angles or sharp turns.
By studying the design features of Bravo I XC, we noted some important differences compared to Bravo I FS:
- it has a longer tube by about 12 mm, which became wider at its end
- it features a tremendous amount of cup on both the trailing edge of its blade and its blade’s tip
- it has a slightly larger diameter, which means a larger blade area
- the trailing edge of its blades is more convex
The above differentiations of the Bravo I XC are responsible for its impressive performance in terms of acceleration, stern lift, ride attitude, slip and cruising speed, factors that make it ideal for rough seas, for ribs which have their bow high during the ride or when carrying heavy loads. All of these, of course, are not offered without cost: the load for the engine is heavier, resulting in much less engine rpm at WOT compared to Bravo I FS.
So when we switch from a Bravo I FS propeller to a Bravo I XC propeller, we should expect a reduction in engine rpm of about 550 rpm.
In other words: A 20" pitch Bravo I XC offers almost the same engine rpm at WOT as a 23" pitch Bravo I FS.