Bravo I LT and Bravo I FS belong to the series of pro-finished Bravo I propellers, each one of which has undergone highly specialized modifications so as to match in specific uses and in hulls with equally specific requirements.
The wide range of Bravo I FS has been enriched with new numbers of pitch and is now available in pitch sizes: 20.5", 21.5", 22.5", 24.5", 25.5", 26.5" and 28.5".
The same thing is happening for Bravo I LT too, which is available in pitch sizes, ranging from 19" to 29", in one-half inch increments.
Both propellers have a diameter of 15 ¼", and their availability in one-half inch increments gives us the ability to approach with great precision our engine's maximum engine speed at WOT.
We tested the Bravo I LT 15 ¼" x 19" and the Bravo I FS 15 ¼" x 20", in the Shearwater 860 on the transom of which a Verado 350hp was mounted, trying to compare their performance.
Observing the above table we see that Bravo I LT:
- got the rib on plane at lower rpm
- was offering a lightly shorter time to plane
- was faster by 3 knots at 3500 and 4000 rpm despite its shorter pitch, while after 4500 rpm where the rib’s wet surfaces are reduced, the FS managed to catch it reaching almost the same speeds until the WOT
- increased mid-range economy by 0.2 litters per nautical mile, while over 4500 rpm it showed the same fuel consumptions as FS
- showed lower slip values across the whole rpm range, causing the higher cruising speeds it achieves
- reached exactly at the same WOT rpm but achieved a higher top speed despite its shorter pitch.
By comparing the behaviour of the two props, η Bravo I LT:
- provided impressive overall lift, raising the Shearwater’s stern much more higher than the FS
- was bringing the rib in a more level ride at the medium rpm
- offered a quicker throttle response and faster accelerations throughout the engine rpm range
- improved handling and showed greater stability at high trim angles and sharp turns.
Observing the design features of Bravo I LT we noted three important differences compared to Bravo I FS:
- it has longer tube
- its tube became wider at its end
- it is maintaining the same rake angle along its blade length, thus leading more water to its barrel, giving it the ability to hold the bow high.
The above differentiations of Bravo I LT are responsible for its outstanding performance on the particular rib, making it ideal for Shearwater's hull and centre of gravity!
The combination of its longer and framed tube with the higher rake angle at the tip of its blades create more overall lift that makes it unique for applications in heavier or longer single engine outboard powered ribs, in hulls unable to lift their stern or when we carry heavy loads.
So while Bravo I FS is unable to perform because «it does not like loads» and is designed to keep the stern low, LT comes to replenish it.
Undoubtedly, both of them are great propellers, but when it is time to choose between them, we first have to take into account the geometry and length of our hull, the total load of the rib and the weight distribution.
So when we switch from a Bravo I FS propeller to a Bravo I LT one, we should expect a reduction in engine’s rpm by about 100 to 150 rpm.
In other words: a 20" Bravo I FS offers almost the same engine’s rpm at WOT as a 19" Bravo I LT.